c++: more constrained nested partial specialization

When checking that a constrained partial specialization is more
constrained than the primary template, we pass only the innermost level
of generic template arguments to strictly_subsumes.  This leads to us
doing a nonsensical substitution from normalize_concept_check if the
full set of template arguments has multiple levels, and it ultimately
causes strictly_subsumes to sometimes erroneously return false as in the
testcase below.

gcc/cp/ChangeLog:

	* pt.c (process_partial_specialization): Pass the full set of
	generic template arguments to strictly_subsumes.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-partial-spec8.C: New test.

(cherry picked from commit c06280ac4c34b0aff8cfa2e74ae8c7afd759d52f)

gcc/cp/pt.c

@@ -5053,7 +5053,7 @@ process_partial_specialization (tree decl)
   if (comp_template_args (inner_args, INNERMOST_TEMPLATE_ARGS (main_args))
       && (!flag_concepts
 	  || !strictly_subsumes (current_template_constraints (),
-				 inner_args, maintmpl)))
+				 main_args, maintmpl)))
     {
       if (!flag_concepts)
         error ("partial specialization %q+D does not specialize "

gcc/testsuite/g++.dg/cpp2a/concepts-partial-spec8.C

+// { dg-do compile { target c++20 } }
+
+template<int M, int N>
+concept equal = M == N;
+
+template<int M>
+struct traits
+{
+  template<int N> requires equal<M, N>
+    struct foo {};
+
+  template<int N> requires equal<M, N> && (M >= 0) // { dg-bogus "not more constrained" }
+    struct foo<N> {};
+};