The function either returns 1 or loops infinitely. So if you assume that infinite loops are undefined behavior, the optimization is valid. I only know what the

The function either returns 1 or loops infinitely. So if you assume that infinite loops are undefined behavior, the optimization is valid. I only know what the C11 standard says about infinite loops: > An iteration statement whose controlling expression is not a constant expression, that performs no input/output operations, does not access volatile objects, and performs no synchronization or atomic operations in its body, controlling expression, or (in the case of a for statement) its expression-3, may be assumed by the implementation to terminate. But this only applies to some iteration statements, not recursive functions. I don't know about C++ rules but since this optimization is also applied when compiling as C code (https://godbolt.org/z/K3uT2G https://godbolt.org/z/K3uT2G), it seems to be invalid.