Lightning Talk: The UB Detector: constexpr
summary
Claim that a constexpr evaluation forbids undefined behaviour is false: C++20 code using std::views::filter, mutating elements so they no longer satisfy the predicate, triggers UB at compile time and yields a surprising odd value where evens were expected. more than zero undefined behaviours. Compiler Explorer: clang and gcc agree and a compile-time static_assert fails with odd value 5. A second example shows a [[noreturn]] function returning a value is UB yet compilers sometimes accept it.
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