[Bug c++/59873] The value of char32_t U'\u0000' and char16_t u'\u000' is 1, instead of 0.

[glisse at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59873

--- Comment #4 from Marc Glisse <glisse at gcc dot gnu.org> ---
Seems to be on purpose, see the comment before _cpp_valid_ucn in
libcpp/charset.c, and the last instruction in that function.

[lex.charset] is a bit hard to read for me.