[cxx1z-constexpr-lambda] Implement constant evaluation of non-capturing lambda expressions.

Add a visitor for lambda expressions to RecordExprEvaluator in ExprConstant.cpp that creates an empty APValue of Struct type to represent the closure object. Additionally, add a LambdaExpr visitor to the TemporaryExprEvaluator that forwards constant evaluation of immediately-called-lambda-expressions to the one in RecordExprEvaluator through VisitConstructExpr.

This patch supports:
constexpr auto ID = [] (auto a) { return a; };
static_assert(ID(3.14) == 3.14);
static_assert([](auto a) { return a + 1; }(10) == 11);

Lambda captures are still not supported for constexpr lambdas.

llvm-svn: 291416

1 files changed, 4 insertions, 2 deletions

diff --git a/clang/lib/Sema/SemaExpr.cpp b/clang/lib/Sema/SemaExpr.cpp
index 023d6f678d5..65e8236952f 100644
--- a/clang/lib/Sema/SemaExpr.cpp
+++ b/clang/lib/Sema/SemaExpr.cpp
@@ -13097,8 +13097,10 @@ void Sema::PopExpressionEvaluationContext() {
         //   evaluate [...] a lambda-expression.
         D = diag::err_lambda_in_constant_expression;
       }
-      for (const auto *L : Rec.Lambdas)
-        Diag(L->getLocStart(), D);
+      // C++1z allows lambda expressions as core constant expressions.
+      if (Rec.Context != ConstantEvaluated || !getLangOpts().CPlusPlus1z)
+        for (const auto *L : Rec.Lambdas)
+          Diag(L->getLocStart(), D);
     } else {
       // Mark the capture expressions odr-used. This was deferred
       // during lambda expression creation.