Context: C compilers deliberately do not support some features while being conforming implementations. Is it ok?
Some examples. All of the compilers down below are conforming implementations (__STDC__ is defined to 1). However:
gcc does not support #pragma STDC FENV_ACCESS and issues warning: ignoring ‘#pragma STDC FENV_ACCESS’ [-Wunknown-pragmas]. However, fenv.h is a standard header, and support of #pragma STDC FENV_ACCESS ON is required by the standard, because ON is one of the choices in on-off-switch: ON OFF DEFAULT.
msvc does not support #pragma STDC FP_CONTRACT and rather supports its own version #pragma fp_contract ( { on | off } ). However, the standard version is #pragma STDC FP_CONTRACT { ON | OFF | DEFAULT }.
gcc on Cygwin: sscanf doesn't handle hexadecimal floating-point input. And libc is a part of the standard.
Question: is it allowed for an implementation to deliberately provide a limited functionality (or its own version of such functionality) and still be conforming implementation at the same time?
Reason of the question: better understanding of the definition of conforming implementation and the state of affairs around existing conforming implementations.
Note: here the area of interest is conforming hosted implementation only. However, the question is applicable for conforming freestanding implementation as well.
UPD. Again: __STDC__ is defined to 1 means maybe conforming implementation rather than conforming implementation. Any discrepancy with the standard automatically makes such implementation be "nonconforming implementation that defines __STDC__ to 1".
If a standard has a rule that an implementation does X, and an implementation does not do X, then it is not conforming.