[cxx1z-constexpr-lambda] Make a lambda's closure type eligible as a literal-type in C++1z

Additionally, for pre-C++1z, instead of forbidding a lambda's closure type from being a literal type through circumlocutorily setting HasNonLiteralTypeFieldsOrBases falsely to true -- handle lambda's more directly in CXXRecordDecl::isLiteral().

One additional small step towards implementing constexpr-lambdas.

Thanks to Richard Smith for his review! 
https://reviews.llvm.org/D22662

llvm-svn: 276514

clang/include/clang/AST/DeclCXX.h

@@ -535,11 +535,10 @@ class CXXRecordDecl : public RecordDecl {
         MethodTyInfo(Info) {
       IsLambda = true;
 
-      // C++11 [expr.prim.lambda]p3:
-      //   This class type is neither an aggregate nor a literal type.
+      // C++1z [expr.prim.lambda]p4:
+      //   This class type is not an aggregate type.
       Aggregate = false;
       PlainOldData = false;
-      HasNonLiteralTypeFieldsOrBases = true;
     }
 
     /// \brief Whether this lambda is known to be dependent, even if its
@@ -1338,11 +1337,15 @@ public:
   ///
   /// We resolve DR1361 by ignoring the second bullet. We resolve DR1452 by
   /// treating types with trivial default constructors as literal types.
+  ///
+  /// Only in C++1z and beyond, are lambdas literal types.
   bool isLiteral() const {
     return hasTrivialDestructor() &&
-           (isAggregate() || hasConstexprNonCopyMoveConstructor() ||
-            hasTrivialDefaultConstructor()) &&
-           !hasNonLiteralTypeFieldsOrBases();
+           (!isLambda() || getASTContext().getLangOpts().CPlusPlus1z) &&
+           !hasNonLiteralTypeFieldsOrBases() &&
+           (isAggregate() || isLambda() ||
+            hasConstexprNonCopyMoveConstructor() ||
+            hasTrivialDefaultConstructor());
   }
 
   /// \brief If this record is an instantiation of a member class,

clang/test/SemaCXX/cxx1z-constexpr-lambdas.cpp

@@ -1,8 +1,8 @@
 // RUN: %clang_cc1 -std=c++1z -verify -fsyntax-only -fblocks %s
 // RUN: %clang_cc1 -std=c++1z -verify -fsyntax-only -fblocks -fdelayed-template-parsing %s 
-// RUN: %clang_cc1 -std=c++1z -verify -fsyntax-only -fblocks -fms-extensions %s 
-// RUN: %clang_cc1 -std=c++1z -verify -fsyntax-only -fblocks -fdelayed-template-parsing -fms-extensions %s 
+// RUN: %clang_cc1 -std=c++14 -verify -fsyntax-only -fblocks %s -DCPP14_AND_EARLIER
 
+#ifndef CPP14_AND_EARLIER
 namespace test_constexpr_checking {
 
 namespace ns1 {
@@ -33,4 +33,16 @@ namespace ns3 {
       L(3); //expected-note{{non-constexpr function}}
 } 
 
-} // end ns test_constexpr_call
+} // end ns test_constexpr_call
+
+#endif
+
+namespace test_lambda_is_literal {
+#ifdef CPP14_AND_EARLIER
+//expected-error@+4{{not a literal type}}
+//expected-note@+2{{not an aggregate and has no constexpr constructors}}
+#endif
+auto L = [] { };
+constexpr int foo(decltype(L) l) { return 0; }
+
+}