Dear Joseph,

On Thu, Oct 8, 2020 at 1:36 PM Joseph Myers <joseph@codesourcery.com> wrote:
>
> This documentation doesn't seem sufficient to use the macros.  Do they
> expand to (narrow) string literals?  To an unquoted sequence of
> characters?  I think from the implementation that the answer is strings
> (so, in particular, not usable for testing anything in #if conditionals),
> but the documentation ought to say so.  The test ought to verify the form
> of the expansion as well (even if it can't do anything useful at execution
> time, because if you make the macros reflect the command-line options they
> are character set names that are meaningful on the host, and any
> conversion functionality on the target may not use the same names as the
> host).

     You're right; sorry about that, I should have been more thorough!
I thought about adding a test to check the name itself (e.g, for
"UTF-8"), but that might make tests fail on platforms where the
default SOURCE_CHARSET from the dev files is not, in fact, UTF-8. I
could also try to pass some options but then I'd have to guarantee
that the encoding was available on all testable platforms, too...!

    In the end, for the tests, I just initialize two "const char[]"
directly from the macro expansions to make sure we are getting
strings. It seeeeeems to work okay. Attached is the revised patch with
better docs and test!

Sincerely,
JeanHeyd

2020-10-08  JeanHeyd "ThePhD" Meneide  <phdofthehouse@gmail.com>

        * gcc/c-family/c-cppbuiltin.c: Add predefined macro
definitions for charsets
        * gcc/doc/cpp.texi: Document new predefined macro.
        * gcc/testsuite/c-c++-common/cpp/wide-narrow-predef-macros.c (new):
          New test for macro definitions to always exist.
        * libcpp/include/cpplib.h: Add functions declarations for
          retrieving charset names
        * libcpp/directives.c: Add function definitions to retrieve charset
          names.
        * libcpp/internal.h: Add to/from name preservations