[Cxx1z-constexpr-lambda-P0170R1] Support parsing of constexpr specifier (and its inference) on lambda expressions

Support the constexpr specifier on lambda expressions - and support its inference from the lambda call operator's body.

i.e.
  auto L = [] () constexpr { return 5; };
  static_assert(L() == 5); // OK
  auto Implicit = [] (auto a) { return a; };
  static_assert(Implicit(5) == 5); 

We do not support evaluation of lambda's within constant expressions just yet.

Implementation Strategy:
  - teach ParseLambdaExpressionAfterIntroducer to expect a constexpr specifier and mark the invented function call operator's declarator's decl-specifier with it; Have it emit fixits for multiple decl-specifiers (mutable or constexpr) in this location.
  - for cases where constexpr is not explicitly specified, have buildLambdaExpr check whether the invented function call operator satisfies the requirements of a constexpr function, by calling CheckConstexprFunctionDecl/Body.

Much obliged to Richard Smith for his patience and his care, in ensuring the code is clang-worthy.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@264513 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 6 insertions, 0 deletions

diff --git a/include/clang/Basic/DiagnosticASTKinds.td b/include/clang/Basic/DiagnosticASTKinds.td
index 0b37030d7e..d2b0618600 100644
--- a/include/clang/Basic/DiagnosticASTKinds.td
+++ b/include/clang/Basic/DiagnosticASTKinds.td
@@ -153,6 +153,12 @@ def warn_integer_constant_overflow : Warning<
   "overflow in expression; result is %0 with type %1">,
   InGroup<DiagGroup<"integer-overflow">>;
 
+// This is a temporary diagnostic, and shall be removed once our 
+// implementation is complete, and like the preceding constexpr notes belongs
+// in Sema.
+def note_unimplemented_constexpr_lambda_feature_ast : Note<
+    "unimplemented constexpr lambda feature: %0 (coming soon!)">;
+
 // inline asm related.
 let CategoryName = "Inline Assembly Issue" in {
   def err_asm_invalid_escape : Error<