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

[Patrick Palka via Gcc-patches]

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.

Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look for
trunk?

gcc/cp/ChangeLog:

        PR c++/71879
        * semantics.c (finish_decltype_type): Temporarily increment
        cp_unevaluated_operand during call to resolve_nondeduced_context.

gcc/testsuite/ChangeLog:

        PR c++/71879
        * g++.dg/cpp0x/decltype-71879.C: New test.
---
 gcc/cp/semantics.c                          | 2 ++
 gcc/testsuite/g++.dg/cpp0x/decltype-71879.C | 5 +++++
 2 files changed, 7 insertions(+)
 create mode 100644 gcc/testsuite/g++.dg/cpp0x/decltype-71879.C

diff --git a/gcc/cp/semantics.c b/gcc/cp/semantics.c
index c8a6283b120..cad55665ce8 100644
--- a/gcc/cp/semantics.c
+++ b/gcc/cp/semantics.c
@@ -10098,7 +10098,9 @@ finish_decltype_type (tree expr, bool 
id_expression_or_member_access_p,
 
   /* The type denoted by decltype(e) is defined as follows:  */
 
+  ++cp_unevaluated_operand;
   expr = resolve_nondeduced_context (expr, complain);
+  --cp_unevaluated_operand;
 
   if (invalid_nonstatic_memfn_p (input_location, expr, complain))
     return error_mark_node;
diff --git a/gcc/testsuite/g++.dg/cpp0x/decltype-71879.C 
b/gcc/testsuite/g++.dg/cpp0x/decltype-71879.C
new file mode 100644
index 00000000000..9da4d40ca70
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/decltype-71879.C
@@ -0,0 +1,5 @@
+// PR c++/71879
+// { dg-do compile { target c++11 } }
+
+template <class T> void f(T x) { x.fail(); }
+using R = decltype(&f<int>);
-- 
2.30.0.155.g66e871b664