> On 19 Mar 2025, at 00:16, Andre Kostur <andre_at_[hidden]> wrote:
>
> Please provide a specific concrete example of this in play to showcase how this attribute would be significantly better than all of the existing alternatives.

The example I mentioned first in this thread is:
https://gcc.gnu.org/pipermail/gcc-help/2025-January/143945.html
GCC has the type std::float128_t on MacOS, but not the functions, as they rely on the C library which comes from Clang which does not have this type implemented.

Assume that GCC has the declaration:
namespace std{
  [[unimplemented]] std::float128_t modf(std::float128_t, std::float128_t*);
}

Then it is possible to implement it in GCC, assuming there is also a new “unimplemented” macro, by:
#if QUADMATH_H // Or something
#include <quadmath.h>
#endif

namespace std {
#if unimplemented(modf(std::float128_t, std::float128_t*)) // Problem with “,” here
  std::float128_t modf(std::float128_t x, std::float128_t* y) {
    __float128 y0;
    __float128 r = modfq(x, &y0);
    *y = y0;
    return r;
  }
#endif
}

Then it will add the function definition on platforms where it is unimplemented but having the <quadmath.h> library, but add nothing if it is implemented.

If Clang later adds support for float128 in its C library, then GCC will also will have it from this library, rather than the added implementation.