[Bug c/83584] New: "ISO C forbids conversion of object pointer to function pointer type" -- no, not really

["Keith.S.Thompson at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83584

            Bug ID: 83584
           Summary: "ISO C forbids conversion of object pointer to
                    function pointer type" -- no, not really
           Product: gcc
           Version: 7.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: Keith.S.Thompson at gmail dot com
  Target Milestone: ---

$ gcc --version
gcc (GCC) 7.1.0
Copyright (C) 2017 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ cat c.c
int main(void) {
    typedef void (func)(void);
    void *ptr = 0;
    func *fptr = (func*)ptr;
}
$ gcc -std=c11 -pedantic-errors -c c.c
c.c: In function ‘main’:
c.c:4:18: error: ISO C forbids conversion of object pointer to function pointer
type [-Wpedantic]
     func *fptr = (func*)ptr;
                  ^
$ 

In fact the conversion does not violate any syntax rule or constraint,
though its behavior is undefined.

N1570 6.5.4 p2-4 specifies the constraints on the cast operator. It
permits conversions between scalar types (which include pointer types). p4
specifically forbids conversions between pointer types and floating-point
types. There's nothing about pointer-to-function types.

6.3.2.3 defines the semantics of pointer conversions. It says nothing
about conversions between void* and pointer-to-function types -- which
means the behavior is undefined by omission.

The diagnostic should be a non-fatal warning when "-pedantic-errors"
is used, and it should be rephrased.