[IR] Path dependent types

[joroKr21]

Currently, the IR cannot deal with path dependent types, because it relies on transformations that decompose paths (ANF). That's why Type.fix always calls widen.

In Scala, a (stable) path is

• either x where x is a stable identifier (i.e. val or object)
• or p.x where p is a path and x is a stable identifier (member of the object p).

A path-dependent type can be one of:

1. p.type where p is a path;
2. p.T where T is a type member of the path p;
3. p.T[A1, ..., An] where T is a polymorphic type member of the path p.

The problem with this is that a definition val x = y.z can have one of two types - y.z.type or the concrete type of z. In some contexts, the path-dependent type is better (e.g. instantiation of inner classes or method calls with dependent type signatures), in others the concrete type (e.g. arguments to a generic method, esp. if the result is some F[A] and not covariant).

Note that path dependent types are sometimes inferred (LHS of methods calls, arguments to method calls depending on the parameters, etc.)

[joroKr21]

It appears this was magically fixed by removing most .dealias.widen calls on types. I will add a test to ANFSpec that verifies the fix.

[joroKr21]

Ok it turns out that AlphaEq cannot handle path-dependent types 😞 Which is very logical, but that means I can't write such a test.

[joroKr21]

Resolved in #306