[gcc(refs/users/jason/heads/alias-ctad)] c++: CTAD for less-specialized alias template [PR102529]

[Jason Merrill <jason@gcc.gnu.org>]

https://gcc.gnu.org/g:e88e0337bac56ee087314fc26206bc86332c0dfc

commit e88e0337bac56ee087314fc26206bc86332c0dfc
Author: Jason Merrill <jason@redhat.com>
Date:   Thu Feb 9 12:58:35 2023 -0800

    c++: CTAD for less-specialized alias template [PR102529]
    
            DR 2664
            PR c++/102529
    
    gcc/cp/ChangeLog:
    
            * pt.cc (alias_ctad_tweaks): Continue after deduction failure.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/DRs/dr2664.C: New test.
            * g++.dg/cpp2a/class-deduction-alias15.C: New test.

Diff:
---
 gcc/cp/pt.cc                                         |  4 +++-
 gcc/testsuite/g++.dg/DRs/dr2664.C                    | 17 +++++++++++++++++
 gcc/testsuite/g++.dg/cpp2a/class-deduction-alias15.C | 18 ++++++++++++++++++
 3 files changed, 38 insertions(+), 1 deletion(-)

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 35d8ce709c3..68488a0d018 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -30057,7 +30057,9 @@ alias_ctad_tweaks (tree tmpl, tree uguides)
 	  tree targs = make_tree_vec (len);
 	  int err = unify (ftparms, targs, ret, utype, UNIFY_ALLOW_NONE, false);
 	  if (err)
-	    continue;
+	    /* CWG2664: Discard any deductions, still build the guide.  */
+	    for (unsigned i = 0; i < len; ++i)
+	      TREE_VEC_ELT (targs, i) = NULL_TREE;
 
 	  /* The number of parms for f' is the number of parms for A plus
 	     non-deduced parms of f.  */
diff --git a/gcc/testsuite/g++.dg/DRs/dr2664.C b/gcc/testsuite/g++.dg/DRs/dr2664.C
new file mode 100644
index 00000000000..f6bf8e2ecef
--- /dev/null
+++ b/gcc/testsuite/g++.dg/DRs/dr2664.C
@@ -0,0 +1,17 @@
+// CWG 2664
+// { dg-do compile { target c++20 } }
+
+template <class S1, class S2> struct C {
+  C(...);
+};
+
+template<class T1> C(T1) -> C<T1, T1>;
+template<class T1, class T2> C(T1, T2) -> C<T1 *, T2>;
+
+template<class V1, class V2> using A = C<V1, V2>;
+
+C c1{""};
+A a1{""};
+
+C c2{"", 1};
+A a2{"", 1};
diff --git a/gcc/testsuite/g++.dg/cpp2a/class-deduction-alias15.C b/gcc/testsuite/g++.dg/cpp2a/class-deduction-alias15.C
new file mode 100644
index 00000000000..db615faf8e2
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/class-deduction-alias15.C
@@ -0,0 +1,18 @@
+// PR c++/102529
+// { dg-do compile { target c++20 } }
+
+template <typename T>
+struct C {
+    template <typename U>
+    C(U);
+};
+
+template <typename U>
+C(U) -> C<U*>;
+
+template <typename T>
+    requires true
+using A = C<T>;
+
+C ok(1);   // ok, a is a C<int*>
+A bad(2);  // fails