[Bug c++/106753] New: Give -Wunused-value warning for user-defined literal expressions with discarded result

[Bug c++/106753] New: Give -Wunused-value warning for user-defined literal expressions with discarded result

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106753

            Bug ID: 106753
           Summary: Give -Wunused-value warning for user-defined literal
                    expressions with discarded result
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Keywords: diagnostic
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
  Target Milestone: ---

Testcase:

// { dg-do compile { target c++14 } }

#include <string>
#include <complex>
#include <chrono>

void test()
{
  using namespace std::literals;
  2i;  // { dg-warning "Wunused-value" }
  2s;  // { dg-warning "Wunused-value" }
  ""s; // { dg-warning "Wunused-value" }
#if __cplusplus >= 201703L
  ""sv; // { dg-warning "Wunused-value" "" { target c++17 } }
#endif
}


I could add [[nodiscard]] to all these UDLs, but that wouldn't help users who
write their own.

It's theoretically possible that a UDL could modify global variables, and be
called only for its side effects, but that seems disgusting. If we think that's
a real concern, we could only warned for 'constexpr' UDLs, which would avoid
_most_ disgusting global-state-modifying UDLs (but not ones that use
std::is_constant_evaluated() to decide whether to be disgusting).

I think we should just warn for all UDLs though.

Or I could just decorate the ones in libstdc++ with [[nodiscard]].

[Bug c++/106753] Give -Wunused-value warning for user-defined literal expressions with discarded result

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106753

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |mpolacek at gcc dot gnu.org
   Last reconfirmed|                            |2022-08-26
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW

[Bug c++/106753] Give -Wunused-value warning for user-defined literal expressions with discarded result

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106753

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|normal                      |enhancement

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
>but that seems disgusting. 

You know if it works, someone will do it, especially with C++ code.

[Bug c++/106753] Give -Wunused-value warning for user-defined literal expressions with discarded result

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106753

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
But it seems reasonable that they should write:

(void) 123_disgusting;

to silence a warning if they're really creating a literal just for its side
effects.