c++: Fix SFINAE for deleted explicit specializations [PR119343]

When checking a deleted explicit specialization in a SFINAE context,
we were incorrectly selecting a partial specialization because
resolve_nondeduced_context was calling mark_used.  But resolving an
overload to a single function (per DR 115) does not constitute ODR-use,
so mark_used shouldn't be called there.  Instead callers should call
mark_used or mark_single_function on the result to uniformly handle all
resolvable overloads (even non-template-id ones).

This turns out to fix the below testcase because it causes convert_to_void
for void(X::template g<0>) to properly propagate ODR-use failure (due to
deleted g<0>) and return error_mark_node instead of returning void_node.

	PR c++/119343

gcc/cp/ChangeLog:

	* pt.cc (resolve_nondeduced_context): Remove mark_used call.

gcc/testsuite/ChangeLog:

	* g++.dg/template/sfinae-deleted-pr119343.C: New test.

Signed-off-by: Egas Ribeiro <egas.g.ribeiro@tecnico.ulisboa.pt>
Reviewed-by: Patrick Palka <ppalka@redhat.com>

2 files changed, 31 insertions, 2 deletions

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 8498730..a9b311b 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -24816,8 +24816,6 @@ resolve_nondeduced_context (tree orig_expr, tsubst_flags_t complain)
 	}
       if (good == 1)
 	{
-	  if (!mark_used (goodfn, complain) && !(complain & tf_error))
-	    return error_mark_node;
 	  expr = goodfn;
 	  if (baselink)
 	    expr = build_baselink (BASELINK_BINFO (baselink),
diff --git a/gcc/testsuite/g++.dg/template/sfinae-deleted-pr119343.C b/gcc/testsuite/g++.dg/template/sfinae-deleted-pr119343.C
new file mode 100644
index 0000000..065ad60
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/sfinae-deleted-pr119343.C
@@ -0,0 +1,31 @@
+// { dg-do compile { target c++11 } }
+// PR c++/119343 - No SFINAE for deleted explicit specializations
+
+struct true_type { static constexpr bool value = true; };
+struct false_type { static constexpr bool value = false; };
+
+struct X {
+  static void f()=delete;
+  template<int> static void g();
+};
+template<> void X::g<0>()=delete;
+struct Y {
+  static void f();
+  template<int> static void g();
+};
+
+template<class T,class=void>
+struct has_f : false_type {};
+template<class T>
+struct has_f<T,decltype(void(T::f))> : true_type {};
+
+static_assert(!has_f<X>::value, "");
+static_assert(has_f<Y>::value, "");
+
+template<class T,class=void>
+struct has_g0 : false_type {};
+template<class T>
+struct has_g0<T,decltype(void(T::template g<0>))> : true_type {};
+
+static_assert(!has_g0<X>::value, "");
+static_assert(has_g0<Y>::value, "");