[C++2a] Add __builtin_bit_cast, used to implement std::bit_cast

This commit adds a new builtin, __builtin_bit_cast(T, v), which performs a
bit_cast from a value v to a type T. This expression can be evaluated at
compile time under specific circumstances.

The compile time evaluation currently doesn't support bit-fields, but I'm
planning on fixing this in a follow up (some of the logic for figuring this out
is in CodeGen). I'm also planning follow-ups for supporting some more esoteric
types that the constexpr evaluator supports, as well as extending
__builtin_memcpy constexpr evaluation to use the same infrastructure.

rdar://44987528

Differential revision: https://reviews.llvm.org/D62825

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@364954 91177308-0d34-0410-b5e6-96231b3b80d8

include/clang-c/Index.h

@@ -2546,7 +2546,11 @@ enum CXCursorKind {
    */
   CXCursor_OMPTargetTeamsDistributeSimdDirective = 279,
 
-  CXCursor_LastStmt = CXCursor_OMPTargetTeamsDistributeSimdDirective,
+  /** C++2a std::bit_cast expression.
+   */
+  CXCursor_BuiltinBitCastExpr = 280,
+
+  CXCursor_LastStmt = CXCursor_BuiltinBitCastExpr,
 
   /**
    * Cursor that represents the translation unit itself.

include/clang/AST/ExprCXX.h

@@ -4716,6 +4716,35 @@ public:
   }
 };
 
+/// Represents a C++2a __builtin_bit_cast(T, v) expression. Used to implement
+/// std::bit_cast. These can sometimes be evaluated as part of a constant
+/// expression, but otherwise CodeGen to a simple memcpy in general.
+class BuiltinBitCastExpr final
+    : public ExplicitCastExpr,
+      private llvm::TrailingObjects<BuiltinBitCastExpr, CXXBaseSpecifier *> {
+  friend class ASTStmtReader;
+  friend class CastExpr;
+  friend class TrailingObjects;
+
+  SourceLocation KWLoc;
+  SourceLocation RParenLoc;
+
+public:
+  BuiltinBitCastExpr(QualType T, ExprValueKind VK, CastKind CK, Expr *SrcExpr,
+                     TypeSourceInfo *DstType, SourceLocation KWLoc,
+                     SourceLocation RParenLoc)
+      : ExplicitCastExpr(BuiltinBitCastExprClass, T, VK, CK, SrcExpr, 0,
+                         DstType),
+        KWLoc(KWLoc), RParenLoc(RParenLoc) {}
+
+  SourceLocation getBeginLoc() const LLVM_READONLY { return KWLoc; }
+  SourceLocation getEndLoc() const LLVM_READONLY { return RParenLoc; }
+
+  static bool classof(const Stmt *T) {
+    return T->getStmtClass() == BuiltinBitCastExprClass;
+  }
+};
+
 } // namespace clang
 
 #endif // LLVM_CLANG_AST_EXPRCXX_H

include/clang/AST/OperationKinds.def

@@ -66,6 +66,10 @@ CAST_OPERATION(BitCast)
 ///    bool b; reinterpret_cast<char&>(b) = 'a';
 CAST_OPERATION(LValueBitCast)
 
+/// CK_LValueToRValueBitCast - A conversion that causes us to reinterpret an
+/// lvalue as an rvalue of a different type. Created by __builtin_bit_cast.
+CAST_OPERATION(LValueToRValueBitCast)
+
 /// CK_LValueToRValue - A conversion which causes the extraction of
 /// an r-value from the operand gl-value.  The result of an r-value
 /// conversion is always unqualified.

include/clang/AST/RecursiveASTVisitor.h

@@ -2282,6 +2282,10 @@ DEF_TRAVERSE_STMT(CXXStaticCastExpr, {
   TRY_TO(TraverseTypeLoc(S->getTypeInfoAsWritten()->getTypeLoc()));
 })
 
+DEF_TRAVERSE_STMT(BuiltinBitCastExpr, {
+  TRY_TO(TraverseTypeLoc(S->getTypeInfoAsWritten()->getTypeLoc()));
+})
+
 template <typename Derived>
 bool RecursiveASTVisitor<Derived>::TraverseSynOrSemInitListExpr(
     InitListExpr *S, DataRecursionQueue *Queue) {

include/clang/Basic/DiagnosticASTKinds.td

@@ -215,6 +215,19 @@ def note_constexpr_memcpy_unsupported : Note<
   "size to copy (%4) is not a multiple of size of element type %3 (%5)|"
   "source is not a contiguous array of at least %4 elements of type %3|"
   "destination is not a contiguous array of at least %4 elements of type %3}2">;
+def note_constexpr_bit_cast_unsupported_type : Note<
+  "constexpr bit_cast involving type %0 is not yet supported">;
+def note_constexpr_bit_cast_unsupported_bitfield : Note<
+  "constexpr bit_cast involving bit-field is not yet supported">;
+def note_constexpr_bit_cast_invalid_type : Note<
+  "bit_cast %select{from|to}0 a %select{|type with a }1"
+  "%select{union|pointer|member pointer|volatile|reference}2 "
+  "%select{type|member}1 is not allowed in a constant expression">;
+def note_constexpr_bit_cast_invalid_subtype : Note<
+  "invalid type %0 is a %select{member|base}1 of %2">;
+def note_constexpr_bit_cast_indet_dest : Note<
+  "indeterminate value can only initialize an object of type 'unsigned char'"
+  "%select{, 'char',|}1 or 'std::byte'; %0 is invalid">;
 
 def warn_integer_constant_overflow : Warning<
   "overflow in expression; result is %0 with type %1">,

include/clang/Basic/DiagnosticSemaKinds.td

@@ -9734,4 +9734,8 @@ def err_builtin_launder_invalid_arg : Error<
   "%select{non-pointer|function pointer|void pointer}0 argument to "
   "'__builtin_launder' is not allowed">;
 
+def err_bit_cast_non_trivially_copyable : Error<
+  "__builtin_bit_cast %select{source|destination}0 type must be trivially copyable">;
+def err_bit_cast_type_size_mismatch : Error<
+  "__builtin_bit_cast source size does not equal destination size (%0 vs %1)">;
 } // end of sema component.

include/clang/Basic/StmtNodes.td

@@ -192,6 +192,7 @@ def ConvertVectorExpr : DStmt<Expr>;
 def BlockExpr : DStmt<Expr>;
 def OpaqueValueExpr : DStmt<Expr>;
 def TypoExpr : DStmt<Expr>;
+def BuiltinBitCastExpr : DStmt<ExplicitCastExpr>;
 
 // Microsoft Extensions.
 def MSPropertyRefExpr : DStmt<Expr>;

include/clang/Basic/TokenKinds.def

@@ -670,7 +670,7 @@ ALIAS("_pascal"      , __pascal   , KEYBORLAND)
 KEYWORD(__builtin_convertvector   , KEYALL)
 ALIAS("__char16_t"   , char16_t   , KEYCXX)
 ALIAS("__char32_t"   , char32_t   , KEYCXX)
-
+KEYWORD(__builtin_bit_cast        , KEYALL)
 KEYWORD(__builtin_available       , KEYALL)
 
 // Clang-specific keywords enabled only in testing.

include/clang/Parse/Parser.h

@@ -1780,6 +1780,9 @@ private:
   // C++ 5.2p1: C++ Casts
   ExprResult ParseCXXCasts();
 
+  /// Parse a __builtin_bit_cast(T, E), used to implement C++2a std::bit_cast.
+  ExprResult ParseBuiltinBitCast();
+
   //===--------------------------------------------------------------------===//
   // C++ 5.2p1: C++ Type Identification
   ExprResult ParseCXXTypeid();

include/clang/Sema/Sema.h

@@ -5236,6 +5236,13 @@ public:
                                SourceRange AngleBrackets,
                                SourceRange Parens);
 
+  ExprResult ActOnBuiltinBitCastExpr(SourceLocation KWLoc, Declarator &Dcl,
+                                     ExprResult Operand,
+                                     SourceLocation RParenLoc);
+
+  ExprResult BuildBuiltinBitCastExpr(SourceLocation KWLoc, TypeSourceInfo *TSI,
+                                     Expr *Operand, SourceLocation RParenLoc);
+
   ExprResult BuildCXXTypeId(QualType TypeInfoType,
                             SourceLocation TypeidLoc,
                             TypeSourceInfo *Operand,

lib/AST/Expr.cpp

@@ -1862,6 +1862,7 @@ bool CastExpr::CastConsistency() const {
   case CK_FloatingComplexToBoolean:
   case CK_IntegralComplexToBoolean:
   case CK_LValueBitCast:            // -> bool&
+  case CK_LValueToRValueBitCast:
   case CK_UserDefinedConversion:    // operator bool()
   case CK_BuiltinFnToFnPtr:
   case CK_FixedPointToBoolean:
@@ -3506,7 +3507,8 @@ bool Expr::HasSideEffects(const ASTContext &Ctx,
   case CXXStaticCastExprClass:
   case CXXReinterpretCastExprClass:
   case CXXConstCastExprClass:
-  case CXXFunctionalCastExprClass: {
+  case CXXFunctionalCastExprClass:
+  case BuiltinBitCastExprClass: {
     // While volatile reads are side-effecting in both C and C++, we treat them
     // as having possible (not definite) side-effects. This allows idiomatic
     // code to behave without warning, such as sizeof(*v) for a volatile-

lib/AST/ExprClassification.cpp

@@ -343,6 +343,7 @@ static Cl::Kinds ClassifyInternal(ASTContext &Ctx, const Expr *E) {
   case Expr::CXXReinterpretCastExprClass:
   case Expr::CXXConstCastExprClass:
   case Expr::ObjCBridgedCastExprClass:
+  case Expr::BuiltinBitCastExprClass:
     // Only in C++ can casts be interesting at all.
     if (!Lang.CPlusPlus) return Cl::CL_PRValue;
     return ClassifyUnnamed(Ctx, cast<ExplicitCastExpr>(E)->getTypeAsWritten());

lib/AST/ExprConstant.cpp

@@ -47,6 +47,7 @@
 #include "clang/Basic/Builtins.h"
 #include "clang/Basic/FixedPoint.h"
 #include "clang/Basic/TargetInfo.h"
+#include "llvm/ADT/Optional.h"
 #include "llvm/ADT/SmallBitVector.h"
 #include "llvm/Support/SaveAndRestore.h"
 #include "llvm/Support/raw_ostream.h"
@@ -56,8 +57,10 @@
 #define DEBUG_TYPE "exprconstant"
 
 using namespace clang;
+using llvm::APInt;
 using llvm::APSInt;
 using llvm::APFloat;
+using llvm::Optional;
 
 static bool IsGlobalLValue(APValue::LValueBase B);
 
