[ashearer]

It looks like a general optimization for tail-recursive functions that assumes they terminate (because not terminating would be undefined behavior). The parameters to the recursive calls don't matter: Substitute other expressions or constants for `n / 2` and `3 * n + 1`, and the compiled result remains the same. So it's not Collatz-specific.

clang appears to correctly detect that `collatz` only directly defines a result for `1`, and any other input expands to yet another recursive call to `collatz` (the parameter is irrelevant). To avoid infinite recursion, `collatz` must eventually be called with the value 1, so that's what clang concludes.