[Bug c/94379] New: Feature request: like clang, support __attribute((__warn_unused_result__)) on structs, unions, and enums

[Bug c/94379] New: Feature request: like clang, support __attribute((__warn_unused_result__)) on structs, unions, and enums

[pskocik at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94379

            Bug ID: 94379
           Summary: Feature request: like clang, support
                    __attribute((__warn_unused_result__)) on structs,
                    unions, and enums
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pskocik at gmail dot com
  Target Milestone: ---

Clang supports applying the warn_unused_result attribute to enums, structs, and
unions, which has the effect that functions returning such an attributed
enum/struct/union behaves as if it itself had the warn_unused_result attribute.


Example:

typedef struct __attribute__((__warn_unused_result__)) aStructType{ int x; }
aStructType;
aStructType getStruct(void);

typedef union __attribute__((__warn_unused_result__)) aUnionType{ int x; }
aUnionType;
aUnionType getUnion(void);

typedef enum __attribute__((__warn_unused_result__)) anEnumType{
anEnumarationConstant } anEnumType;
anEnumType getEnum(void);

int main()
{
        getEnum();
        getStruct();
        getUnion();
}
// https://gcc.godbolt.org/z/jyHhLx

I find this to be a very useful feature, and it would be nice if gcc had it
(along with its current un-void-able warn_unused_result
(https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66425)).

[Bug c/94379] Feature request: like clang, support __attribute((__warn_unused_result__)) on structs, unions, and enums

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94379

Martin Sebor <msebor at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2020-03-28
                 CC|                            |msebor at gcc dot gnu.org
     Ever confirmed|0                           |1
           Severity|normal                      |enhancement
             Status|UNCONFIRMED                 |NEW

--- Comment #1 from Martin Sebor <msebor at gcc dot gnu.org> ---
Confirmed.

C2X will likely have the same [[nodiscard]] attribute as C++.  GCC 10
recognizes [[nodiscard]] but ignores it with a warning.  In both languages the
attribute can be applied to enums, structures, and unions as well.  The latest
proposal is http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2267.pdf.  But I
think it makes sense to extend warn_unused_result so that it can be used in
earlier language conformance modes, and for compatibility with Clang.

[Bug c/94379] Feature request: like clang, support __attribute((__warn_unused_result__)) on structs, unions, and enums

[pskocik at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94379

--- Comment #2 from Petr Skocik <pskocik at gmail dot com> ---
Excellent! 

For optional super extra coolness, this might work (and clang doesn't do this)
with statement expressions too so that statement expression-based macros could
be
marked warn_unused_result through it too.


typedef struct  __attribute((__warn_unused_result__)) { int x; } 
    wur_retval_t;

wur_retval_t foo(void){ int x=41; return (wur_retval_t){x+1}; }
#define foo_macro() ({  int x=41; (wur_retval_t){x+1}; })

void use(void){
    foo();  //warn unused result ✓
    foo_macro(); //perhaps should "warn unused result" too?
}