Summary:
X-link: https://github.com/facebook/react-native/pull/39358
This adds a function polyfilling C++ 20's `std::bit_cast`, using `memcpy()` to be safe with strict aliasing rules.
This replaces the conditional code in CompactValue for type punning, an unsafe place in YGJNI where we do it unsafely, and is used in ValuePool. The polyfill can be switched to `std::bit_cast` whenever we adopt C++ 20.
Note that this doesn't actually call into `memcpy()`, as verified by Godbolt. Compilers are aware of the memcpy type punning pattern and optimize it, but it's ugly and confusing to folks who haven't seen it before.
Reviewed By: javache
Differential Revision: D49082997
fbshipit-source-id: b848775a68286bdb11b2a3a95bef8069364ac9b5
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