[Bug c++/88815] [9 Regression] is_constexpr (based on narrowing conversion and expression SFINAE) broken

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88815

--- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
I believe it is the

@@ -25361,7 +25486,10 @@
       return NULL_TREE;

     case TEMPLATE_PARM_INDEX:
-      return *tp;
+      if (dependent_type_p (TREE_TYPE (*tp)))
+       return *tp;
+      /* We'll check value-dependence separately.  */
+      return NULL_TREE;

       /* Handle expressions with type operands.  */
     case SIZEOF_EXPR:

pt.c (instantiation_dependent_r) hunk that matters for this testcase, at least
reverting it makes the testcase succeed.