[cxx2a] P0624R2 fix: only lambdas with no lambda-capture are default-constructible and assignable.

This is a fix for rG864949 which only disabled default construction and
assignment for lambdas with capture-defaults, where the C++2a draft
disables them for lambdas with any lambda-capture at all.

Patch by Logan Smith!

Differential Revision: https://reviews.llvm.org/D64058

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@365406 91177308-0d34-0410-b5e6-96231b3b80d8

lib/AST/DeclCXX.cpp

@@ -645,7 +645,8 @@ bool CXXRecordDecl::lambdaIsDefaultConstructibleAndAssignable() const {
   // C++17 [expr.prim.lambda]p21:
   //   The closure type associated with a lambda-expression has no default
   //   constructor and a deleted copy assignment operator.
-  if (getLambdaCaptureDefault() != LCD_None)
+  if (getLambdaCaptureDefault() != LCD_None || 
+      getLambdaData().NumCaptures != 0)
     return false;
   return getASTContext().getLangOpts().CPlusPlus2a;
 }

lib/Sema/SemaDeclCXX.cpp

@@ -7238,7 +7238,7 @@ bool Sema::ShouldDeleteSpecialMember(CXXMethodDecl *MD, CXXSpecialMember CSM,
   //   The closure type associated with a lambda-expression has a
   //   deleted (8.4.3) default constructor and a deleted copy
   //   assignment operator.
-  // C++2a adds back these operators if the lambda has no capture-default.
+  // C++2a adds back these operators if the lambda has no lambda-capture.
   if (RD->isLambda() && !RD->lambdaIsDefaultConstructibleAndAssignable() &&
       (CSM == CXXDefaultConstructor || CSM == CXXCopyAssignment)) {
     if (Diagnose)

test/SemaCXX/cxx2a-lambda-default-ctor-assign.cpp

@@ -1,37 +1,22 @@
 // RUN: %clang_cc1 -std=c++2a -verify %s
 
-auto a = []{};
-decltype(a) a2;
-void f(decltype(a) x, decltype(a) y) {
-  x = y;
-  x = static_cast<decltype(a)&&>(y);
+void no_capture() {
+  auto x = [] {};
+  decltype(x) y;
+  x = x;
+  x = static_cast<decltype(x)&&>(x);
 }
 
-template<typename T>
-struct X {
-  constexpr X() { T::error; } // expected-error {{'::'}}
-  X(const X&);
-  constexpr X &operator=(const X&) { T::error; } // expected-error {{'::'}}
-  constexpr X &operator=(X&&) { T::error; } // expected-error {{'::'}}
-};
-extern X<int> x;
-auto b = [x = x]{}; // expected-note 3{{in instantiation of}}
-decltype(b) b2; // expected-note {{in implicit default constructor}}
-void f(decltype(b) x, decltype(b) y) {
-  x = y; // expected-note {{in implicit copy assignment}}
-  x = static_cast<decltype(b)&&>(y); // expected-note {{in implicit move assignment}}
+void capture_default(int i) {
+  auto x = [=] {}; // expected-note 2{{candidate constructor}} expected-note 2{{lambda expression begins here}}
+  decltype(x) y; // expected-error {{no matching constructor}}
+  x = x; // expected-error {{cannot be assigned}}
+  x = static_cast<decltype(x)&&>(x); // expected-error {{cannot be assigned}}
 }
 
-struct Y {
-  Y() = delete; // expected-note {{deleted}}
-  Y(const Y&);
-  Y &operator=(const Y&) = delete; // expected-note 2{{deleted}}
-  Y &operator=(Y&&) = delete;
-};
-extern Y y;
-auto c = [y = y]{}; // expected-note 3{{deleted because}}
-decltype(c) c2; // expected-error {{deleted}}
-void f(decltype(c) x, decltype(c) y) {
-  x = y; // expected-error {{deleted}}
-  x = static_cast<decltype(c)&&>(y); // expected-error {{deleted}}
+void explicit_capture(int i) {
+  auto x = [i] {}; // expected-note 2{{candidate constructor}} expected-note 2{{lambda expression begins here}}
+  decltype(x) y; // expected-error {{no matching constructor}}
+  x = x; // expected-error {{cannot be assigned}}
+  x = static_cast<decltype(x)&&>(x); // expected-error {{cannot be assigned}}
 }