[Bug c/57345] New: Preprocessor fails to evaluation string token not valid

[signupnathan at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57345

            Bug ID: 57345
           Summary: Preprocessor fails to evaluation string token not
                    valid
           Product: gcc
           Version: 4.7.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: signupnathan at gmail dot com

This error

23L03.c: In function ‘main’:
23L03.c:16:10: error: token ""I know the C language.\n"" is not valid in
preprocessor expressions
23L03.c:20:10: error: token ""I know BASIC.\n"" is not valid in preprocessor
expressions

Is generated by the following code that compliles fine with clang. Code comes
from a book teaching about C in a preprocessor chapter.

#include <stdio.h>

#define C_LANG    'C'
#define B_LANG    'B'
#define NO_ERROR  0

int main(void)
{
   #if C_LANG == 'C' && B_LANG == 'B'
     #undef C_LANG
     #define C_LANG "I know the C language.\n"
     #undef B_LANG
     #define B_LANG "I know BASIC.\n"
     printf("%s%s", C_LANG, B_LANG);
   #elif C_LANG == 'C'
     #undef C_LANG
     #define C_LANG "I only know C language.\n"
     printf("%s", C_LANG);
   #elif B_LANG == 'B'
     #undef B_LANG
     #define B_LANG "I only know BASIC.\n"
     printf("%s", B_LANG);
   #else
     printf("I don't know C or BASIC.\n");
   #endif

   return NO_ERROR;
}


The preprocessor balks at this type of statement:

#if "foo" == "foo"
   printf("foo matches");
#endif