[Bug c++/99287] std::source_location::function_name breaks constexpr

["ppalka at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99287

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Assignee|unassigned at gcc dot gnu.org      |ppalka at gcc dot gnu.org
             Status|NEW                         |ASSIGNED

--- Comment #3 from Patrick Palka <ppalka at gcc dot gnu.org> ---
IIUC, those two types are actually the same, it's just that one of them was
obtained through the char_type alias, and it seems debug_tree prefers to show
the name of the alias when the type came from one.

It seems the issue ultimately is in cxx_eval_increment_expression: during
evaluation of ++__first with __first = &"mystr"[n] + m and with lval=false,
since cxx_fold_pointer_plus_expression and not fold_build2 is responsible for
simplifying this POINTER_PLUS_EXPR to &"mystr"[n+m], returning 'mod' actually
returns the unreduced &"mystr"[n] + m rather than the reduced &"mystr"[n+m]
that we obtain as part of constexpr evaluation of the temporary MODIFY_EXPR. 
This unreduced return value of cxx_eval_increment interfers with later
constexpr evaluation, e.g. folding of the POINTER_DIFF_EXPR.

I'm testing the following which updates 'mod' with the result of evaluation of
the MODIFY_EXPR:

--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -5582,20 +5582,14 @@ cxx_eval_increment_expression (const constexpr_ctx
*ctx, tree t,
   /* Storing the modified value.  */
   tree store = build2_loc (cp_expr_loc_or_loc (t, input_location),
                           MODIFY_EXPR, type, op, mod);
-  cxx_eval_constant_expression (ctx, store,
-                               true, non_constant_p, overflow_p);
+  mod = cxx_eval_constant_expression (ctx, store, false,
+                                     non_constant_p, overflow_p);
   ggc_free (store);

   /* And the value of the expression.  */