[PATCH v2] c++: resolvedness of resolve_nondeduced_context result [PR126406]

Changes in v2:
  - Outright revert r16-5967, and instead fix PR119343 more directly
    for sake of backports.

Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look
OK for trunk/16?

-- >8 --

In r16-5967-gbae0ed69e1862a we removed the mark_used call from
resolve_nondeduced_context under the rationale that it should be
the callers' responsiblity to do so.c

Removing this call however now means that resolve_nondeduced_context
could return a specialization whose type is not yet fully resolved
(i.e. has an uninstantiated noexcept or undeduced return type), and
callers that immediately inspect TREE_TYPE of the result (such as
standard_conversion and build_conditional_expr) now misbehave.

In light of such callers, this patch reverts r16-5967 and more
directly fixes the PR119343 bug by just propagating error_mark_node
from resolve_nondeduced_context during convert_to_void.

	PR c++/126406
	PR c++/119343

gcc/cp/ChangeLog:

	* cvt.cc (convert_to_void): Propagate error_mark_node result
	from resolve_nondeduced_context.
	* pt.cc (resolve_nondeduced_context): Revert r16-5967 change.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/cond2a.C: New test.
	* g++.dg/cpp1y/auto-fn67.C: New test.
	* g++.dg/cpp1z/noexcept-type29.C: New test.
---
 gcc/cp/cvt.cc                                |  3 ++-
 gcc/cp/pt.cc                                 |  2 ++
 gcc/testsuite/g++.dg/cpp0x/cond2a.C          | 17 +++++++++++++++++
 gcc/testsuite/g++.dg/cpp1y/auto-fn67.C       | 11 +++++++++++
 gcc/testsuite/g++.dg/cpp1z/noexcept-type29.C | 11 +++++++++++
 5 files changed, 43 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp0x/cond2a.C
 create mode 100644 gcc/testsuite/g++.dg/cpp1y/auto-fn67.C
 create mode 100644 gcc/testsuite/g++.dg/cpp1z/noexcept-type29.C

diff --git a/gcc/cp/cvt.cc b/gcc/cp/cvt.cc
index a378b51a2bed..3b68b8723afb 100644
--- a/gcc/cp/cvt.cc
+++ b/gcc/cp/cvt.cc
@@ -1573,7 +1573,8 @@ convert_to_void (tree expr, impl_conv_void implicit, tsubst_flags_t complain)
     default:;
     }
   expr = resolve_nondeduced_context (expr, complain);
-  if (!mark_single_function (expr, complain))
+  if (expr == error_mark_node
+      || !mark_single_function (expr, complain))
     return error_mark_node;
 
   {
diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 6a081b838e06..1d22563bc586 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -25653,6 +25653,8 @@ 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/cpp0x/cond2a.C b/gcc/testsuite/g++.dg/cpp0x/cond2a.C
new file mode 100644
index 000000000000..a42e198291b2
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/cond2a.C
@@ -0,0 +1,17 @@
+// PR c++/126406
+// { dg-do compile { target c++14 } }
+// A version of cond2.C where f has a deduced return type
+// and g is instantiated.
+
+bool b;
+
+template < class T > auto f ()
+{
+}
+
+template < class T > auto g () -> decltype (b ? f < int > : throw 0)
+{
+  return b ? f<int> : throw 0;
+}
+
+using type = decltype(g<int>());
diff --git a/gcc/testsuite/g++.dg/cpp1y/auto-fn67.C b/gcc/testsuite/g++.dg/cpp1y/auto-fn67.C
new file mode 100644
index 000000000000..20ff86459c3d
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/auto-fn67.C
@@ -0,0 +1,11 @@
+// PR c++/126406
+// { dg-do compile { target c++14 } }
+
+template<class T> auto g(T) { }
+static_assert(g<int>);
+
+template<class T>
+struct B {
+  static auto g(T) { }
+};
+static_assert(B<int>::g);
diff --git a/gcc/testsuite/g++.dg/cpp1z/noexcept-type29.C b/gcc/testsuite/g++.dg/cpp1z/noexcept-type29.C
new file mode 100644
index 000000000000..c620b180fc83
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/noexcept-type29.C
@@ -0,0 +1,11 @@
+// PR c++/126406
+// { dg-do compile { target c++11 } }
+
+template<class T> void f(T) noexcept(noexcept(T())) { }
+static_assert(f<int>);
+
+template<class T>
+struct A {
+  static void f(T) noexcept(noexcept(T())) { }
+};
+static_assert(A<int>::f);
-- 
2.55.0.424.g13c7afec21