[Bug preprocessor/69792] New: spurious warning for UDL declaration with parenthesized literal-operator-id

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69792

            Bug ID: 69792
           Summary: spurious warning for UDL declaration with
                    parenthesized literal-operator-id
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Keywords: diagnostic
          Severity: normal
          Priority: P3
         Component: preprocessor
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
  Target Milestone: ---

This example surrounds operator""_min in parentheses, to protect it from
macros:

#define _min(a, b) (((a) < (b)) ? (a) : (b))
long double (operator""_min)(long double m) { return m; }
long double d = 1.0_min;

But the preprocessor thinks that (operator""_min) is a string literal followed
by a token, and so complains there is no whitespace:

bug.cc:2:22: warning: invalid suffix on literal; C++11 requires a space between
literal and string macro [-Wliteral-suffix]
 long double (operator""_min)(long double m) { return m; }
                      ^

This isn't a literal with an invalid suffix, but a literal-operator-id.

There's a simple workaround, which is to add the suggested whitespace, which
works, but not for the reason the preprocessor thinks it works :-)