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The C++23 std::unreachable is a tool for injecting undefined behaviour into a program. A typical use case is in an else branch or the default case of a switch.

Compilers can use this undefined behaviour and optimise the corresponding branch away.

While we can achieve the same behaviour with [[assume(false)]], std::unreachable provides a more readable and logical name.

Compiler Explorer link: https://compiler-explorer.com/z/xh6h3EMqv