@@ -2657,10 +2660,6 @@ static bool HandleLValueComplexElement(EvalInfo &Info, const Expr *E,
   return true;
 }
 
-static bool handleLValueToRValueConversion(EvalInfo &Info, const Expr *Conv,
-                                           QualType Type, const LValue &LVal,
-                                           APValue &RVal);
-
 /// Try to evaluate the initializer for a variable declaration.
 ///
 /// \param Info   Information about the ongoing evaluation.
@@ -5376,6 +5375,489 @@ static bool HandleConstructorCall(const Expr *E, const LValue &This,
 //===----------------------------------------------------------------------===//
 namespace {
 
+class BitCastBuffer {
+  // FIXME: We're going to need bit-level granularity when we support
+  // bit-fields.
+  // FIXME: Its possible under the C++ standard for 'char' to not be 8 bits, but
+  // we don't support a host or target where that is the case. Still, we should
+  // use a more generic type in case we ever do.
+  SmallVector<Optional<unsigned char>, 32> Bytes;
+
+  static_assert(std::numeric_limits<unsigned char>::digits >= 8,
+                "Need at least 8 bit unsigned char");
+
+  bool TargetIsLittleEndian;
+
+public:
+  BitCastBuffer(CharUnits Width, bool TargetIsLittleEndian)
+      : Bytes(Width.getQuantity()),
+        TargetIsLittleEndian(TargetIsLittleEndian) {}
+
+  LLVM_NODISCARD
+  bool readObject(CharUnits Offset, CharUnits Width,
+                  SmallVectorImpl<unsigned char> &Output) const {
+    for (CharUnits I = Offset, E = Offset + Width; I != E; ++I) {
+      // If a byte of an integer is uninitialized, then the whole integer is
+      // uninitalized.
+      if (!Bytes[I.getQuantity()])
+        return false;
+      Output.push_back(*Bytes[I.getQuantity()]);
+    }
+    if (llvm::sys::IsLittleEndianHost != TargetIsLittleEndian)
+      std::reverse(Output.begin(), Output.end());
+    return true;
+  }
+
+  void writeObject(CharUnits Offset, SmallVectorImpl<unsigned char> &Input) {
+    if (llvm::sys::IsLittleEndianHost != TargetIsLittleEndian)
+      std::reverse(Input.begin(), Input.end());
+
+    size_t Index = 0;
+    for (unsigned char Byte : Input) {
+      assert(!Bytes[Offset.getQuantity() + Index] && "overwriting a byte?");
+      Bytes[Offset.getQuantity() + Index] = Byte;
+      ++Index;
+    }
+  }
+
+  size_t size() { return Bytes.size(); }
+};
+
+/// Traverse an APValue to produce an BitCastBuffer, emulating how the current
+/// target would represent the value at runtime.
+class APValueToBufferConverter {
+  EvalInfo &Info;
+  BitCastBuffer Buffer;
+  const CastExpr *BCE;
+
+  APValueToBufferConverter(EvalInfo &Info, CharUnits ObjectWidth,
+                           const CastExpr *BCE)
+      : Info(Info),
+        Buffer(ObjectWidth, Info.Ctx.getTargetInfo().isLittleEndian()),
+        BCE(BCE) {}
+
+  bool visit(const APValue &Val, QualType Ty) {
+    return visit(Val, Ty, CharUnits::fromQuantity(0));
+  }
+
+  // Write out Val with type Ty into Buffer starting at Offset.
+  bool visit(const APValue &Val, QualType Ty, CharUnits Offset) {
+    assert((size_t)Offset.getQuantity() <= Buffer.size());
+
+    // As a special case, nullptr_t has an indeterminate value.
+    if (Ty->isNullPtrType())
+      return true;
+
+    // Dig through Src to find the byte at SrcOffset.
+    switch (Val.getKind()) {
+    case APValue::Indeterminate:
+    case APValue::None:
+      return true;
+
+    case APValue::Int:
+      return visitInt(Val.getInt(), Ty, Offset);
+    case APValue::Float:
+      return visitFloat(Val.getFloat(), Ty, Offset);
+    case APValue::Array:
+      return visitArray(Val, Ty, Offset);
+    case APValue::Struct:
+      return visitRecord(Val, Ty, Offset);
+
+    case APValue::ComplexInt:
+    case APValue::ComplexFloat:
+    case APValue::Vector:
+    case APValue::FixedPoint:
+      // FIXME: We should support these.
+
+    case APValue::Union:
+    case APValue::MemberPointer:
+    case APValue::AddrLabelDiff: {
+      Info.FFDiag(BCE->getBeginLoc(),
+                  diag::note_constexpr_bit_cast_unsupported_type)
+          << Ty;
+      return false;
+    }
+
+    case APValue::LValue:
+      llvm_unreachable("LValue subobject in bit_cast?");
+    }
+  }
+
+  bool visitRecord(const APValue &Val, QualType Ty, CharUnits Offset) {
+    const RecordDecl *RD = Ty->getAsRecordDecl();
+    const ASTRecordLayout &Layout = Info.Ctx.getASTRecordLayout(RD);
+
+    // Visit the base classes.
+    if (auto *CXXRD = dyn_cast<CXXRecordDecl>(RD)) {
+      for (size_t I = 0, E = CXXRD->getNumBases(); I != E; ++I) {
+        const CXXBaseSpecifier &BS = CXXRD->bases_begin()[I];
+        CXXRecordDecl *BaseDecl = BS.getType()->getAsCXXRecordDecl();
+
+        if (!visitRecord(Val.getStructBase(I), BS.getType(),
+                         Layout.getBaseClassOffset(BaseDecl) + Offset))
+          return false;
+      }
+    }
+
+    // Visit the fields.
+    unsigned FieldIdx = 0;
+    for (FieldDecl *FD : RD->fields()) {
+      if (FD->isBitField()) {
+        Info.FFDiag(BCE->getBeginLoc(),
+                    diag::note_constexpr_bit_cast_unsupported_bitfield);
+        return false;
+      }
+
+      uint64_t FieldOffsetBits = Layout.getFieldOffset(FieldIdx);
+
+      assert(FieldOffsetBits % Info.Ctx.getCharWidth() == 0 &&
+             "only bit-fields can have sub-char alignment");
+      CharUnits FieldOffset =
+          Info.Ctx.toCharUnitsFromBits(FieldOffsetBits) + Offset;
+      QualType FieldTy = FD->getType();
+      if (!visit(Val.getStructField(FieldIdx), FieldTy, FieldOffset))
+        return false;
+      ++FieldIdx;
+    }
+
+    return true;
+  }
+
+  bool visitArray(const APValue &Val, QualType Ty, CharUnits Offset) {
+    const auto *CAT =
+        dyn_cast_or_null<ConstantArrayType>(Ty->getAsArrayTypeUnsafe());
+    if (!CAT)
+      return false;
+
+    CharUnits ElemWidth = Info.Ctx.getTypeSizeInChars(CAT->getElementType());
+    unsigned NumInitializedElts = Val.getArrayInitializedElts();
+    unsigned ArraySize = Val.getArraySize();
+    // First, initialize the initialized elements.
+    for (unsigned I = 0; I != NumInitializedElts; ++I) {
+      const APValue &SubObj = Val.getArrayInitializedElt(I);
+      if (!visit(SubObj, CAT->getElementType(), Offset + I * ElemWidth))
+        return false;
+    }
+
+    // Next, initialize the rest of the array using the filler.
+    if (Val.hasArrayFiller()) {
+      const APValue &Filler = Val.getArrayFiller();
+      for (unsigned I = NumInitializedElts; I != ArraySize; ++I) {
+        if (!visit(Filler, CAT->getElementType(), Offset + I * ElemWidth))
+          return false;
+      }
+    }
+
+    return true;
+  }
+
+  bool visitInt(const APSInt &Val, QualType Ty, CharUnits Offset) {
+    CharUnits Width = Info.Ctx.getTypeSizeInChars(Ty);
+    SmallVector<unsigned char, 8> Bytes(Width.getQuantity());
+    llvm::StoreIntToMemory(Val, &*Bytes.begin(), Width.getQuantity());
+    Buffer.writeObject(Offset, Bytes);
+    return true;
+  }
+
+  bool visitFloat(const APFloat &Val, QualType Ty, CharUnits Offset) {
+    APSInt AsInt(Val.bitcastToAPInt());
+    return visitInt(AsInt, Ty, Offset);
+  }
+
+public:
+  static Optional<BitCastBuffer> convert(EvalInfo &Info, const APValue &Src,
+                                         const CastExpr *BCE) {
+    CharUnits DstSize = Info.Ctx.getTypeSizeInChars(BCE->getType());
+    APValueToBufferConverter Converter(Info, DstSize, BCE);
+    if (!Converter.visit(Src, BCE->getSubExpr()->getType()))
+      return None;
+    return Converter.Buffer;
+  }
+};
+
+/// Write an BitCastBuffer into an APValue.
+class BufferToAPValueConverter {
+  EvalInfo &Info;
+  const BitCastBuffer &Buffer;
+  const CastExpr *BCE;
+
+  BufferToAPValueConverter(EvalInfo &Info, const BitCastBuffer &Buffer,
+                           const CastExpr *BCE)
+      : Info(Info), Buffer(Buffer), BCE(BCE) {}
+
+  // Emit an unsupported bit_cast type error. Sema refuses to build a bit_cast
+  // with an invalid type, so anything left is a deficiency on our part (FIXME).
+  // Ideally this will be unreachable.
+  llvm::NoneType unsupportedType(QualType Ty) {
+    Info.FFDiag(BCE->getBeginLoc(),
+                diag::note_constexpr_bit_cast_unsupported_type)
+        << Ty;
+    return None;
+  }
+
+  Optional<APValue> visit(const BuiltinType *T, CharUnits Offset,
+                          const EnumType *EnumSugar = nullptr) {
+    if (T->isNullPtrType()) {
+      uint64_t NullValue = Info.Ctx.getTargetNullPointerValue(QualType(T, 0));
+      return APValue((Expr *)nullptr,
+                     /*Offset=*/CharUnits::fromQuantity(NullValue),
+                     APValue::NoLValuePath{}, /*IsNullPtr=*/true);
+    }
+
+    CharUnits SizeOf = Info.Ctx.getTypeSizeInChars(T);
+    SmallVector<uint8_t, 8> Bytes;
+    if (!Buffer.readObject(Offset, SizeOf, Bytes)) {
+      // If this is std::byte or unsigned char, then its okay to store an
+      // indeterminate value.
+      bool IsStdByte = EnumSugar && EnumSugar->isStdByteType();
+      bool IsUChar =
+          !EnumSugar && (T->isSpecificBuiltinType(BuiltinType::UChar) ||
+                         T->isSpecificBuiltinType(BuiltinType::Char_U));
+      if (!IsStdByte && !IsUChar) {
+        QualType DisplayType(EnumSugar ? (Type *)EnumSugar : T, 0);
+        Info.FFDiag(BCE->getExprLoc(),
+                    diag::note_constexpr_bit_cast_indet_dest)
+            << DisplayType << Info.Ctx.getLangOpts().CharIsSigned;
+        return None;
+      }
+
