Hindley-Milner monomorphizes the type of a function inside the body of the function, so if you make a recursive call on a different type the type checker will complain. There are many subtle "gotchas" with type inference that are often not encountered by most programmers. Unless you've implemented a type inference engine yourself, you probably don't know about these sort of restrictions.

Here is an example of the type error (I used the datatype from the Wikipedia article on polymorphic recursion): https://repl.it/@CalebHelbling/PolymorphicRecursionError