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

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

[Keith.S.Thompson at gmail dot com]

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.

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

[pinskia at gcc dot gnu.org]

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

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2022-09-29
             Status|UNCONFIRMED                 |NEW

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
clang produces:
<source>:3:12: error: returning 'void *' from a function with incompatible
result type 'double'
    return (void *)0;
           ^~~~~~~~~

MSVC produces:

<source>(3): error C2440: 'return': cannot convert from 'void *' to 'double'

ICC Produces:
<source>(3): error: return value type does not match the function type
      return (void *)0;
             ^

cproc (https://github.com/michaelforney/cproc) produces:
<source>:2:22: error: assignment to arithmetic type must be from arithmetic
type


MSVC seems similar to what is produced for GCC's C++ front-end.
I don't know the best wording here really because convert might even be wrong.