+      return APValue::IndeterminateValue();
+    }
+
+    APSInt Val(SizeOf.getQuantity() * Info.Ctx.getCharWidth(), true);
+    llvm::LoadIntFromMemory(Val, &*Bytes.begin(), Bytes.size());
+
+    if (T->isIntegralOrEnumerationType()) {
+      Val.setIsSigned(T->isSignedIntegerOrEnumerationType());
+      return APValue(Val);
+    }
+
+    if (T->isRealFloatingType()) {
+      const llvm::fltSemantics &Semantics =
+          Info.Ctx.getFloatTypeSemantics(QualType(T, 0));
+      return APValue(APFloat(Semantics, Val));
+    }
+
+    return unsupportedType(QualType(T, 0));
+  }
+
+  Optional<APValue> visit(const RecordType *RTy, CharUnits Offset) {
+    const RecordDecl *RD = RTy->getAsRecordDecl();
+    const ASTRecordLayout &Layout = Info.Ctx.getASTRecordLayout(RD);
+
+    unsigned NumBases = 0;
+    if (auto *CXXRD = dyn_cast<CXXRecordDecl>(RD))
+      NumBases = CXXRD->getNumBases();
+
+    APValue ResultVal(APValue::UninitStruct(), NumBases,
+                      std::distance(RD->field_begin(), RD->field_end()));
+
+    // Visit the base classes.
+    if (auto *CXXRD = dyn_cast<CXXRecordDecl>(RD)) {
+      for (size_t I = 0, E = CXXRD->getNumBases(); I != E; ++I) {
+        const CXXBaseSpecifier &BS = CXXRD->bases_begin()[I];
+        CXXRecordDecl *BaseDecl = BS.getType()->getAsCXXRecordDecl();
+        if (BaseDecl->isEmpty() ||
+            Info.Ctx.getASTRecordLayout(BaseDecl).getNonVirtualSize().isZero())
+          continue;
+
+        Optional<APValue> SubObj = visitType(
+            BS.getType(), Layout.getBaseClassOffset(BaseDecl) + Offset);
+        if (!SubObj)
+          return None;
+        ResultVal.getStructBase(I) = *SubObj;
+      }
+    }
+
+    // Visit the fields.
+    unsigned FieldIdx = 0;
+    for (FieldDecl *FD : RD->fields()) {
+      // FIXME: We don't currently support bit-fields. A lot of the logic for
+      // this is in CodeGen, so we need to factor it around.
+      if (FD->isBitField()) {
+        Info.FFDiag(BCE->getBeginLoc(),
+                    diag::note_constexpr_bit_cast_unsupported_bitfield);
+        return None;
+      }
+
+      uint64_t FieldOffsetBits = Layout.getFieldOffset(FieldIdx);
+      assert(FieldOffsetBits % Info.Ctx.getCharWidth() == 0);
+
+      CharUnits FieldOffset =
+          CharUnits::fromQuantity(FieldOffsetBits / Info.Ctx.getCharWidth()) +
+          Offset;
+      QualType FieldTy = FD->getType();
+      Optional<APValue> SubObj = visitType(FieldTy, FieldOffset);
+      if (!SubObj)
+        return None;
+      ResultVal.getStructField(FieldIdx) = *SubObj;
+      ++FieldIdx;
+    }
+
+    return ResultVal;
+  }
+
+  Optional<APValue> visit(const EnumType *Ty, CharUnits Offset) {
+    QualType RepresentationType = Ty->getDecl()->getIntegerType();
+    assert(!RepresentationType.isNull() &&
+           "enum forward decl should be caught by Sema");
+    const BuiltinType *AsBuiltin =
+        RepresentationType.getCanonicalType()->getAs<BuiltinType>();
+    assert(AsBuiltin && "non-integral enum underlying type?");
+    // Recurse into the underlying type. Treat std::byte transparently as
+    // unsigned char.
+    return visit(AsBuiltin, Offset, /*EnumTy=*/Ty);
+  }
+
+  Optional<APValue> visit(const ConstantArrayType *Ty, CharUnits Offset) {
+    size_t Size = Ty->getSize().getLimitedValue();
+    CharUnits ElementWidth = Info.Ctx.getTypeSizeInChars(Ty->getElementType());
+
+    APValue ArrayValue(APValue::UninitArray(), Size, Size);
+    for (size_t I = 0; I != Size; ++I) {
+      Optional<APValue> ElementValue =
+          visitType(Ty->getElementType(), Offset + I * ElementWidth);
+      if (!ElementValue)
+        return None;
+      ArrayValue.getArrayInitializedElt(I) = std::move(*ElementValue);
+    }
+
+    return ArrayValue;
+  }
+
+  Optional<APValue> visit(const Type *Ty, CharUnits Offset) {
+    return unsupportedType(QualType(Ty, 0));
+  }
+
+  Optional<APValue> visitType(QualType Ty, CharUnits Offset) {
+    QualType Can = Ty.getCanonicalType();
+
+    switch (Can->getTypeClass()) {
+#define TYPE(Class, Base)                                                      \
+  case Type::Class:                                                            \
+    return visit(cast<Class##Type>(Can.getTypePtr()), Offset);
+#define ABSTRACT_TYPE(Class, Base)
+#define NON_CANONICAL_TYPE(Class, Base)                                        \
+  case Type::Class:                                                            \
+    llvm_unreachable("non-canonical type should be impossible!");
+#define DEPENDENT_TYPE(Class, Base)                                            \
+  case Type::Class:                                                            \
+    llvm_unreachable(                                                          \
+        "dependent types aren't supported in the constant evaluator!");
+#define NON_CANONICAL_UNLESS_DEPENDENT(Class, Base)                            \
+  case Type::Class:                                                            \
+    llvm_unreachable("either dependent or not canonical!");
+#include "clang/AST/TypeNodes.def"
+    }
+  }
+
+public:
+  // Pull out a full value of type DstType.
+  static Optional<APValue> convert(EvalInfo &Info, BitCastBuffer &Buffer,
+                                   const CastExpr *BCE) {
+    BufferToAPValueConverter Converter(Info, Buffer, BCE);
+    return Converter.visitType(BCE->getType(), CharUnits::fromQuantity(0));
+  }
+};
+
+static bool checkBitCastConstexprEligibilityType(SourceLocation Loc,
+                                                 QualType Ty, EvalInfo *Info,
+                                                 const ASTContext &Ctx,
+                                                 bool CheckingDest) {
+  Ty = Ty.getCanonicalType();
+
+  auto diag = [&](int Reason) {
+    if (Info)
+      Info->FFDiag(Loc, diag::note_constexpr_bit_cast_invalid_type)
+          << CheckingDest << (Reason == 4) << Reason;
+    return false;
+  };
+  auto note = [&](int Construct, QualType NoteTy, SourceLocation NoteLoc) {
+    if (Info)
+      Info->Note(NoteLoc, diag::note_constexpr_bit_cast_invalid_subtype)
+          << NoteTy << Construct << Ty;
+    return false;
+  };
+
+  if (Ty->isUnionType())
+    return diag(0);
+  if (Ty->isPointerType())
+    return diag(1);
+  if (Ty->isMemberPointerType())
+    return diag(2);
+  if (Ty.isVolatileQualified())
+    return diag(3);
+
+  if (RecordDecl *Record = Ty->getAsRecordDecl()) {
+    if (auto *CXXRD = dyn_cast<CXXRecordDecl>(Record)) {
+      for (CXXBaseSpecifier &BS : CXXRD->bases())
+        if (!checkBitCastConstexprEligibilityType(Loc, BS.getType(), Info, Ctx,
+                                                  CheckingDest))
+          return note(1, BS.getType(), BS.getBeginLoc());
+    }
+    for (FieldDecl *FD : Record->fields()) {
+      if (FD->getType()->isReferenceType())
+        return diag(4);
+      if (!checkBitCastConstexprEligibilityType(Loc, FD->getType(), Info, Ctx,
+                                                CheckingDest))
+        return note(0, FD->getType(), FD->getBeginLoc());
+    }
+  }
+
+  if (Ty->isArrayType() &&
+      !checkBitCastConstexprEligibilityType(Loc, Ctx.getBaseElementType(Ty),
+                                            Info, Ctx, CheckingDest))
+    return false;
+
+  return true;
+}
+
+static bool checkBitCastConstexprEligibility(EvalInfo *Info,
+                                             const ASTContext &Ctx,
+                                             const CastExpr *BCE) {
+  bool DestOK = checkBitCastConstexprEligibilityType(
+      BCE->getBeginLoc(), BCE->getType(), Info, Ctx, true);
+  bool SourceOK = DestOK && checkBitCastConstexprEligibilityType(
+                                BCE->getBeginLoc(),
+                                BCE->getSubExpr()->getType(), Info, Ctx, false);
+  return SourceOK;
+}
+
+static bool handleLValueToRValueBitCast(EvalInfo &Info, APValue &DestValue,
+                                        APValue &SourceValue,
+                                        const CastExpr *BCE) {
+  assert(CHAR_BIT == 8 && Info.Ctx.getTargetInfo().getCharWidth() == 8 &&
+         "no host or target supports non 8-bit chars");
+  assert(SourceValue.isLValue() &&
+         "LValueToRValueBitcast requires an lvalue operand!");
+
+  if (!checkBitCastConstexprEligibility(&Info, Info.Ctx, BCE))
+    return false;
+
+  LValue SourceLValue;
+  APValue SourceRValue;
+  SourceLValue.setFrom(Info.Ctx, SourceValue);
+  if (!handleLValueToRValueConversion(Info, BCE,
+                                      BCE->getSubExpr()->getType().withConst(),
+                                      SourceLValue, SourceRValue))
+    return false;
+
+  // Read out SourceValue into a char buffer.
+  Optional<BitCastBuffer> Buffer =
+      APValueToBufferConverter::convert(Info, SourceRValue, BCE);
+  if (!Buffer)
+    return false;
+
+  // Write out the buffer into a new APValue.
+  Optional<APValue> MaybeDestValue =
+      BufferToAPValueConverter::convert(Info, *Buffer, BCE);
+  if (!MaybeDestValue)
+    return false;
+
+  DestValue = std::move(*MaybeDestValue);
+  return true;
+}
+
 template <class Derived>
 class ExprEvaluatorBase
   : public ConstStmtVisitor<Derived, bool> {
@@ -5510,6 +5992,9 @@ public:
       CCEDiag(E, diag::note_constexpr_invalid_cast) << 1;
     return static_cast<Derived*>(this)->VisitCastExpr(E);
   }
+  bool VisitBuiltinBitCastExpr(const BuiltinBitCastExpr *E) {
+    return static_cast<Derived*>(this)->VisitCastExpr(E);
+  }
 
