[Bug] Default TypeMutator creates a new Global Type Var but the type checker relies on pointer equality

[slyubomirsky]

The default behavior of the Relay type mutator when it encounters Global Type Variables is to create a new Global Type Var instead of passing along the existing one. This can lead to type checking errors, as the type checker relies on pointer equality for global type vars (the type solver unifies them by checking for structural equality, and the structural equality check looks at pointer equality).

Proposed fix: Have the default return the global type var unmodified. Alternatively, we shouldn't use pointer equality for global type vars.

[StrongerXi]

Any update on this?

I feel like returning the global type var unmodified might make the semantics a bit unintuitive, because we always construct new ones for other base types.

Alternatively, we shouldn't use pointer equality for global type vars.

If this approach sounds good, I wouldn't mind giving it a shot.

Could you also send a quick repro for the type checking error you are describing, if possible?