[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.

llvm-svn: 264513

1 files changed, 3 insertions, 1 deletions

diff --git a/clang/lib/Sema/TreeTransform.h b/clang/lib/Sema/TreeTransform.h
index 7ed52ebb12a..58cf67ccc81 100644
--- a/clang/lib/Sema/TreeTransform.h
+++ b/clang/lib/Sema/TreeTransform.h
@@ -10054,7 +10054,9 @@ TreeTransform<Derived>::TransformLambdaExpr(LambdaExpr *E) {
   CXXMethodDecl *NewCallOperator = getSema().startLambdaDefinition(
       Class, E->getIntroducerRange(), NewCallOpTSI,
       E->getCallOperator()->getLocEnd(),
-      NewCallOpTSI->getTypeLoc().castAs<FunctionProtoTypeLoc>().getParams());
+      NewCallOpTSI->getTypeLoc().castAs<FunctionProtoTypeLoc>().getParams(),
+      E->getCallOperator()->isConstexpr());
+
   LSI->CallOperator = NewCallOperator;
 
   getDerived().transformAttrs(E->getCallOperator(), NewCallOperator);