[Bug c/107091] New: Misleading error message "incompatible types when returning ..."

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107091

            Bug ID: 107091
           Summary: Misleading error message "incompatible types when
                    returning ..."
           Product: gcc
           Version: 12.2.0
            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: ---

When I compile this source file:

double bad(void) {
    return (void*)0;
}

double good(void) {
    return 42;
}

I get this output from gcc 12.2.0 (and from earlier versions):

c.c: In function ‘bad’:
c.c:2:12: error: incompatible types when returning type ‘void *’ but ‘double’
was expected
    2 |     return (void*)0;
      |            ^

It's true that it's a constraint violation, and it's true that void* and double
are incompatible, but type compatibility is not required here. The problem is
that there is no implicit conversion from void* to double.

In the "good" function, int and double are also incompatible types, but there
is an implicit conversion so the statement is valid.

Note that g++ produces this correct message for the equivalent C++ code:

c.cpp: In function ‘double bad()’:
c.cpp:2:19: error: cannot convert ‘void*’ to ‘double’ in return
    2 |     return (void*)0;
      |          

This was brought to my attention by this post on Stack Overflow:
https://stackoverflow.com/q/73899947/827263

Reference for compatible types: C11 (or N1570) 6.2.7.