From: Patrick Palka <ppalka@redhat.com>
Date: Tue, 10 Feb 2026 01:32:39 +0000 (-0500)
Subject: c++: tf_partial and alias_ctad_tweaks [PR122621]
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=bf2657d9d45e50c4eb82da3f6f8d9d26e288890f;p=thirdparty%2Fgcc.git

c++: tf_partial and alias_ctad_tweaks [PR122621]

Like in r15-6740-g27d620d6769715 for instantiate_template with dependent
arguments, we also need to set tf_partial during the alias_ctad_tweaks
transformation mainly for benefit of properly handling extra-args trees.

In this testcase during alias_ctad_tweaks we substitute the dependent
ElemTs={Tuple<Us...>}, Ts={Ts...} into the requires-clause, which
tsubst_pack_expansion decides to defer via extra-args, and for the
subsequent add_extra_args (during guide overload resolution) to merge
the deferred dependent arguments correctly, tf_partial has to have been
set.

	PR c++/122621

gcc/cp/ChangeLog:

	* pt.cc (tsubst_pack_expansion): Use tf_partial instead of
	tf_none, except when substituting outer (non-dependent) template
	arguments.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/class-deduction-alias26.C: New test.

Reviewed-by: Jason Merrill <jason@redhat.com>
---

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 049bbf07e01..7c6577b48ef 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -31718,7 +31718,7 @@ alias_ctad_tweaks (tree tmpl, tree uguides)
 			  (INNERMOST_TEMPLATE_PARMS (fullatparms)));
     }
 
-  tsubst_flags_t complain = tf_none;
+  tsubst_flags_t complain = tf_partial;
   tree aguides = NULL_TREE;
   tree atparms = INNERMOST_TEMPLATE_PARMS (fullatparms);
   unsigned natparms = TREE_VEC_LENGTH (atparms);
@@ -31825,7 +31825,8 @@ alias_ctad_tweaks (tree tmpl, tree uguides)
 	  if (ci)
 	    {
 	      if (tree outer_targs = outer_template_args (f))
-		ci = tsubst_constraint_info (ci, outer_targs, complain, in_decl);
+		ci = tsubst_constraint_info (ci, outer_targs,
+					     complain & ~tf_partial, in_decl);
 	      ci = tsubst_constraint_info (ci, targs, complain, in_decl);
 	    }
 	  if (ci == error_mark_node)
diff --git a/gcc/testsuite/g++.dg/cpp2a/class-deduction-alias26.C b/gcc/testsuite/g++.dg/cpp2a/class-deduction-alias26.C
new file mode 100644
index 00000000000..01fd1a4d5e2
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/class-deduction-alias26.C
@@ -0,0 +1,17 @@
+// PR c++/122621
+// { dg-do compile { target c++20 } }
+
+template <class, class>
+concept constructible_from = true;
+
+template <class... ElemTs>
+struct Tuple {
+    template <class... Ts> requires (... && constructible_from<ElemTs, Ts>)
+    Tuple(Ts &&...) {}
+};
+
+template <class... Us>
+using A = Tuple<Tuple<Us...>>;
+
+using type = decltype(A{0});
+using type = Tuple<Tuple<>>;