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In the interests of keeping subtyping decidable [1], 152563b
added some bookkeeping to `isSubType` to detect cycles.
However, this was based on a hash set containing instances of
`SubTypePair`, and that class had inconsistencies between its
`hashCode` (in terms of `Type#hashCode`) and `equals`
(in terms of `=:=`).
This inconsistency can be seen in:
scala> trait C { def apply: (Int @unchecked) }
defined trait C
scala> val intUnchecked = typeOf[C].decls.head.info.finalResultType
intUnchecked: $r.intp.global.Type = Int @unchecked
scala> val p1 = new SubTypePair(intUnchecked, intUnchecked)
p1: $r.intp.global.SubTypePair = Int @unchecked <:<? Int @unchecked
scala> val p2 = new SubTypePair(intUnchecked.withoutAnnotations, intUnchecked.withoutAnnotations)
p2: $r.intp.global.SubTypePair = Int <:<? Int
scala> p1 == p2
res0: Boolean = true
scala> p1.hashCode == p2.hashCode
res1: Boolean = false
This commit switches to using `Type#==`, by way of the standard
case class equality.
The risk here is that you could find a subtyping computation that
progresses in such a manner that we don't detect the cycle. It would
need to produce an infinite stream of representations for types that
were `=:=` but not `==`. If that happened, we'd fail to terminate,
rather than judging the relationship as `false`.
[1] http://research.microsoft.com/pubs/64041/fool2007.pdf
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