   bool VisitBinaryOperator(const BinaryOperator *E) {
     switch (E->getOpcode()) {
@@ -5802,6 +6287,14 @@ public:
         return false;
       return DerivedSuccess(RVal, E);
     }
+    case CK_LValueToRValueBitCast: {
+      APValue DestValue, SourceValue;
+      if (!Evaluate(SourceValue, Info, E->getSubExpr()))
+        return false;
+      if (!handleLValueToRValueBitCast(Info, DestValue, SourceValue, E))
+        return false;
+      return DerivedSuccess(DestValue, E);
+    }
     }
 
     return Error(E);
@@ -6651,7 +7144,6 @@ bool PointerExprEvaluator::VisitCastExpr(const CastExpr *E) {
   switch (E->getCastKind()) {
   default:
     break;
-
   case CK_BitCast:
   case CK_CPointerToObjCPointerCast:
   case CK_BlockPointerToObjCPointerCast:
@@ -8176,7 +8668,7 @@ public:
   }
 
   bool Success(const APValue &V, const Expr *E) {
-    if (V.isLValue() || V.isAddrLabelDiff()) {
+    if (V.isLValue() || V.isAddrLabelDiff() || V.isIndeterminate()) {
       Result = V;
       return true;
     }
@@ -10598,6 +11090,7 @@ bool IntExprEvaluator::VisitCastExpr(const CastExpr *E) {
   case CK_LValueToRValue:
   case CK_AtomicToNonAtomic:
   case CK_NoOp:
+  case CK_LValueToRValueBitCast:
     return ExprEvaluatorBaseTy::VisitCastExpr(E);
 
   case CK_MemberPointerToBoolean:
@@ -11210,6 +11703,7 @@ bool ComplexExprEvaluator::VisitCastExpr(const CastExpr *E) {
   case CK_LValueToRValue:
   case CK_AtomicToNonAtomic:
   case CK_NoOp:
+  case CK_LValueToRValueBitCast:
     return ExprEvaluatorBaseTy::VisitCastExpr(E);
 
   case CK_Dependent:
@@ -12512,6 +13006,11 @@ static ICEDiag CheckICE(const Expr* E, const ASTContext &Ctx) {
   case Expr::ChooseExprClass: {
     return CheckICE(cast<ChooseExpr>(E)->getChosenSubExpr(), Ctx);
   }
+  case Expr::BuiltinBitCastExprClass: {
+    if (!checkBitCastConstexprEligibility(nullptr, Ctx, cast<CastExpr>(E)))
+      return ICEDiag(IK_NotICE, E->getBeginLoc());
+    return CheckICE(cast<CastExpr>(E)->getSubExpr(), Ctx);
+  }
   }
 
   llvm_unreachable("Invalid StmtClass!");

lib/AST/ItaniumMangle.cpp

@@ -3634,6 +3634,7 @@ recurse:
   case Expr::AtomicExprClass:
   case Expr::SourceLocExprClass:
   case Expr::FixedPointLiteralClass:
+  case Expr::BuiltinBitCastExprClass:
   {
     if (!NullOut) {
       // As bad as this diagnostic is, it's better than crashing.

lib/AST/StmtPrinter.cpp

@@ -1703,6 +1703,14 @@ void StmtPrinter::VisitCXXConstCastExpr(CXXConstCastExpr *Node) {
   VisitCXXNamedCastExpr(Node);
 }
 
+void StmtPrinter::VisitBuiltinBitCastExpr(BuiltinBitCastExpr *Node) {
+  OS << "__builtin_bit_cast(";
+  Node->getTypeInfoAsWritten()->getType().print(OS, Policy);
+  OS << ", ";
+  PrintExpr(Node->getSubExpr());
+  OS << ")";
+}
+
 void StmtPrinter::VisitCXXTypeidExpr(CXXTypeidExpr *Node) {
   OS << "typeid(";
   if (Node->isTypeOperand()) {

lib/AST/StmtProfile.cpp

@@ -1569,6 +1569,11 @@ void StmtProfiler::VisitCXXConstCastExpr(const CXXConstCastExpr *S) {
   VisitCXXNamedCastExpr(S);
 }
 
+void StmtProfiler::VisitBuiltinBitCastExpr(const BuiltinBitCastExpr *S) {
+  VisitExpr(S);
+  VisitType(S->getTypeInfoAsWritten()->getType());
+}
+
 void StmtProfiler::VisitUserDefinedLiteral(const UserDefinedLiteral *S) {
   VisitCallExpr(S);
 }

lib/CodeGen/CGExpr.cpp

@@ -4209,6 +4209,7 @@ LValue CodeGenFunction::EmitCastLValue(const CastExpr *E) {
   switch (E->getCastKind()) {
   case CK_ToVoid:
   case CK_BitCast:
+  case CK_LValueToRValueBitCast:
   case CK_ArrayToPointerDecay:
   case CK_FunctionToPointerDecay:
   case CK_NullToMemberPointer:

lib/CodeGen/CGExprAgg.cpp

@@ -711,6 +711,25 @@ void AggExprEmitter::VisitCastExpr(CastExpr *E) {
     break;
   }
 
+  case CK_LValueToRValueBitCast: {
+    if (Dest.isIgnored()) {
+      CGF.EmitAnyExpr(E->getSubExpr(), AggValueSlot::ignored(),
+                      /*ignoreResult=*/true);
+      break;
+    }
+
+    LValue SourceLV = CGF.EmitLValue(E->getSubExpr());
+    Address SourceAddress =
+        Builder.CreateElementBitCast(SourceLV.getAddress(), CGF.Int8Ty);
+    Address DestAddress =
+        Builder.CreateElementBitCast(Dest.getAddress(), CGF.Int8Ty);
+    llvm::Value *SizeVal = llvm::ConstantInt::get(
+        CGF.SizeTy,
+        CGF.getContext().getTypeSizeInChars(E->getType()).getQuantity());
+    Builder.CreateMemCpy(DestAddress, SourceAddress, SizeVal);
+    break;
+  }
+
   case CK_DerivedToBase:
   case CK_BaseToDerived:
   case CK_UncheckedDerivedToBase: {

lib/CodeGen/CGExprComplex.cpp

@@ -464,6 +464,15 @@ ComplexPairTy ComplexExprEmitter::EmitCast(CastKind CK, Expr *Op,
     return EmitLoadOfLValue(CGF.MakeAddrLValue(V, DestTy), Op->getExprLoc());
   }
 
