[nortiero]

You are absolutely correct, "mandates" is too strong a word. Annex F, which is normative, have a way out by not setting __STDC_IEC_559__, GCC6.3.0 does set a slightly different GCC_IEC_559. Yet I think that my argument still holds:

floating point calculations can be executed at wider range (§5.1.2.3.12);

assignments and casts are under obligation to wipe out that extra precision (§ same paragraph);

I am no language lawyer, but .. given the issue of program observable effects (§5.1.2.3.12) and the "as is" implied rule that governs optimizations, how possibly could equality be stable over time?

[leni536]

Yeah, you are right. It seems to be that the problem is that we cram two standards into too few types. Additionally to float, double, and long double there could be optional iec559_single_t, iec559_double_t and iec559_extended_t (similarly to stdint.h integer types). The operations on variables of these types could be strictly iec559 conformant.