The contents and meaning of the header <cfenv>
are a subset of the C standard library header <fenv.h> and
only the declarations shown in the synopsis above are present.
This document does not require an implementation to support the
FENV_ACCESS pragma;
it is implementation-defined ([cpp.pragma])
whether the pragma is supported.
As a consequence,
it is implementation-defined
whether these functions can be used to test floating-point status flags,
set floating-point control modes, or run under non-default mode settings.
If the pragma is used to enable control over the floating-point environment,
this document does not specify the effect on
floating-point evaluation in constant expressions.