+  case CK_LValueToRValueBitCast: {
+    LValue SourceLVal = CGF.EmitLValue(Op);
+    Address Addr = Builder.CreateElementBitCast(SourceLVal.getAddress(),
+                                                CGF.ConvertTypeForMem(DestTy));
+    LValue DestLV = CGF.MakeAddrLValue(Addr, DestTy);
+    DestLV.setTBAAInfo(TBAAAccessInfo::getMayAliasInfo());
+    return EmitLoadOfLValue(DestLV, Op->getExprLoc());
+  }
+
   case CK_BitCast:
   case CK_BaseToDerived:
   case CK_DerivedToBase:

lib/CodeGen/CGExprConstant.cpp

@@ -1115,6 +1115,7 @@ public:
     case CK_ToVoid:
     case CK_Dynamic:
     case CK_LValueBitCast:
+    case CK_LValueToRValueBitCast:
     case CK_NullToMemberPointer:
     case CK_UserDefinedConversion:
     case CK_CPointerToObjCPointerCast:

lib/CodeGen/CGExprScalar.cpp

@@ -2033,6 +2033,15 @@ Value *ScalarExprEmitter::VisitCastExpr(CastExpr *CE) {
     return EmitLoadOfLValue(LV, CE->getExprLoc());
   }
 
+  case CK_LValueToRValueBitCast: {
+    LValue SourceLVal = CGF.EmitLValue(E);
+    Address Addr = Builder.CreateElementBitCast(SourceLVal.getAddress(),
+                                                CGF.ConvertTypeForMem(DestTy));
+    LValue DestLV = CGF.MakeAddrLValue(Addr, DestTy);
+    DestLV.setTBAAInfo(TBAAAccessInfo::getMayAliasInfo());
+    return EmitLoadOfLValue(DestLV, CE->getExprLoc());
+  }
+
   case CK_CPointerToObjCPointerCast:
   case CK_BlockPointerToObjCPointerCast:
   case CK_AnyPointerToBlockPointerCast:

lib/Edit/RewriteObjCFoundationAPI.cpp

@@ -1042,6 +1042,7 @@ static bool rewriteToNumericBoxedExpression(const ObjCMessageExpr *Msg,
     case CK_Dependent:
     case CK_BitCast:
     case CK_LValueBitCast:
+    case CK_LValueToRValueBitCast:
     case CK_BaseToDerived:
     case CK_DerivedToBase:
     case CK_UncheckedDerivedToBase:

lib/Lex/PPMacroExpansion.cpp

@@ -1630,6 +1630,7 @@ void Preprocessor::ExpandBuiltinMacro(Token &Tok) {
                       .Case("__builtin_FILE", true)
                       .Case("__builtin_FUNCTION", true)
                       .Case("__builtin_COLUMN", true)
+                      .Case("__builtin_bit_cast", true)
                       .Default(false);
         }
       });

lib/Parse/ParseExpr.cpp

@@ -1223,6 +1223,9 @@ ExprResult Parser::ParseCastExpression(bool isUnaryExpression,
   case tok::kw_static_cast:
     Res = ParseCXXCasts();
     break;
+  case tok::kw___builtin_bit_cast:
+    Res = ParseBuiltinBitCast();
+    break;
   case tok::kw_typeid:
     Res = ParseCXXTypeid();
     break;

lib/Parse/ParseExprCXX.cpp

@@ -3513,3 +3513,37 @@ Parser::ParseCXXAmbiguousParenExpression(ParenParseOption &ExprType,
   ConsumeAnyToken();
   return Result;
 }
+
+/// Parse a __builtin_bit_cast(T, E).
+ExprResult Parser::ParseBuiltinBitCast() {
+  SourceLocation KWLoc = ConsumeToken();
+
+  BalancedDelimiterTracker T(*this, tok::l_paren);
+  if (T.expectAndConsume(diag::err_expected_lparen_after, "__builtin_bit_cast"))
+    return ExprError();
+
+  // Parse the common declaration-specifiers piece.
+  DeclSpec DS(AttrFactory);
+  ParseSpecifierQualifierList(DS);
+
+  // Parse the abstract-declarator, if present.
+  Declarator DeclaratorInfo(DS, DeclaratorContext::TypeNameContext);
+  ParseDeclarator(DeclaratorInfo);
+
+  if (ExpectAndConsume(tok::comma)) {
+    Diag(Tok.getLocation(), diag::err_expected) << tok::comma;
+    SkipUntil(tok::r_paren, StopAtSemi);
+    return ExprError();
+  }
+
+  ExprResult Operand = ParseExpression();
+
+  if (T.consumeClose())
+    return ExprError();
+
+  if (Operand.isInvalid() || DeclaratorInfo.isInvalidType())
+    return ExprError();
+
+  return Actions.ActOnBuiltinBitCastExpr(KWLoc, DeclaratorInfo, Operand,
+                                         T.getCloseLocation());
+}

lib/Sema/SemaCast.cpp

@@ -87,6 +87,7 @@ namespace {
     void CheckDynamicCast();
     void CheckCXXCStyleCast(bool FunctionalCast, bool ListInitialization);
     void CheckCStyleCast();
+    void CheckBuiltinBitCast();
 
     void updatePartOfExplicitCastFlags(CastExpr *CE) {
       // Walk down from the CE to the OrigSrcExpr, and mark all immediate
@@ -331,6 +332,38 @@ Sema::BuildCXXNamedCast(SourceLocation OpLoc, tok::TokenKind Kind,
   }
 }
 
+ExprResult Sema::ActOnBuiltinBitCastExpr(SourceLocation KWLoc, Declarator &D,
+                                         ExprResult Operand,
+                                         SourceLocation RParenLoc) {
+  assert(!D.isInvalidType());
+
+  TypeSourceInfo *TInfo = GetTypeForDeclaratorCast(D, Operand.get()->getType());
+  if (D.isInvalidType())
+    return ExprError();
+
+  return BuildBuiltinBitCastExpr(KWLoc, TInfo, Operand.get(), RParenLoc);
+}
+
+ExprResult Sema::BuildBuiltinBitCastExpr(SourceLocation KWLoc,
+                                         TypeSourceInfo *TSI, Expr *Operand,
+                                         SourceLocation RParenLoc) {
+  CastOperation Op(*this, TSI->getType(), Operand);
+  Op.OpRange = SourceRange(KWLoc, RParenLoc);
+  TypeLoc TL = TSI->getTypeLoc();
+  Op.DestRange = SourceRange(TL.getBeginLoc(), TL.getEndLoc());
+
+  if (!Operand->isTypeDependent() && !TSI->getType()->isDependentType()) {
+    Op.CheckBuiltinBitCast();
+    if (Op.SrcExpr.isInvalid())
+      return ExprError();
+  }
+
+  BuiltinBitCastExpr *BCE =
+      new (Context) BuiltinBitCastExpr(Op.ResultType, Op.ValueKind, Op.Kind,
+                                       Op.SrcExpr.get(), TSI, KWLoc, RParenLoc);
+  return Op.complete(BCE);
+}
+
 /// Try to diagnose a failed overloaded cast.  Returns true if
 /// diagnostics were emitted.
 static bool tryDiagnoseOverloadedCast(Sema &S, CastType CT,
@@ -2764,6 +2797,43 @@ void CastOperation::CheckCStyleCast() {
     checkCastAlign();
 }
 
+void CastOperation::CheckBuiltinBitCast() {
+  QualType SrcType = SrcExpr.get()->getType();
+  if (SrcExpr.get()->isRValue())
+    SrcExpr = Self.CreateMaterializeTemporaryExpr(SrcType, SrcExpr.get(),
+                                                  /*IsLValueReference=*/false);
+
+  CharUnits DestSize = Self.Context.getTypeSizeInChars(DestType);
+  CharUnits SourceSize = Self.Context.getTypeSizeInChars(SrcType);
+  if (DestSize != SourceSize) {
+    Self.Diag(OpRange.getBegin(), diag::err_bit_cast_type_size_mismatch)
+        << (int)SourceSize.getQuantity() << (int)DestSize.getQuantity();
+    SrcExpr = ExprError();
+    return;
+  }
+
+  if (!DestType.isTriviallyCopyableType(Self.Context)) {
+    Self.Diag(OpRange.getBegin(), diag::err_bit_cast_non_trivially_copyable)
+        << 1;
+    SrcExpr = ExprError();
+    return;
+  }
+
+  if (!SrcType.isTriviallyCopyableType(Self.Context)) {
+    Self.Diag(OpRange.getBegin(), diag::err_bit_cast_non_trivially_copyable)
+        << 0;
+    SrcExpr = ExprError();
+    return;
+  }
+
+  if (Self.Context.hasSameUnqualifiedType(DestType, SrcType)) {
+    Kind = CK_NoOp;
+    return;
+  }
+
+  Kind = CK_LValueToRValueBitCast;
+}
+
 /// DiagnoseCastQual - Warn whenever casts discards a qualifiers, be it either
 /// const, volatile or both.
 static void DiagnoseCastQual(Sema &Self, const ExprResult &SrcExpr,

lib/Sema/SemaExceptionSpec.cpp

@@ -1201,6 +1201,7 @@ CanThrowResult Sema::canThrow(const Expr *E) {
   case Expr::CoyieldExprClass:
   case Expr::CXXConstCastExprClass:
   case Expr::CXXReinterpretCastExprClass:
+  case Expr::BuiltinBitCastExprClass:
   case Expr::CXXStdInitializerListExprClass:
   case Expr::DesignatedInitExprClass:
   case Expr::DesignatedInitUpdateExprClass:

lib/Sema/TreeTransform.h

@@ -2652,6 +2652,16 @@ public:
                                                ListInitialization);
   }
 
