c++/1056: spurious warning on implicit conversion followed by promotion and comparison

[martin.dorey@synaxia.com]

>Number:         1056
>Category:       c++
>Synopsis:       spurious warning on implicit conversion followed by promotion and comparison
>Confidential:   no
>Severity:       non-critical
>Priority:       low
>Responsible:    unassigned
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Wed Dec 13 09:16:00 PST 2000
>Closed-Date:
>Last-Modified:
>Originator:     martin.dorey@synaxia.com
>Release:        gcc version 2.97 20001211 (experimental)
>Organization:
>Environment:
http://www.codesourcery.com/gcc-compile.shtml
>Description:
Builtin operator==() produces a spurious warning when comparing an unsigned short with an unsigned long where the unsigned short is obtained via an implicit conversion from a class object.
>How-To-Repeat:

typedef unsigned short uint16;

struct be16
{
  uint16 get () const;
  operator uint16 () const;
};

be16 b;
unsigned long s;

bool cat (int, int);
bool cat (int, unsigned int);
bool cat (unsigned int, int);
bool cat (unsigned int, unsigned int);
bool cat (long, long);
bool cat (long, unsigned long);
bool cat (unsigned long, unsigned long);
bool cat (unsigned long, long);

bool cat (long, int);
bool cat (long, unsigned int);
bool cat (unsigned long, int);
bool cat (unsigned long, unsigned int);
bool cat (int, long);
bool cat (int, unsigned long);
bool cat (unsigned int, unsigned long);
bool cat (unsigned int, long);

#if 0
#define CAT(a, b) cat (a, b)
#else
#define CAT(a, b) a == b
#endif

int
main()
{
    extern uint16 fn();
    uint16 a = fn();

    if (CAT (a, b))
        ;
    if (CAT (b, a))
        ;
    if (CAT (a, s))
        ;
    if (CAT (s, a))
        ;
    if (CAT (b, s))
        ;
    if (CAT (s, b))
        ;
    if (CAT (b.get(), s))
        ;
    if (CAT (s, b.get()))
        ;
    return 0;
}

gcc -c -W -Wall produces:

/tmp/@7541.7.cc: In function `int main()':
/tmp/@7541.7.cc:51: warning: comparison between signed and unsigned integer 
expressions
/tmp/@7541.7.cc:53: warning: comparison between signed and unsigned integer 
expressions

Changing the #if around causes a compilation with no warnings.  I haven't been able to find anything in the Standard which makes me think that the warning is justified.

>Fix:

>Release-Note:
>Audit-Trail:
>Unformatted: