Does C++11, 14, 17 or 20 introduce a standard constant for pi?

Question

There is a rather silly problem with the number pi in C and C++. As far as I know M_PI defined in math.h is not required by any standard.

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Answer 1

M_PI is defined by "a standard", if not a language standard: POSIX with the X/Open System Interfaces extension (which is very commonly supported and required for official UNIX branding).

It's (still) not certain what will be in C++20, but since you asked: it probably will have such constants. The paper was merged in the last round of C++20 features (for the Committee Draft in August 2019).

Specifically, there will both be std::numbers::pi (of type double) and a variable template that you can use if you want a different floating point type, e.g. std::numbers::pi_v. The full list of constants can be see in [numbers.syn].