[PATCH] c++: ignore explicit dguides during NTTP CTAD [PR101883]

[Patrick Palka via Gcc-patches]

Since (template) argument passing is a copy-initialization context,
we mustn't consider explicit deduction guides when deducing a CTAD
placeholder type of an NTTP.

Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
trunk/11?

        PR c++/101883

gcc/cp/ChangeLog:

        * pt.c (convert_template_argument): Pass LOOKUP_IMPLICIT to
        convert_template_argument.

gcc/testsuite/ChangeLog:

        * g++.dg/cpp2a/nontype-class49.C: New test.
---
 gcc/cp/pt.c                                  | 3 ++-
 gcc/testsuite/g++.dg/cpp2a/nontype-class49.C | 8 ++++++++
 2 files changed, 10 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp2a/nontype-class49.C

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 0870ccdc9f6..5ac89901e22 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -8486,7 +8486,8 @@ convert_template_argument (tree parm,
           can happen in the context of -fnew-ttp-matching.  */;
       else if (tree a = type_uses_auto (t))
        {
-         t = do_auto_deduction (t, arg, a, complain, adc_unify, args);
+         t = do_auto_deduction (t, arg, a, complain, adc_unify, args,
+                                LOOKUP_IMPLICIT);
          if (t == error_mark_node)
            return error_mark_node;
        }
diff --git a/gcc/testsuite/g++.dg/cpp2a/nontype-class49.C 
b/gcc/testsuite/g++.dg/cpp2a/nontype-class49.C
new file mode 100644
index 00000000000..c83e4075ed0
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/nontype-class49.C
@@ -0,0 +1,8 @@
+// PR c++/101883
+// { dg-do compile { target c++20 } }
+
+template<class T> struct C { constexpr C(int) { } };
+explicit C(int) -> C<int>;
+
+template<C c> struct X { };
+X<1> x; // { dg-error "deduction|no match" }
-- 
2.33.0.rc1