P2231R1 Completing `constexpr` In `optional` And `variant`

[StephanTLavavej]

P2231R1 Completing constexpr In optional And variant

Feature-test macros:

#define __cpp_lib_optional INCREASED_VALUE_TO_BE_DETERMINED
#define __cpp_lib_variant  INCREASED_VALUE_TO_BE_DETERMINED

At the June 2021 virtual plenary meeting, this was accepted as a defect report for C++20, which means that this paper applies retroactively to the C++20 Standard.