+  /// Build a new C++ __builtin_bit_cast expression.
+  ///
+  /// By default, performs semantic analysis to build the new expression.
+  /// Subclasses may override this routine to provide different behavior.
+  ExprResult RebuildBuiltinBitCastExpr(SourceLocation KWLoc,
+                                       TypeSourceInfo *TSI, Expr *Sub,
+                                       SourceLocation RParenLoc) {
+    return getSema().BuildBuiltinBitCastExpr(KWLoc, TSI, Sub, RParenLoc);
+  }
+
   /// Build a new C++ typeid(type) expression.
   ///
   /// By default, performs semantic analysis to build the new expression.
@@ -10245,6 +10255,22 @@ TreeTransform<Derived>::TransformCXXNamedCastExpr(CXXNamedCastExpr *E) {
       E->getAngleBrackets().getEnd(), SubExpr.get(), E->getRParenLoc());
 }
 
+template<typename Derived>
+ExprResult
+TreeTransform<Derived>::TransformBuiltinBitCastExpr(BuiltinBitCastExpr *BCE) {
+  TypeSourceInfo *TSI =
+      getDerived().TransformType(BCE->getTypeInfoAsWritten());
+  if (!TSI)
+    return ExprError();
+
+  ExprResult Sub = getDerived().TransformExpr(BCE->getSubExpr());
+  if (Sub.isInvalid())
+    return ExprError();
+
+  return getDerived().RebuildBuiltinBitCastExpr(BCE->getBeginLoc(), TSI,
+                                                Sub.get(), BCE->getEndLoc());
+}
+
 template<typename Derived>
 ExprResult
 TreeTransform<Derived>::TransformCXXStaticCastExpr(CXXStaticCastExpr *E) {

lib/Serialization/ASTReaderStmt.cpp

@@ -1510,6 +1510,12 @@ void ASTStmtReader::VisitCXXFunctionalCastExpr(CXXFunctionalCastExpr *E) {
   E->setRParenLoc(ReadSourceLocation());
 }
 
+void ASTStmtReader::VisitBuiltinBitCastExpr(BuiltinBitCastExpr *E) {
+  VisitExplicitCastExpr(E);
+  E->KWLoc = ReadSourceLocation();
+  E->RParenLoc = ReadSourceLocation();
+}
+
 void ASTStmtReader::VisitUserDefinedLiteral(UserDefinedLiteral *E) {
   VisitCallExpr(E);
   E->UDSuffixLoc = ReadSourceLocation();

lib/Serialization/ASTWriterStmt.cpp

@@ -1451,6 +1451,12 @@ void ASTStmtWriter::VisitCXXFunctionalCastExpr(CXXFunctionalCastExpr *E) {
   Code = serialization::EXPR_CXX_FUNCTIONAL_CAST;
 }
 
+void ASTStmtWriter::VisitBuiltinBitCastExpr(BuiltinBitCastExpr *E) {
+  VisitExplicitCastExpr(E);
+  Record.AddSourceLocation(E->getBeginLoc());
+  Record.AddSourceLocation(E->getEndLoc());
+}
+
 void ASTStmtWriter::VisitUserDefinedLiteral(UserDefinedLiteral *E) {
   VisitCallExpr(E);
   Record.AddSourceLocation(E->UDSuffixLoc);

lib/StaticAnalyzer/Core/ExprEngine.cpp

@@ -1687,6 +1687,7 @@ void ExprEngine::Visit(const Stmt *S, ExplodedNode *Pred,
     case Stmt::CXXReinterpretCastExprClass:
     case Stmt::CXXConstCastExprClass:
     case Stmt::CXXFunctionalCastExprClass:
+    case Stmt::BuiltinBitCastExprClass:
     case Stmt::ObjCBridgedCastExprClass: {
       Bldr.takeNodes(Pred);
       const auto *C = cast<CastExpr>(S);

lib/StaticAnalyzer/Core/ExprEngineC.cpp

@@ -380,6 +380,7 @@ void ExprEngine::VisitCast(const CastExpr *CastE, const Expr *Ex,
       case CK_Dependent:
       case CK_ArrayToPointerDecay:
       case CK_BitCast:
+      case CK_LValueToRValueBitCast:
       case CK_AddressSpaceConversion:
       case CK_BooleanToSignedIntegral:
       case CK_IntegralToPointer:

test/CodeGenCXX/builtin-bit-cast-no-tbaa.cpp

@@ -0,0 +1,19 @@
+// RUN: %clang_cc1 -O3 -std=c++2a -S -emit-llvm -o - -disable-llvm-passes -triple x86_64-apple-macos10.14 %s | FileCheck %s
+
+void test_scalar() {
+  // CHECK-LABEL: define void @_Z11test_scalarv
+  __builtin_bit_cast(float, 42);
+
+  // CHECK: load float, float* {{.*}}, align 4, !tbaa ![[MAY_ALIAS_TBAA:.*]]
+}
+
+void test_scalar2() {
+  // CHECK-LABEL: define void @_Z12test_scalar2v
+  struct S {int m;};
+  __builtin_bit_cast(int, S{42});
+
+  // CHECK: load i32, i32* {{.*}}, align 4, !tbaa ![[MAY_ALIAS_TBAA]]
+}
+
+// CHECK: ![[CHAR_TBAA:.*]] = !{!"omnipotent char", {{.*}}, i64 0}
+// CHECK: ![[MAY_ALIAS_TBAA]] = !{![[CHAR_TBAA]], ![[CHAR_TBAA]], i64 0}

test/CodeGenCXX/builtin-bit-cast.cpp

@@ -0,0 +1,106 @@
+// RUN: %clang_cc1 -std=c++2a -S -emit-llvm -o - -disable-llvm-passes -triple x86_64-apple-macos10.14 %s | FileCheck %s
+
+void test_scalar(int &oper) {
+  // CHECK-LABEL: define void @_Z11test_scalarRi
+  __builtin_bit_cast(float, oper);
+
+  // CHECK: [[OPER:%.*]] = alloca i32*
+  // CHECK: [[REF:%.*]] = load i32*, i32**
+  // CHECK-NEXT: [[CASTED:%.*]] = bitcast i32* [[REF]] to float*
+  // CHECK-NEXT: load float, float* [[CASTED]]
+}
+
+struct two_ints {
+  int x;
+  int y;
+};
+
+unsigned long test_aggregate_to_scalar(two_ints &ti) {
+  // CHECK-LABEL: define i64 @_Z24test_aggregate_to_scalarR8two_ints
+  return __builtin_bit_cast(unsigned long, ti);
+
+  // CHECK: [[TI_ADDR:%.*]] = alloca %struct.two_ints*, align 8
+  // CHECK: [[TI_LOAD:%.*]] = load %struct.two_ints*, %struct.two_ints** [[TI_ADDR]]
+  // CHECK-NEXT: [[CASTED:%.*]] = bitcast %struct.two_ints* [[TI_LOAD]] to i64*
+  // CHECK-NEXT: load i64, i64* [[CASTED]]
+}
+
+struct two_floats {
+  float x;
+  float y;
+};
+
+two_floats test_aggregate_record(two_ints& ti) {
+  // CHECK-LABEL: define <2 x float> @_Z21test_aggregate_recordR8two_int
+   return __builtin_bit_cast(two_floats, ti);
+
+  // CHECK: [[RETVAL:%.*]] = alloca %struct.two_floats, align 4
+  // CHECK: [[TI:%.*]] = alloca %struct.two_ints*, align 8
+
+  // CHECK: [[LOAD_TI:%.*]] = load %struct.two_ints*, %struct.two_ints** [[TI]]
+  // CHECK: [[MEMCPY_SRC:%.*]] = bitcast %struct.two_ints* [[LOAD_TI]] to i8*
+  // CHECK: [[MEMCPY_DST:%.*]] = bitcast %struct.two_floats* [[RETVAL]] to i8*
+  // CHECK: call void @llvm.memcpy.p0i8.p0i8.i64(i8* align 4 [[MEMCPY_DST]], i8* align 4 [[MEMCPY_SRC]]
+}
+
+two_floats test_aggregate_array(int (&ary)[2]) {
+  // CHECK-LABEL: define <2 x float> @_Z20test_aggregate_arrayRA2_i
+  return __builtin_bit_cast(two_floats, ary);
+
+  // CHECK: [[RETVAL:%.*]] = alloca %struct.two_floats, align 4
+  // CHECK: [[ARY:%.*]] = alloca [2 x i32]*, align 8
+
+  // CHECK: [[LOAD_ARY:%.*]] = load [2 x i32]*, [2 x i32]** [[ARY]]
+  // CHECK: [[MEMCPY_SRC:%.*]] = bitcast [2 x i32]* [[LOAD_ARY]] to i8*
+  // CHECK: [[MEMCPY_DST:%.*]] = bitcast %struct.two_floats* [[RETVAL]] to i8*
+  // CHECK: call void @llvm.memcpy.p0i8.p0i8.i64(i8* align 4 [[MEMCPY_DST]], i8* align 4 [[MEMCPY_SRC]]
+}
+
+two_ints test_scalar_to_aggregate(unsigned long ul) {
+  // CHECK-LABEL: define i64 @_Z24test_scalar_to_aggregatem
+  return __builtin_bit_cast(two_ints, ul);
+
+  // CHECK: [[TI:%.*]] = alloca %struct.two_ints, align 4
+  // CHECK: [[TITMP:%.*]] = bitcast %struct.two_ints* [[TI]] to i8*
+  // CHECK-NEXT: call void @llvm.memcpy.p0i8.p0i8.i64(i8* align 4 [[TITMP]]
+}
+
+unsigned long test_complex(_Complex unsigned &cu) {
+  // CHECK-LABEL: define i64 @_Z12test_complexRCj
+  return __builtin_bit_cast(unsigned long, cu);
+
+  // CHECK: [[REF_ALLOCA:%.*]] = alloca { i32, i32 }*, align 8
+  // CHECK-NEXT: store { i32, i32 }* {{.*}}, { i32, i32 }** [[REF_ALLOCA]]
+  // CHECK-NEXT: [[REF:%.*]] = load { i32, i32 }*, { i32, i32 }** [[REF_ALLOCA]]
+  // CHECK-NEXT: [[CASTED:%.*]] = bitcast { i32, i32 }* [[REF]] to i64*
+  // CHECK-NEXT: load i64, i64* [[CASTED]], align 4
+}
+
+_Complex unsigned test_to_complex(unsigned long &ul) {
+  // CHECK-LABEL: define i64 @_Z15test_to_complexRm
+
+  return __builtin_bit_cast(_Complex unsigned, ul);
+
+  // CHECK: [[REF:%.*]] = alloca i64*
+  // CHECK: [[LOAD_REF:%.*]] = load i64*, i64** [[REF]]
+  // CHECK: [[CASTED:%.*]] = bitcast i64* [[LOAD_REF]] to { i32, i32 }*
+}
+
+unsigned long test_array(int (&ary)[2]) {
+  // CHECK-LABEL: define i64 @_Z10test_arrayRA2_i
+  return __builtin_bit_cast(unsigned long, ary);
+
+  // CHECK: [[REF_ALLOCA:%.*]] = alloca [2 x i32]*
+  // CHECK: [[LOAD_REF:%.*]] = load [2 x i32]*, [2 x i32]** [[REF_ALLOCA]]
+  // CHECK: [[CASTED:%.*]] = bitcast [2 x i32]* [[LOAD_REF]] to i64*
+  // CHECK: load i64, i64* [[CASTED]], align 4
+}
+
+two_ints test_rvalue_aggregate() {
+  // CHECK-LABEL: define i64 @_Z21test_rvalue_aggregate
+  return __builtin_bit_cast(two_ints, 42ul);
+
+  // CHECK: [[TI:%.*]] = alloca %struct.two_ints, align 4
+  // CHECK: [[CASTED:%.*]] = bitcast %struct.two_ints* [[TI]] to i8*
+  // CHECK: call void @llvm.memcpy.p0i8.p0i8.i64(i8* align 4 [[CASTED]]
+}

