[Sema] Fix a multiple definition bug with friends and templates

The problem was that MergeFunctionDecl sometimes needs the injected template
arguments of a FunctionTemplateDecl, but is called before adding the new
template to the redecl chain. This leads to multiple common pointers in the same
redecl chain, each with their own identical instantiation. Fix this by merging
the the common state before inserting the new template into the redecl chain.

rdar://44810129

Differential revision: https://reviews.llvm.org/D53046

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@344157 91177308-0d34-0410-b5e6-96231b3b80d8

include/clang/AST/DeclTemplate.h

@@ -1093,6 +1093,9 @@ public:
   /// template.
   ArrayRef<TemplateArgument> getInjectedTemplateArgs();
 
+  /// Merge our RedeclarableTemplateDecl::Common with \param Prev.
+  void mergePrevDecl(FunctionTemplateDecl *Prev);
+
   /// Create a function template node.
   static FunctionTemplateDecl *Create(ASTContext &C, DeclContext *DC,
                                       SourceLocation L,

lib/AST/DeclTemplate.cpp

@@ -300,6 +300,42 @@ ArrayRef<TemplateArgument> FunctionTemplateDecl::getInjectedTemplateArgs() {
   return llvm::makeArrayRef(CommonPtr->InjectedArgs, Params->size());
 }
 
+void FunctionTemplateDecl::mergePrevDecl(FunctionTemplateDecl *Prev) {
+  using Base = RedeclarableTemplateDecl;
+
+  // If we haven't created a common pointer yet, then it can just be created
+  // with the usual method.
+  if (!Base::Common)
+    return;
+
+  Common *ThisCommon = static_cast<Common *>(Base::Common);
+  Common *PrevCommon = nullptr;
+  SmallVector<FunctionTemplateDecl *, 8> PreviousDecls;
+  for (; Prev; Prev = Prev->getPreviousDecl()) {
+    if (Prev->Base::Common) {
+      PrevCommon = static_cast<Common *>(Prev->Base::Common);
+      break;
+    }
+    PreviousDecls.push_back(Prev);
+  }
+
+  // If the previous redecl chain hasn't created a common pointer yet, then just
+  // use this common pointer.
+  if (!PrevCommon) {
+    for (auto *D : PreviousDecls)
+      D->Base::Common = ThisCommon;
+    return;
+  }
+
+  // Ensure we don't leak any important state.
+  assert(ThisCommon->Specializations.size() == 0 &&
+         !ThisCommon->InstantiatedFromMember.getPointer() &&
+         !ThisCommon->InstantiatedFromMember.getInt() &&
+         "Can't merge incompatible declarations!");
+
+  Base::Common = PrevCommon;
+}
+
 //===----------------------------------------------------------------------===//
 // ClassTemplateDecl Implementation
 //===----------------------------------------------------------------------===//

lib/Sema/SemaDecl.cpp

@@ -10022,11 +10022,17 @@ bool Sema::CheckFunctionDeclaration(Scope *S, FunctionDecl *NewFD,
     if (FunctionTemplateDecl *OldTemplateDecl =
             dyn_cast<FunctionTemplateDecl>(OldDecl)) {
       auto *OldFD = OldTemplateDecl->getTemplatedDecl();
-      NewFD->setPreviousDeclaration(OldFD);
-      adjustDeclContextForDeclaratorDecl(NewFD, OldFD);
       FunctionTemplateDecl *NewTemplateDecl
         = NewFD->getDescribedFunctionTemplate();
       assert(NewTemplateDecl && "Template/non-template mismatch");
+
+      // The call to MergeFunctionDecl above may have created some state in
+      // NewTemplateDecl that needs to be merged with OldTemplateDecl before we
+      // can add it as a redeclaration.
+      NewTemplateDecl->mergePrevDecl(OldTemplateDecl);
+
+      NewFD->setPreviousDeclaration(OldFD);
+      adjustDeclContextForDeclaratorDecl(NewFD, OldFD);
       if (NewFD->isCXXClassMember()) {
         NewFD->setAccess(OldTemplateDecl->getAccess());
         NewTemplateDecl->setAccess(OldTemplateDecl->getAccess());

test/SemaCXX/friend-template-redecl.cpp

@@ -0,0 +1,20 @@
+// RUN: %clang_cc1 -std=c++17 -verify -emit-llvm-only %s
+
+// expected-no-diagnostics
+
+template <class T> void bar(const T &t) { foo(t); }
+
+template <class>
+struct HasFriend {
+  template <class T>
+  friend void foo(const HasFriend<T> &m) noexcept(false);
+};
+
+template <class T>
+void foo(const HasFriend<T> &m) noexcept(false) {}
+
+void f() {
+  HasFriend<int> x;
+  foo(x);
+  bar(x);
+}