But, for example, LLVM does not fully support IEEE 754 [0].
And nor does GCC - who list it as unsupported, despite defining the macro and having partial support. [1]
The biggest caveat is in Annex F of the C standard:
> The C functions in the following table correspond to mathematical operations recommended by IEC 60559. However, correct rounding, which IEC 60559 specifies for its operations, is not required for the C functions in the table.
The C++ standard [2] barely covers support, but if a type supports any of the properties of ISO 60559, then it gets is_iec559 - even if that support is _incomplete_.
This paper [3] is a much deeper dive - but the current state for C++ is worse than C. Its underspecified.
> When built with version 18.1.0 of the clang C++ compiler, without specifying any compiler options, the output is:
> distance: 0.0999999
> proj_vector_y: -0.0799999
> Worse, if -march=skylake is passed to the clang C++ compiler, the output is:
> distance: 0.1
> proj_vector_y: -0.08
[0] https://github.com/llvm/llvm-project/issues/17379
[1] https://www.gnu.org/software/gcc/projects/c-status.html
[2] https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2023/n49...