C++ PATCH for c++/64647 (wrong error with constexpr template)

[Jason Merrill]

An instantiation of a constexpr template that does not satisfy the rules 
for a constexpr function is still well-formed, just not constexpr.

Tested x86_64-pc-linux-gnu, applying to trunk.
commit f31ad5fe09e8d973926e32fa9617970e53b1ad19
Author: Jason Merrill <ja...@redhat.com>
Date:   Tue Jan 20 17:51:30 2015 -0500
    	PR c++/64647
    	* constexpr.c (ensure_literal_type_for_constexpr_object): Don't
    	give a hard error in a template instantiation.

diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index 0d47424..decc84d 100644
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -102,15 +102,21 @@ ensure_literal_type_for_constexpr_object (tree decl)
       else if (!literal_type_p (type))
 	{
 	  if (DECL_DECLARED_CONSTEXPR_P (decl))
-	    error ("the type %qT of constexpr variable %qD is not literal",
-		   type, decl);
+	    {
+	      error ("the type %qT of constexpr variable %qD is not literal",
+		     type, decl);
+	      explain_non_literal_class (type);
+	    }
 	  else
 	    {
-	      error ("variable %qD of non-literal type %qT in %<constexpr%> "
-		     "function", decl, type);
+	      if (!DECL_TEMPLATE_INSTANTIATION (current_function_decl))
+		{
+		  error ("variable %qD of non-literal type %qT in %<constexpr%> "
+			 "function", decl, type);
+		  explain_non_literal_class (type);
+		}
 	      cp_function_chain->invalid_constexpr = true;
 	    }
-	  explain_non_literal_class (type);
 	  return NULL;
 	}
     }
diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-local3.C b/gcc/testsuite/g++.dg/cpp1y/constexpr-local3.C
new file mode 100644
index 0000000..76d4b55
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-local3.C
@@ -0,0 +1,20 @@
+// PR c++/64647
+// { dg-do compile { target c++14 } }
+
+template<typename T>
+constexpr T foo(T t)
+{
+  T tt = t;
+  return tt;
+}
+
+struct X
+{
+  X() { }
+};
+
+int main()
+{
+  X x;
+  foo(x);
+}