[gcc r14-418] c++: fix TTP level reduction cache

[Jason Merrill <jason@gcc.gnu.org>]

https://gcc.gnu.org/g:0bc2a1dc327af9817163cc8df78b9f9be2ad0f90

commit r14-418-g0bc2a1dc327af9817163cc8df78b9f9be2ad0f90
Author: Jason Merrill <jason@redhat.com>
Date:   Tue Mar 14 15:16:46 2023 -0400

    c++: fix TTP level reduction cache
    
    We try to cache the result of reduce_template_parm_level so that when we
    reduce the same parm multiple times we get the same result, but this wasn't
    working for template template parms because in that case TYPE is a
    TEMPLATE_TEMPLATE_PARM, and so same_type_p was false because of the same
    level mismatch that we're trying to adjust for.  So in that case compare the
    template parms of the template template parms instead.
    
    The result can be seen in nontype12.C, where we previously gave three
    duplicate errors on line 7 and now give only one because subsequent
    substitutions use the cache.
    
    gcc/cp/ChangeLog:
    
            * pt.cc (reduce_template_parm_level): Fix comparison of
            template template parm to cached version.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/template/nontype12.C: Check for duplicate error.

Diff:
---
 gcc/cp/pt.cc                              | 7 ++++++-
 gcc/testsuite/g++.dg/template/nontype12.C | 3 ++-
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 471fc20bc5b..5446b5058b7 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -4550,7 +4550,12 @@ reduce_template_parm_level (tree index, tree type, int levels, tree args,
   if (TEMPLATE_PARM_DESCENDANTS (index) == NULL_TREE
       || (TEMPLATE_PARM_LEVEL (TEMPLATE_PARM_DESCENDANTS (index))
 	  != TEMPLATE_PARM_LEVEL (index) - levels)
-      || !same_type_p (type, TREE_TYPE (TEMPLATE_PARM_DESCENDANTS (index))))
+      || !(TREE_CODE (type) == TEMPLATE_TEMPLATE_PARM
+	   ? (comp_template_parms
+	      (DECL_TEMPLATE_PARMS (TYPE_NAME (type)),
+	       DECL_TEMPLATE_PARMS (TEMPLATE_PARM_DECL
+				    (TEMPLATE_PARM_DESCENDANTS (index)))))
+	   : same_type_p (type, TREE_TYPE (TEMPLATE_PARM_DESCENDANTS (index)))))
     {
       tree orig_decl = TEMPLATE_PARM_DECL (index);
 
diff --git a/gcc/testsuite/g++.dg/template/nontype12.C b/gcc/testsuite/g++.dg/template/nontype12.C
index e37cf8f7646..6642ffd0a13 100644
--- a/gcc/testsuite/g++.dg/template/nontype12.C
+++ b/gcc/testsuite/g++.dg/template/nontype12.C
@@ -4,7 +4,8 @@
 template<typename T> struct A
 {
   template<T> int foo();                        // { dg-error "double" "" { target c++17_down } }
-  template<template<T> class> int bar();        // { dg-error "double" "" { target c++17_down } }
+  template<template<T> class> int bar();        // { dg-bogus {double.*C:7:[^\n]*double} }
+  // { dg-error "double" "" { target c++17_down } .-1 }
   template<T> struct X;                         // { dg-error "double" "" { target c++17_down } }
 };