HK reduction: Remove special-case for typerefs #1400
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Now that we implement partial higher-order unification (SI-2712 fix) i1181.scala will compile even if `Alias[Int]` gets dealiased to `(Int, Int)` because we can unify the latter with `M[_]` where `M = [X] -> (Int, X)`. The new test will only succeed if `Alias[Int, Int]` is not dealiased because we will never unify `Foo[Int]` with `M[_,_]`.
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The special case:
case stripped: TypeRef =>
stripped.symbol.is(BaseTypeArg)
is wrong because you might still want to reduce applications involving
TypeRefs which are not base class parameters, like in:
class Foo[A]
type Alias[X] = Foo[X]
val x: Alias[Int] = ???
`Alias` is a TypeRef so before this commit `Alias[Int]` was never reduced
to `Foo[Int]`. It should have been:
case stripped: TypeRef if stripped.symbol.is(BaseTypeArg) =>
true
But even this is incorrect: it assumes that we can always safely reduce HK
applications involving base class parameters, this is not the case when
the parameter kind is different from the rhs kind as illustrated by
`i1181c.scala`. We fix this by simply dropping the special case.
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Oops, had a typo in a test, should be all good now. |
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LGTM |
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The special case:
is wrong because you might still want to reduce applications involving
TypeRefs which are not base class parameters, like in:
Aliasis a TypeRef so before this commitAlias[Int]was never reducedto
Foo[Int]. It should have been:But even this is incorrect: it assumes that we can always safely reduce HK
applications involving base class parameters, this is not the case when
the parameter kind is different from the rhs kind as illustrated by
i1181c.scala. We fix this by simply dropping the special case.Review by @odersky