Linux Kernel's __is_constexpr Macro

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How does the __is_constexpr(x) macro of the Linux Kernel work? What is its purpose? When was it introduced? Why was it introduced?

For a discussion on different approaches to solve the same problem, see instead: Detecting Integer Constant Expressions in Macros

It was introduced during the merge window for Linux Kernel v4.17, commit 3c8ba0d61d04 on 2018-04-05; although the discussions around it started a month before.

The macro is notable for taking advantage of subtle details of the C standard: the conditional operator's rules for determining its returned type (6.5.15.6) and the definition of a null pointer constant (6.3.2.3.3).

Note: the (long)(x) cast is intended to allow x to have pointer types and to avoid warnings on u64 types on 32-bit platforms. However, this detail is not important for understanding the key points of the macro.

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