Weird assertion in unification

[osa1]

In the type checker we have this assertion when setting link of an unification variable:

fir/src/type_checker/unification.rs

Lines 165 to 168 in a5229e3

	Ty::Var(var) => {
	assert!(var.link().is_none());
	var.prune_level(max_level);
	}

I don't remember why I thought that assertion was necessary. Consider unifying types of a value and a pattern:

let x = Option.Some(123)        # Option[I32]
match x:
    Option.Some(i): ...
    Option.None: ...

Here the unifications we do:

• I32 ~ _0: argument of instantiated type of Option.Some with type of 123.
• _1 ~ Option[_0]: type of Option.Some(123) with type of x.

In the second step, we link _1 -> Option[_0], and prune the level of of Option[_0] to _1s level. That prune opeartion fails because _0 is linked to I32 in the previous unification step.

For now I'll comment-out the assertion.