[Bug c++/99534] New: bogus UDL diagnostic for header-name followed by macro

[Bug c++/99534] New: bogus UDL diagnostic for header-name followed by macro

[richard-gccbugzilla at metafoo dot co.uk]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99534

            Bug ID: 99534
           Summary: bogus UDL diagnostic for header-name followed by macro
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: richard-gccbugzilla at metafoo dot co.uk
  Target Milestone: ---

Testcase:

#define M
#include "foo.h"M
#include <foo.h>M

This produces two warnings

warning: invalid suffix on literal; C++11 requires a space between literal and
string macro [-Wliteral-suffix]

At least the second diagnostic appears to be incorrect: <foo.h> is not a string
literal and lexically can't have a UDL suffix applied.

The first diagnostic might also be incorrect (the header-name production for
double-quoted strings also can't have a UDL suffix), but the max munch rule
appears to require that we lex a string-literal rather than a header-name here!
I think that's probably a wording bug, and the lexer should prefer to produce a
header-name token whenever possible. (I'm taking that part to WG21 CWG; Clang
also diagnoses this, but EDG does not.)