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

["redi at gcc dot gnu.org" <gcc-bugzilla@gcc.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]].