[ Brno 2026-06-09; Change status New → Core ]

In C++14 these macros were defined in a table showing them as macros in <cuchar>, and referring to the C Unicode TR, ISO/IEC TR 19769:2004, where the macros were defined in the header. But in the final C11 standard, the macros were predefined by the compiler not in the header. So when C++17 rebased on C11 and referred to the `uchar.h` from the C standard, we lost any reference to those macros.

An editorial change for C++17 removed the table mentioning those macros, following an arguably incorrect editorial change to "remove an outdated reference to the C Unicode TR". Although `uchar.h` is in C11, we lost any mention of the macros by saying that our `uchar.h` is the same as C `uchar.h`. So those macros now have no existence at all in C++. They're not inherited from the C version of `uchar.h` and they're not predefined by a C++ compiler.

If we want to copy those macros from C, they should be in [cpp.predefined], so Core should decide on that.

Apparently C1X changes __STDC_UTF_16__ and __STDC_UTF_32__ from macros defined in uchar.h (and reflected in C++ by Table 79) to be predefined by the compiler. Do we want to do the same?