[gcc r13-2340] c++: set TYPE_STRING_FLAG for char8_t

[gcc r13-2340] c++: set TYPE_STRING_FLAG for char8_t

[Jason Merrill]

https://gcc.gnu.org/g:c5699dd82aa053a8597a69f3331f11aff3e2823d

commit r13-2340-gc5699dd82aa053a8597a69f3331f11aff3e2823d
Author: Jason Merrill <jason@redhat.com>
Date:   Wed Aug 24 16:31:11 2022 -0400

    c++: set TYPE_STRING_FLAG for char8_t
    
    While looking at the DWARF handling of char8_t I wondered why we weren't
    setting TREE_STRING_FLAG on it.  I hoped that setting that flag would be an
    easy fix for PR102958, but it doesn't seem to be sufficicent.  But it still
    seems correct.
    
    I also tried setting the flag on char16_t and char32_t, but that broke
    because braced_list_to_string assumes char-sized elements.  Since we don't
    set the flag on wchar_t, I abandoned that idea.
    
    gcc/c-family/ChangeLog:
    
            * c-common.cc (c_common_nodes_and_builtins): Set TREE_STRING_FLAG on
            char8_t.
            (braced_list_to_string): Check for char-sized elements.

Diff:
---
 gcc/c-family/c-common.cc | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/gcc/c-family/c-common.cc b/gcc/c-family/c-common.cc
index 1eb842e1c7b..9746504195b 100644
--- a/gcc/c-family/c-common.cc
+++ b/gcc/c-family/c-common.cc
@@ -4550,6 +4550,7 @@ c_common_nodes_and_builtins (void)
   if (c_dialect_cxx ())
     {
       char8_type_node = make_unsigned_type (char8_type_size);
+      TYPE_STRING_FLAG (char8_type_node) = true;
 
       if (flag_char8_t)
         record_builtin_type (RID_CHAR8, "char8_t", char8_type_node);
@@ -9345,12 +9346,15 @@ braced_list_to_string (tree type, tree ctor, bool member)
   if (!member && !tree_fits_uhwi_p (typesize))
     return ctor;
 
-  /* If the target char size differes from the host char size, we'd risk
+  /* If the target char size differs from the host char size, we'd risk
      loosing data and getting object sizes wrong by converting to
      host chars.  */
   if (TYPE_PRECISION (char_type_node) != CHAR_BIT)
     return ctor;
 
+  /* STRING_CST doesn't support wide characters.  */
+  gcc_checking_assert (TYPE_PRECISION (TREE_TYPE (type)) == CHAR_BIT);
+
   /* If the array has an explicit bound, use it to constrain the size
      of the string.  If it doesn't, be sure to create a string that's
      as long as implied by the index of the last zero specified via