Annex C (informative) Compatibility [diff]

C.1 C++ and ISO C [diff.iso]

C.1.10 [cpp]: preprocessing directives [diff.cpp]

[cpp.predefined]
Change: Whether __STDC__ is defined and if so, what its value is, are implementation-defined.

Rationale: C++ is not identical to ISO C.
Mandating that __STDC__ be defined would require that translators make an incorrect claim.
Each implementation must choose the behavior that will be most useful to its marketplace.

Effect on original feature: Change to semantics of well-defined feature.

Difficulty of converting: Semantic transformation.

How widely used: Programs and headers that reference __STDC__ are quite common.