[PATCH] c++: deduction for dependent class type of NTTP [PR105110]

[Patrick Palka via Gcc-patches]

Here deduction for the P/A pair V/a spuriously fails with

  types ‘A<T>’ and ‘const A<int>’ have incompatible cv-qualifiers

because the argument type is const, whereas the parameter type is
non-const.

Since the type of an NTTP is always cv-unqualified, it seems natural to
ignore cv-qualifiers on the argument type before attempting to unify the
two types.

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

        PR c++/105110

gcc/cp/ChangeLog:

        * pt.cc (unify) <case TEMPLATE_PARM_INDEX>: Ignore cv-quals on
        on the argument type of an NTTP before deducing from it.

gcc/testsuite/ChangeLog:

        * g++.dg/cpp2a/nontype-class52.C: New test.
---
 gcc/cp/pt.cc                                 |  5 +++--
 gcc/testsuite/g++.dg/cpp2a/nontype-class52.C | 13 +++++++++++++
 2 files changed, 16 insertions(+), 2 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp2a/nontype-class52.C

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 1acb5990c5c..cdd75d3b6ac 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -24271,8 +24271,9 @@ unify (tree tparms, tree targs, tree parm, tree arg, 
int strict,
              && !(strict & UNIFY_ALLOW_INTEGER)
              && TEMPLATE_PARM_LEVEL (parm) <= TMPL_ARGS_DEPTH (targs))
            {
-             /* Deduce it from the non-type argument.  */
-             tree atype = TREE_TYPE (arg);
+             /* Deduce it from the non-type argument.  As above, ignore
+                top-level quals here too.  */
+             tree atype = cv_unqualified (TREE_TYPE (arg));
              RECUR_AND_CHECK_FAILURE (tparms, targs,
                                       tparm, atype,
                                       UNIFY_ALLOW_NONE, explain_p);
diff --git a/gcc/testsuite/g++.dg/cpp2a/nontype-class52.C 
b/gcc/testsuite/g++.dg/cpp2a/nontype-class52.C
new file mode 100644
index 00000000000..56163376afb
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/nontype-class52.C
@@ -0,0 +1,13 @@
+// PR c++/105110
+// { dg-do compile { target c++20 } }
+
+template<class> struct A { };
+
+template<auto> struct B { };
+
+template<class T, A<T> V> void f(B<V>);
+
+int main() {
+  constexpr A<int> a;
+  f(B<a>{});
+}
-- 
2.35.1.693.g805e0a6808