c++: NSDMI instantiation during overload resolution [PR110468]

Here we find ourselves instantiating the NSDMI for A<1>::m when
computing argument conversions during overload resolution, and
thus tf_conv is set.  The flag causes mark_used for the constructor
used in the NSDMI to exit early and not instantiate its noexcept-spec,
which eventually leads to an ICE from nothrow_spec_p.

This patch fixes this by clearing any special tsubst flags during
instantiation of an NSDMI, since the result should be independent of
the context that requires the instantiation.

	PR c++/110468

gcc/cp/ChangeLog:

	* init.cc (maybe_instantiate_nsdmi_init): Mask out all
	tsubst flags except for tf_warning_or_error.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/noexcept79.C: New test.

2 files changed, 22 insertions, 0 deletions

diff --git a/gcc/cp/init.cc b/gcc/cp/init.cc
index af6e30f..ff5014c 100644
--- a/gcc/cp/init.cc
+++ b/gcc/cp/init.cc
@@ -579,6 +579,10 @@ maybe_instantiate_nsdmi_init (tree member, tsubst_flags_t complain)
   /* tsubst_decl uses void_node to indicate an uninstantiated DMI.  */
   if (init == void_node)
     {
+      /* Clear any special tsubst flags; the result of NSDMI instantiation
+	 should be independent of the substitution context.  */
+      complain &= tf_warning_or_error;
+
       init = DECL_INITIAL (DECL_TI_TEMPLATE (member));
       location_t expr_loc
 	= cp_expr_loc_or_loc (init, DECL_SOURCE_LOCATION (member));
diff --git a/gcc/testsuite/g++.dg/cpp0x/noexcept79.C b/gcc/testsuite/g++.dg/cpp0x/noexcept79.C
new file mode 100644
index 0000000..d1f54d1
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/noexcept79.C
@@ -0,0 +1,18 @@
+// PR c++/110468
+// { dg-do compile { target c++11 } }
+
+template<int T>
+struct variant {
+  variant() noexcept(T > 0);
+};
+
+template<int N>
+struct A {
+  variant<N> m = {};
+};
+
+struct B {
+  B(A<1>);
+};
+
+B b = {{}};