test/SemaCXX/builtin-bit-cast.cpp

@@ -0,0 +1,39 @@
+// RUN: %clang_cc1 -verify -std=c++2a -fsyntax-only -triple x86_64-apple-macosx10.14.0 %s
+// RUN: %clang_cc1 -verify -std=c++2a -fsyntax-only -triple x86_64-apple-macosx10.14.0 %s -fno-signed-char
+
+#if !__has_builtin(__builtin_bit_cast)
+#error
+#endif
+
+template <class T, T v>
+T instantiate() {
+  return __builtin_bit_cast(T, v);
+}
+
+int x = instantiate<int, 32>();
+
+struct secret_ctor {
+  char member;
+
+private: secret_ctor() = default;
+};
+
+void test1() {
+  secret_ctor c = __builtin_bit_cast(secret_ctor, (char)0);
+}
+
+void test2() {
+  constexpr int i = 0;
+  // expected-error@+1{{__builtin_bit_cast source size does not equal destination size (4 vs 1)}}
+  constexpr char c = __builtin_bit_cast(char, i);
+}
+
+struct not_trivially_copyable {
+  virtual void foo() {}
+};
+
+// expected-error@+1{{__builtin_bit_cast source type must be trivially copyable}}
+constexpr unsigned long ul = __builtin_bit_cast(unsigned long, not_trivially_copyable{});
+
+// expected-error@+1 {{__builtin_bit_cast destination type must be trivially copyable}}
+constexpr long us = __builtin_bit_cast(unsigned long &, 0L);

