[PATCH] c++: ttp variadic constraint subsumption [PR99904]

[Patrick Palka via Gcc-patches]

Here we're crashing when level-lowering the variadic constraints on the
template template parameter TT because tsubst_pack_expansion expects
processing_template_decl to be set during a partial substitution.

bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
trunk/11?  Also tested on cmcstl2 and range-v3.

        PR c++/99904

gcc/cp/ChangeLog:

        * pt.c (is_compatible_template_arg): Set processing_template_decl
        around tsubst_constraint_info.

gcc/testsuite/ChangeLog:

        * g++.dg/cpp2a/concepts-ttp4.C: New test.
---
 gcc/cp/pt.c                                | 2 ++
 gcc/testsuite/g++.dg/cpp2a/concepts-ttp4.C | 9 +++++++++
 2 files changed, 11 insertions(+)
 create mode 100644 gcc/testsuite/g++.dg/cpp2a/concepts-ttp4.C

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 1dcdffe322a..f950f4a21b7 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -8223,8 +8223,10 @@ is_compatible_template_arg (tree parm, tree arg)
     {
       tree aparms = DECL_INNERMOST_TEMPLATE_PARMS (arg);
       new_args = template_parms_level_to_args (aparms);
+      ++processing_template_decl;
       parm_cons = tsubst_constraint_info (parm_cons, new_args,
                                          tf_none, NULL_TREE);
+      --processing_template_decl;
       if (parm_cons == error_mark_node)
         return false;
     }
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-ttp4.C 
b/gcc/testsuite/g++.dg/cpp2a/concepts-ttp4.C
new file mode 100644
index 00000000000..cf3e71ea974
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-ttp4.C
@@ -0,0 +1,9 @@
+// PR c++/99904
+// { dg-do compile { target c++20 } }
+
+template<class... Ts> concept C = (Ts::value && ...);
+template<class... Ts> requires C<Ts...> struct A;
+template<class T> requires true struct B;
+template<template<class... Ts> requires C<Ts...> class TT> struct S;
+using ty1 = S<A>;
+using ty2 = S<B>; // { dg-error "constraint" } TT's constraints don't subsume 
B's
-- 
2.33.0.610.gcefe983a32