c++: constraint rewriting during ttp coercion [PR111485]

In order to compare the constraints of a ttp with that of its argument,
we rewrite the ttp's constraints in terms of the argument template's
template parameters.  The substitution to achieve this currently uses a
single level of template arguments, but that never does the right thing
because a ttp's template parameters always have level >= 2.  This patch
fixes this by including the outer template arguments in the substitution,
which ought to match the depth of the ttp.

The second testcase demonstrates it's better to substitute the concrete
outer template arguments instead of generic ones since a ttp's constraints
could depend on outer parameters.

	PR c++/111485

gcc/cp/ChangeLog:

	* pt.cc (is_compatible_template_arg): New parameter 'args'.
	Add the outer template arguments 'args' to 'new_args'.
	(convert_template_argument): Pass 'args' to
	is_compatible_template_arg.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-ttp5.C: New test.
	* g++.dg/cpp2a/concepts-ttp6.C: New test.

3 files changed, 44 insertions, 2 deletions

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 5c300f4..9b100e1 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -8361,7 +8361,7 @@ canonicalize_expr_argument (tree arg, tsubst_flags_t complain)
    constrained than the parameter.  */
 
 static bool
-is_compatible_template_arg (tree parm, tree arg)
+is_compatible_template_arg (tree parm, tree arg, tree args)
 {
   tree parm_cons = get_constraints (parm);
 
@@ -8382,6 +8382,7 @@ is_compatible_template_arg (tree parm, tree arg)
     {
       tree aparms = DECL_INNERMOST_TEMPLATE_PARMS (arg);
       new_args = template_parms_level_to_args (aparms);
+      new_args = add_to_template_args (args, new_args);
       ++processing_template_decl;
       parm_cons = tsubst_constraint_info (parm_cons, new_args,
 					  tf_none, NULL_TREE);
@@ -8636,7 +8637,7 @@ convert_template_argument (tree parm,
               // Check that the constraints are compatible before allowing the
               // substitution.
               if (val != error_mark_node)
-                if (!is_compatible_template_arg (parm, arg))
+		if (!is_compatible_template_arg (parm, arg, args))
                   {
 		    if (in_decl && (complain & tf_error))
                       {
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-ttp5.C b/gcc/testsuite/g++.dg/cpp2a/concepts-ttp5.C
new file mode 100644
index 0000000..abc22ce
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-ttp5.C
@@ -0,0 +1,24 @@
+// PR c++/111485
+// { dg-do compile { target c++20 } }
+
+template<class T> constexpr bool always_true = true;
+
+template<class T> concept C = always_true<T>;
+template<class T> concept D = C<T> || true;
+
+template<template<C> class TT> struct example;
+template<template<D> class UU> using example_t = example<UU>;
+
+template<class T>
+struct A {
+  template<template<C> class TT> struct example;
+
+  template<template<D> class UU> using example_t = example<UU>;
+
+  template<class U>
+  struct B {
+    template<template<D> class UU> using example_t = example<UU>;
+  };
+};
+
+template struct A<int>::B<int>;
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-ttp6.C b/gcc/testsuite/g++.dg/cpp2a/concepts-ttp6.C
new file mode 100644
index 0000000..6396e99
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-ttp6.C
@@ -0,0 +1,17 @@
+// PR c++/111485
+// { dg-do compile { target c++20 } }
+
+template<class T, bool V> constexpr bool always_true = true;
+
+template<class T, bool V> concept C = always_true<T, V>;
+
+template<bool V, template<class T> requires C<T, V> class TT>
+void f();
+
+template<class T> requires C<T, true>
+struct A;
+
+int main() {
+  f<true, A>();
+  f<false, A>(); // { dg-error "no match|constraint mismatch" }
+}