[gcc r13-7836] c++: constraint rewriting during ttp coercion [PR111485]

[Patrick Palka <ppalka@gcc.gnu.org>]

https://gcc.gnu.org/g:59f5786c20a42be13ac6fec567ffe840045012ad

commit r13-7836-g59f5786c20a42be13ac6fec567ffe840045012ad
Author: Patrick Palka <ppalka@redhat.com>
Date:   Fri Sep 22 06:25:49 2023 -0400

    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.
    
    (cherry picked from commit 6f902a42b0afe3f3145bcb864695fc290b5acc3e)

Diff:
---
 gcc/cp/pt.cc                               |  5 +++--
 gcc/testsuite/g++.dg/cpp2a/concepts-ttp5.C | 24 ++++++++++++++++++++++++
 gcc/testsuite/g++.dg/cpp2a/concepts-ttp6.C | 17 +++++++++++++++++
 3 files changed, 44 insertions(+), 2 deletions(-)

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index bf2dbd681ac..174567fc1ec 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -8334,7 +8334,7 @@ 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);
 
@@ -8355,6 +8355,7 @@ 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);
@@ -8609,7 +8610,7 @@ 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 00000000000..abc22ce59d8
--- /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 00000000000..6396e99cc05
--- /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" }
+}