c++: Fix excessive instantiation inside decltype [PR71879]

Here after resolving the address of a template-id inside decltype, we
end up instantiating the chosen specialization (from the call to
mark_used in resolve_nondeduced_context), even though only its type is
needed.

This patch sets cp_unevaluated_operand throughout finish_decltype_type,
so that in particular it's set during the call to
resolve_nondeduced_context within.

gcc/cp/ChangeLog:

	PR c++/71879
	* semantics.c (finish_decltype_type): Set up a cp_unevaluated
	sentinel at the start of the function.  Remove a now-redundant
	manual adjustment of cp_unevaluated_operand.

gcc/testsuite/ChangeLog:

	PR c++/71879
	* g++.dg/cpp0x/decltype-71879.C: New test.

1 files changed, 3 insertions, 2 deletions

diff --git a/gcc/cp/semantics.c b/gcc/cp/semantics.c
index 244fc70..0670952 100644
--- a/gcc/cp/semantics.c
+++ b/gcc/cp/semantics.c
@@ -10080,6 +10080,9 @@ finish_decltype_type (tree expr, bool id_expression_or_member_access_p,
       return error_mark_node;
     }
 
+  /* decltype is an unevaluated context.  */
+  cp_unevaluated u;
+
   /* Depending on the resolution of DR 1172, we may later need to distinguish
      instantiation-dependent but not type-dependent expressions so that, say,
      A<decltype(sizeof(T))>::U doesn't require 'typename'.  */
@@ -10095,9 +10098,7 @@ finish_decltype_type (tree expr, bool id_expression_or_member_access_p,
     }
   else if (processing_template_decl)
     {
-      ++cp_unevaluated_operand;
       expr = instantiate_non_dependent_expr_sfinae (expr, complain);
-      --cp_unevaluated_operand;
       if (expr == error_mark_node)
 	return error_mark_node;
     }