Aaron Ballman e5f7123dfe Disable constexpr function body checking in more situations (#94347) ...

Before C++23, we would check a constexpr function body to diagnose if
the function can never be evaluated in a constant expression context.
This was previously required standards behavior, but C++23 relaxed the
restrictions with P2448R2. While this checking is useful, it is also
quite expensive, especially in pathological cases (see #92924 for an
example), because it means the mere presence of a constexpr function
definition will require constant evaluation even if the function is not
used within the TU.

Clang suppresses diagnostics in system headers by default and system
headers (like STL implementations) can be full of constexpr function
bodies. Now we suppress the check for a diagnostic if the function
definition is in a system header or if the `-Winvalid-constexpr`
diagnostic is disabled. This should have some mild compile time
performance improvements.

Also, the previous implementation would disable the diagnostic in C++23
mode entirely. Due to the benefit of the check, this patch now makes it
possible to enable the diagnostic explicitly in C++23 mode.

2024-06-04 13:32:11 -04:00

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