[NFC] Change the evaluation context of a non-type default template argument from Unevaluated to ConstantEvaluated.

This patch emits a more appropriate (but still noisy) diagnostic stream when a lambda-expression is encountered within a non-type default argument. 

For e.g. template<int N = ([] { return 5; }())> int f();

As opposed to complaining that a lambda expression is not allowed in an unevaluated operand, the patch complains about the lambda being forbidden in a constant expression context (which will be allowed in C++17 now that they have been accepted by EWG, unless of course CWG or national bodies (that have so far shown no signs of concern) rise in protest) 

As I start submitting patches for constexpr lambdas (http://wg21.link/P0170R0) under C++1z (OK'd by Richard Smith at Kona), this will be one less change to make.

Thanks!

llvm-svn: 253431

2 files changed, 16 insertions, 1 deletions

diff --git a/clang/lib/Parse/ParseTemplate.cpp b/clang/lib/Parse/ParseTemplate.cpp
index 3a964dd..d661ce0 100644
--- a/clang/lib/Parse/ParseTemplate.cpp
+++ b/clang/lib/Parse/ParseTemplate.cpp
@@ -695,7 +695,8 @@ Parser::ParseNonTypeTemplateParameter(unsigned Depth, unsigned Position) {
     //   end of the template-parameter-list rather than a greater-than
     //   operator.
     GreaterThanIsOperatorScope G(GreaterThanIsOperator, false);
-    EnterExpressionEvaluationContext Unevaluated(Actions, Sema::Unevaluated);
+    EnterExpressionEvaluationContext Unevaluated(Actions,
+                                                 Sema::ConstantEvaluated);
 
     DefaultArg = Actions.CorrectDelayedTyposInExpr(ParseAssignmentExpression());
     if (DefaultArg.isInvalid())
diff --git a/clang/test/CXX/temp/temp.arg/temp.arg.nontype/p1.cpp b/clang/test/CXX/temp/temp.arg/temp.arg.nontype/p1.cpp
index 3f70ca7..22ea018 100644
--- a/clang/test/CXX/temp/temp.arg/temp.arg.nontype/p1.cpp
+++ b/clang/test/CXX/temp/temp.arg/temp.arg.nontype/p1.cpp
@@ -1,4 +1,5 @@
 // RUN: %clang_cc1 -fsyntax-only -verify -triple=x86_64-linux-gnu %s
+// RUN: %clang_cc1 -std=c++11 -fsyntax-only -verify -triple=x86_64-linux-gnu %s -DCPP11ONLY
 
 // C++11 [temp.arg.nontype]p1:
 //
@@ -6,6 +7,8 @@
 //   be one of:
 //   -- an integral constant expression; or
 //   -- the name of a non-type template-parameter ; or
+#ifndef CPP11ONLY 
+
 namespace non_type_tmpl_param {
   template <int N> struct X0 { X0(); };
   template <int N> X0<N>::X0() { }
@@ -95,3 +98,14 @@ namespace bad_args {
   int* iptr = &i;
   X0<iptr> x0b; // expected-error{{non-type template argument for template parameter of pointer type 'int *' must have its address taken}}
 }
+#endif // CPP11ONLY
+
+namespace default_args {
+#ifdef CPP11ONLY
+namespace lambdas {
+template<int I = ([] { return 5; }())> //expected-error 2{{constant expression}} expected-note{{constant expression}}
+int f();
+}
+#endif // CPP11ONLY
+
+}
\ No newline at end of file