Can't test/undef macros named for keywords

[Y-Less]

#define for 42 // Sytactically fine, and actually use by foreach (not as "42" though).
#undef for // error 020: invalid symbol name ""
#if defined for // error 020: invalid symbol name ""
#endif

It seems #define parses the string explicitly. #undef and defined just look for a symbol from the lexer, which those aren't.

[Y-Less]

You literally can't remove them and will get a redefinition warning if you try and change it, even if the change could be made legitimate with #undef.

[YashasSamaga]

Allowing keywords in #undef is quite easy but allowing its use in #if is a bit complicated. The compiler invokes the constexpr function (which handles constant expressions in non-preprocessor code
) to evaluate the expression given to the #if directive.

If the keywords have to work in #if as symbols, then they should work in non-preprocessor code as well.

[Y-Less]

Well they would only be symbols if someone had done #define while if. Otherwise fail as any other undefined symbol would. But I don't know if that would be hard.

[stale]

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