c++: ttp matching with constrained auto parm [PR99909]

Here, when unifying TT with S, processing_template_decl is unset, and
this foils the dependence checks in do_auto_deduction for avoiding
checking constraints on an auto when the initializer is dependent.

This patch fixes this by making sure processing_template_decl is set
around the call to unify from coerce_template_template_parms; this seems
sensible because we're unifying one set of template parameters with
another, so we're dealing with templated trees throughout.

	PR c++/99909

gcc/cp/ChangeLog:

	* pt.c (coerce_template_template_parms): Keep
	processing_template_decl set around the call to unify as well.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-ttp3.C: New test.

gcc/cp/pt.c

@@ -7994,12 +7994,12 @@ coerce_template_template_parms (tree parm_parms,
       /* So coerce P's args to apply to A's parms, and then deduce between A's
 	 args and the converted args.  If that succeeds, A is at least as
 	 specialized as P, so they match.*/
+      processing_template_decl_sentinel ptds (/*reset*/false);
+      ++processing_template_decl;
       tree pargs = template_parms_level_to_args (parm_parms);
       pargs = add_outermost_template_args (outer_args, pargs);
-      ++processing_template_decl;
       pargs = coerce_template_parms (arg_parms, pargs, NULL_TREE, tf_none,
 				     /*require_all*/true, /*use_default*/true);
-      --processing_template_decl;
       if (pargs != error_mark_node)
 	{
 	  tree targs = make_tree_vec (nargs);

gcc/testsuite/g++.dg/cpp2a/concepts-ttp3.C

@@ -0,0 +1,11 @@
+// PR c++/99909
+// { dg-do compile { target c++20 } }
+
+template<class T> constexpr bool always_true = true;
+template<class T> concept C = always_true<T>;
+
+template<C auto> struct S;
+
+template<template<auto> class TT> void f() { }
+
+template void f<S>();