[gcc(refs/vendors/ARM/heads/arm-struct-reorg-wip)] c++: more constrained nested partial specialization

[Tamar Christina]

https://gcc.gnu.org/g:6effd8a7264427785f999536fc37fd771df190ee

commit 6effd8a7264427785f999536fc37fd771df190ee
Author: Patrick Palka <ppalka@redhat.com>
Date:   Thu Jun 4 13:04:01 2020 -0400

    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)

Diff:
---
 gcc/cp/pt.c                                         |  2 +-
 gcc/testsuite/g++.dg/cpp2a/concepts-partial-spec8.C | 14 ++++++++++++++
 2 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 3e786f8032e..732900e4796 100644
--- a/gcc/cp/pt.c
+++ b/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 "
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-partial-spec8.C b/gcc/testsuite/g++.dg/cpp2a/concepts-partial-spec8.C
new file mode 100644
index 00000000000..873cf44e407
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-partial-spec8.C
@@ -0,0 +1,14 @@
+// { 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> {};
+};