Possible resolution:

Change in 15.12 [cpp.predefined] paragraph 1 as follows:

...

__STDC_HOSTED__

The integer literal 1 if the implementation is a hosted implementation or the integer literal 0 if it is a freestanding implementation (4.1 [intro.compliance]).

__STDC_UTF_16__

The integer literal 1, intended to indicate that values of type char16_t are UTF-16 encoded (5.13.3 [lex.ccon], 5.13.5 [lex.string]).

__STDC_UTF_32__

The integer literal 1, intended to indicate that values of type char32_t are UTF-32 encoded (5.13.3 [lex.ccon], 5.13.5 [lex.string]).

(Originally submitted as LWG issue 2237.)

The macros __STDC_UTF_16__ and __STDC_UTF_32__ are predefined by the C implementation. C++ is missing them.