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

> On 7/30/26 3:00 PM, Patrick Palka wrote:
> > 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.
> > 
> > @@ -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))
> 
> I still don't understand why this makes a difference, given that
> mark_single_function has
> 
> >   if (expr == error_mark_node)
> >     return false;
> 
> so for expr == error_mark_node it should return false, and we should already
> return error_mark_node?

Ah indeed, I missed that Egas made mark_used and mark_single_function propagate
error_mark_node as a follow-up.

-- >8 --


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

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

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; it's not
necessary to fix PR119343 because after r16-6276 convert_to_void
now properly propagates an error_mark_node result from
resolve_nondeduced_context.

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

gcc/cp/ChangeLog:

	* 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/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 +++++++++++
 4 files changed, 41 insertions(+)
 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/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