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Before this commit, the added testcase failed because the type of `inv`
was inferred to be `Inv[Any]` instead of `Inv[Int]`. The situation looks
like this:
def inv(cond: Boolean) =
if (cond)
new Inv(1) // : Inv[A] where A >: Int
else
Inv.empty // : Inv[A'] where A' unconstrained
// : Inv[A] | Inv[A']
To get the type of `inv`, we call `harmonizeUnion` which will take the
lub of `Inv[A]` and `Inv[A']`, eventually this mean that we do:
A' <:< A
But since `harmonizeUnion` uses `fluidly`, this does not result in `A'`
getting constrained to be a subtype of `A`, instead we constrain `A` to
the upper bound of `A'`:
Any <:< A
We use `fluidly` to avoid creating OrTypes in `lub`, but it turns out
that there is a less aggressive solution: `lub` calls `mergeIfSuper`
which then calls `isSubTypeWhenFrozen`, if we just make these subtype
calls non-frozen, we can achieve what we want. This is what the new
`lub` parameter `canConstrain` allows.
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