Interactive: Generics fail to unify oddly-bound types

[Matt Giuca]

In interactive mode, some fully general types (e.g. 'a') fail to unify with other types.

It appears the only case this arises is when the "other type" is a complex type with badly-bound type variables, which can only be caused by bug #418161. Therefore, solving that bug may cause this one to disappear, but I still want to investigate.

Observed behaviour:
?> a = [1,2,3]
?> :t a
a :: Array(t_1) # This is caused by bug #41861
?> show(a)
Type error in expression 'show(a)'
        Term: a
        Type: Array(a)
        Expected: a.

Expected behaviour:
?> show(a)
[91, 49, 44, 32, 50, 44, 32, 51, 93]

Note that this bug doesn't occur for array literals:
?> show([1,2,3])
[91, 49, 44, 32, 50, 44, 32, 51, 93]

Or for properly-bound types:

def ai :: Array(Int):
    return [1,2,3]

?> :t ai
ai :: Array(Int)
?> show(ai)
[91, 49, 44, 32, 50, 44, 32, 51, 93]

So it may just be a side-effect of bug #418161.

[Matt Giuca (mgiuca)]

Needs to be fixed (this isn't part of the type system refactor; it's a high-priority bug).

[Matt Giuca (mgiuca)]

This is a duplicate of #418161, as it occurs because local variables in interactive have no varset. Type variables without varsets are not merged correctly when the varsets merge, causing certain type variables to happen to be considered equal.