test/SemaCXX/constexpr-builtin-bit-cast.cpp

@@ -0,0 +1,383 @@
+// RUN: %clang_cc1 -verify -std=c++2a -fsyntax-only -triple x86_64-apple-macosx10.14.0 %s
+// RUN: %clang_cc1 -verify -std=c++2a -fsyntax-only -triple x86_64-apple-macosx10.14.0 %s -fno-signed-char
+// RUN: %clang_cc1 -verify -std=c++2a -fsyntax-only -triple aarch64_be-linux-gnu %s
+
+#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
+#  define LITTLE_END 1
+#elif __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
+#  define LITTLE_END 0
+#else
+#  error "huh?"
+#endif
+
+template <class T, class V> struct is_same {
+  static constexpr bool value = false;
+};
+template <class T> struct is_same<T, T> {
+  static constexpr bool value = true;
+};
+
+static_assert(sizeof(int) == 4);
+static_assert(sizeof(long long) == 8);
+
+template <class To, class From>
+constexpr To bit_cast(const From &from) {
+  static_assert(sizeof(To) == sizeof(From));
+#ifdef __CHAR_UNSIGNED__
+  // expected-note@+4 2 {{indeterminate value can only initialize an object of type 'unsigned char', 'char', or 'std::byte'; 'signed char' is invalid}}
+#else
+  // expected-note@+2 2 {{indeterminate value can only initialize an object of type 'unsigned char' or 'std::byte'; 'signed char' is invalid}}
+#endif
+  return __builtin_bit_cast(To, from);
+}
+
+template <class Intermediate, class Init>
+constexpr bool round_trip(const Init &init) {
+  return bit_cast<Init>(bit_cast<Intermediate>(init)) == init;
+}
+
+void test_int() {
+  static_assert(round_trip<unsigned>((int)-1));
+  static_assert(round_trip<unsigned>((int)0x12345678));
+  static_assert(round_trip<unsigned>((int)0x87654321));
+  static_assert(round_trip<unsigned>((int)0x0C05FEFE));
+}
+
+void test_array() {
+  constexpr unsigned char input[] = {0xCA, 0xFE, 0xBA, 0xBE};
+  constexpr unsigned expected = LITTLE_END ? 0xBEBAFECA : 0xCAFEBABE;
+  static_assert(bit_cast<unsigned>(input) == expected);
+}
+
+void test_record() {
+  struct int_splicer {
+    unsigned x;
+    unsigned y;
+
+    constexpr bool operator==(const int_splicer &other) const {
+      return other.x == x && other.y == y;
+    }
+  };
+
+  constexpr int_splicer splice{0x0C05FEFE, 0xCAFEBABE};
+
+  static_assert(bit_cast<unsigned long long>(splice) == LITTLE_END
+                ? 0xCAFEBABE0C05FEFE
+                : 0x0C05FEFECAFEBABE);
+
+  static_assert(bit_cast<int_splicer>(0xCAFEBABE0C05FEFE).x == LITTLE_END
+                ? 0x0C05FEFE
+                : 0xCAFEBABE);
+
+  static_assert(round_trip<unsigned long long>(splice));
+  static_assert(round_trip<long long>(splice));
+
+  struct base2 {
+  };
+
+  struct base3 {
+    unsigned z;
+  };
+
+  struct bases : int_splicer, base2, base3 {
+    unsigned doublez;
+  };
+
+  struct tuple4 {
+    unsigned x, y, z, doublez;
+
+    constexpr bool operator==(tuple4 const &other) const {
+      return x == other.x && y == other.y &&
+             z == other.z && doublez == other.doublez;
+    }
+  };
+  constexpr bases b = {{1, 2}, {}, {3}, 4};
+  constexpr tuple4 t4 = bit_cast<tuple4>(b);
+  static_assert(t4 == tuple4{1, 2, 3, 4});
+  static_assert(round_trip<tuple4>(b));
+}
+
+void test_partially_initialized() {
+  struct pad {
+    signed char x;
+    int y;
+  };
+
+  struct no_pad {
+    signed char x;
+    signed char p1, p2, p3;
+    int y;
+  };
+
+  static_assert(sizeof(pad) == sizeof(no_pad));
+
+  constexpr pad pir{4, 4};
+  // expected-error@+2 {{constexpr variable 'piw' must be initialized by a constant expression}}
+  // expected-note@+1 {{in call to 'bit_cast(pir)'}}
+  constexpr int piw = bit_cast<no_pad>(pir).x;
+
+  // expected-error@+2 {{constexpr variable 'bad' must be initialized by a constant expression}}
+  // expected-note@+1 {{in call to 'bit_cast(pir)'}}
+  constexpr no_pad bad = bit_cast<no_pad>(pir);
+
+  constexpr pad fine = bit_cast<pad>(no_pad{1, 2, 3, 4, 5});
+  static_assert(fine.x == 1 && fine.y == 5);
+}
+
+void no_bitfields() {
+  // FIXME!
+  struct S {
+    unsigned char x : 8;
+  };
+
+  struct G {
+    unsigned char x : 8;
+  };
+
+  constexpr S s{0};
+  // expected-error@+2 {{constexpr variable 'g' must be initialized by a constant expression}}
+  // expected-note@+1 {{constexpr bit_cast involving bit-field is not yet supported}}
+  constexpr G g = __builtin_bit_cast(G, s);
+}
+
+void array_members() {
+  struct S {
+    int ar[3];
+
+    constexpr bool operator==(const S &rhs) {
+      return ar[0] == rhs.ar[0] && ar[1] == rhs.ar[1] && ar[2] == rhs.ar[2];
+    }
+  };
+
+  struct G {
+    int a, b, c;
+
+    constexpr bool operator==(const G &rhs) {
+      return a == rhs.a && b == rhs.b && c == rhs.c;
+    }
+  };
+
+  constexpr S s{{1, 2, 3}};
+  constexpr G g = bit_cast<G>(s);
+  static_assert(g.a == 1 && g.b == 2 && g.c == 3);
+
+  static_assert(round_trip<G>(s));
+  static_assert(round_trip<S>(g));
+}
+
+void bad_types() {
+  union X {
+    int x;
+  };
+
+  struct G {
+    int g;
+  };
+  // expected-error@+2 {{constexpr variable 'g' must be initialized by a constant expression}}
+  // expected-note@+1 {{bit_cast from a union type is not allowed in a constant expression}}
+  constexpr G g = __builtin_bit_cast(G, X{0});
+  // expected-error@+2 {{constexpr variable 'x' must be initialized by a constant expression}}
+  // expected-note@+1 {{bit_cast to a union type is not allowed in a constant expression}}
+  constexpr X x = __builtin_bit_cast(X, G{0});
+
+  struct has_pointer {
+    // expected-note@+1 2 {{invalid type 'int *' is a member of 'has_pointer'}}
+    int *ptr;
+  };
+
+  // expected-error@+2 {{constexpr variable 'ptr' must be initialized by a constant expression}}
+  // expected-note@+1 {{bit_cast from a pointer type is not allowed in a constant expression}}
+  constexpr unsigned long ptr = __builtin_bit_cast(unsigned long, has_pointer{0});
+  // expected-error@+2 {{constexpr variable 'hptr' must be initialized by a constant expression}}
+  // expected-note@+1 {{bit_cast to a pointer type is not allowed in a constant expression}}
+  constexpr has_pointer hptr =  __builtin_bit_cast(has_pointer, 0ul);
+}
+
+void backtrace() {
+  struct A {
+    // expected-note@+1 {{invalid type 'int *' is a member of 'A'}}
+    int *ptr;
+  };
+
+  struct B {
+    // expected-note@+1 {{invalid type 'A [10]' is a member of 'B'}}
+    A as[10];
+  };
+
+  // expected-note@+1 {{invalid type 'B' is a base of 'C'}}
+  struct C : B {
+  };
+
+  struct E {
+    unsigned long ar[10];
+  };
+
+  // expected-error@+2 {{constexpr variable 'e' must be initialized by a constant expression}}
+  // expected-note@+1 {{bit_cast from a pointer type is not allowed in a constant expression}}
+  constexpr E e = __builtin_bit_cast(E, C{});
+}
+
+void test_array_fill() {
+  constexpr unsigned char a[4] = {1, 2};
+  constexpr unsigned int i = bit_cast<unsigned int>(a);
+  static_assert(i == LITTLE_END ? 0x00000201 : 0x01020000, "");
+}
+
+typedef decltype(nullptr) nullptr_t;
+
+#ifdef __CHAR_UNSIGNED__
+// expected-note@+5 {{indeterminate value can only initialize an object of type 'unsigned char', 'char', or 'std::byte'; 'unsigned long' is invalid}}
+#else
+// expected-note@+3 {{indeterminate value can only initialize an object of type 'unsigned char' or 'std::byte'; 'unsigned long' is invalid}}
+#endif
+// expected-error@+1 {{constexpr variable 'test_from_nullptr' must be initialized by a constant expression}}
+constexpr unsigned long test_from_nullptr = __builtin_bit_cast(unsigned long, nullptr);
+
+constexpr int test_from_nullptr_pass = (__builtin_bit_cast(unsigned char[8], nullptr), 0);
+
+constexpr int test_to_nullptr() {
+  nullptr_t npt = __builtin_bit_cast(nullptr_t, 0ul);
+
+  struct indet_mem {
+    unsigned char data[sizeof(void *)];
+  };
+  indet_mem im = __builtin_bit_cast(indet_mem, nullptr);
+  nullptr_t npt2 = __builtin_bit_cast(nullptr_t, im);
+
+  return 0;
+}
+
+constexpr int ttn = test_to_nullptr();
+
+// expected-warning@+2 {{returning reference to local temporary object}}
+// expected-note@+1 {{temporary created here}}
+constexpr const long &returns_local() { return 0L; }
+
+// expected-error@+2 {{constexpr variable 'test_nullptr_bad' must be initialized by a constant expression}}
+// expected-note@+1 {{read of temporary whose lifetime has ended}}
+constexpr nullptr_t test_nullptr_bad = __builtin_bit_cast(nullptr_t, returns_local());
+
+constexpr int test_indeterminate(bool read_indet) {
+  struct pad {
+    char a;
+    int b;
+  };
+
+  struct no_pad {
+    char a;
+    unsigned char p1, p2, p3;
+    int b;
+  };
+
+  pad p{1, 2};
+  no_pad np = bit_cast<no_pad>(p);
+
+  int tmp = np.a + np.b;
+
+  unsigned char& indet_ref = np.p1;
+
+  if (read_indet) {
+    // expected-note@+1 {{read of uninitialized object is not allowed in a constant expression}}
+    tmp = indet_ref;
+  }
+
+  indet_ref = 0;
+
+  return 0;
+}
+
+constexpr int run_test_indeterminate = test_indeterminate(false);
+// expected-error@+2 {{constexpr variable 'run_test_indeterminate2' must be initialized by a constant expression}}
+// expected-note@+1 {{in call to 'test_indeterminate(true)'}}
+constexpr int run_test_indeterminate2 = test_indeterminate(true);
+
+struct ref_mem {
+  const int &rm;
+};
+
+constexpr int global_int = 0;
+
+// expected-error@+2 {{constexpr variable 'run_ref_mem' must be initialized by a constant expression}}
+// expected-note@+1 {{bit_cast from a type with a reference member is not allowed in a constant expression}}
+constexpr unsigned long run_ref_mem = __builtin_bit_cast(
+    unsigned long, ref_mem{global_int});
+
+union u {
+  int im;
+};
+
+// expected-error@+2 {{constexpr variable 'run_u' must be initialized by a constant expression}}
+// expected-note@+1 {{bit_cast from a union type is not allowed in a constant expression}}
+constexpr int run_u = __builtin_bit_cast(int, u{32});
+
+struct vol_mem {
+  volatile int x;
+};
+
+// expected-error@+2 {{constexpr variable 'run_vol_mem' must be initialized by a constant expression}}
+// expected-note@+1 {{non-literal type 'vol_mem' cannot be used in a constant expression}}
+constexpr int run_vol_mem = __builtin_bit_cast(int, vol_mem{43});
+
+struct mem_ptr {
+  int vol_mem::*x; // expected-note{{invalid type 'int vol_mem::*' is a member of 'mem_ptr'}}
+};
+// expected-error@+2 {{constexpr variable 'run_mem_ptr' must be initialized by a constant expression}}
+// expected-note@+1 {{bit_cast from a member pointer type is not allowed in a constant expression}}
+constexpr int run_mem_ptr = __builtin_bit_cast(unsigned long, mem_ptr{nullptr});
+
+struct A { char c; /* char padding : 8; */ short s; };
+struct B { unsigned char x[4]; };
+
+constexpr B one() {
+  A a = {1, 2};
+  return bit_cast<B>(a);
+}
+constexpr char good_one = one().x[0] + one().x[2] + one().x[3];
+// expected-error@+2 {{constexpr variable 'bad_one' must be initialized by a constant expression}}
+// expected-note@+1 {{read of uninitialized object is not allowed in a constant expression}}
+constexpr char bad_one = one().x[1];
+
+constexpr A two() {
+  B b = one(); // b.x[1] is indeterminate.
+  b.x[0] = 'a';
+  b.x[2] = 1;
+  b.x[3] = 2;
+  return bit_cast<A>(b);
+}
+constexpr short good_two = two().c + two().s;
+
+namespace std {
+enum byte : unsigned char {};
+}
+
+enum my_byte : unsigned char {};
+
+struct pad {
+  char a;
+  int b;
+};
+
+constexpr int ok_byte = (__builtin_bit_cast(std::byte[8], pad{1, 2}), 0);
+constexpr int ok_uchar = (__builtin_bit_cast(unsigned char[8], pad{1, 2}), 0);
+
+#ifdef __CHAR_UNSIGNED__
+// expected-note@+5 {{indeterminate value can only initialize an object of type 'unsigned char', 'char', or 'std::byte'; 'my_byte' is invalid}}}}
+#else
+// expected-note@+3 {{indeterminate value can only initialize an object of type 'unsigned char' or 'std::byte'; 'my_byte' is invalid}}
+#endif
+// expected-error@+1 {{constexpr variable 'bad_my_byte' must be initialized by a constant expression}}
+constexpr int bad_my_byte = (__builtin_bit_cast(my_byte[8], pad{1, 2}), 0);
+#ifndef __CHAR_UNSIGNED__
+// expected-error@+3 {{constexpr variable 'bad_char' must be initialized by a constant expression}}
+// expected-note@+2 {{indeterminate value can only initialize an object of type 'unsigned char' or 'std::byte'; 'char' is invalid}}
+#endif
+constexpr int bad_char =  (__builtin_bit_cast(char[8], pad{1, 2}), 0);
+
+struct pad_buffer { unsigned char data[sizeof(pad)]; };
+constexpr bool test_pad_buffer() {
+  pad x = {1, 2};
+  pad_buffer y = __builtin_bit_cast(pad_buffer, x);
+  pad z = __builtin_bit_cast(pad, y);
+  return x.a == z.a && x.b == z.b;
+}
+static_assert(test_pad_buffer());

tools/libclang/CIndex.cpp

@@ -5194,6 +5194,8 @@ CXString clang_getCursorKindSpelling(enum CXCursorKind Kind) {
       return cxstring::createRef("CallExpr");
   case CXCursor_ObjCMessageExpr:
       return cxstring::createRef("ObjCMessageExpr");
+  case CXCursor_BuiltinBitCastExpr:
+    return cxstring::createRef("BuiltinBitCastExpr");
   case CXCursor_UnexposedStmt:
       return cxstring::createRef("UnexposedStmt");
   case CXCursor_DeclStmt:

tools/libclang/CXCursor.cpp

@@ -716,6 +716,8 @@ CXCursor cxcursor::MakeCXCursor(const Stmt *S, const Decl *Parent,
   case Stmt::OMPTargetTeamsDistributeSimdDirectiveClass:
     K = CXCursor_OMPTargetTeamsDistributeSimdDirective;
     break;
+  case Stmt::BuiltinBitCastExprClass:
+    K = CXCursor_BuiltinBitCastExpr;
   }
 
   CXCursor C = { K, 0, { Parent, S, TU } };