The clue is at the top of the module:
{-
This is a generated file (generated by genprimopcode).
It is not code to actually be used. Its only purpose is to be
consumed by haddock.
-}
Incidentally, let x = x in x
is equivalent to undefined
, and is valid for any data type; x
isn't constrained by being defined in any way other than self-referentially, so can be any type.
These functions are "primitive" meaning operations so basic they're not defined in Haskell code, and quite possibly just translated directly to machine instructions.