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Closes SI-6275, SI-5762.
The comment says is better than I can.
/** On pattern matcher checkability:
*
* Consider a pattern match of this form: (x: X) match { case _: P => }
*
* There are four possibilities to consider:
* [P1] X will always conform to P
* [P2] x will never conform to P
* [P3] X <: P if some runtime test is true
* [P4] X cannot be checked against P
*
* The first two cases correspond to those when there is enough static
* information to say X <: P or that !(X <: P) for all X and P.
* The fourth case includes unknown abstract types or structural
* refinements appearing within a pattern.
*
* The third case is the interesting one. We designate another type, XR,
* which is essentially the intersection of X and |P|, where |P| is
* the erasure of P. If XR <: P, then no warning is emitted.
*
* Examples of how this info is put to use:
* sealed trait A[T] ; class B[T] extends A[T]
* def f(x: B[Int]) = x match { case _: A[Int] if true => }
* def g(x: A[Int]) = x match { case _: B[Int] => }
*
* `f` requires no warning because X=B[Int], P=A[Int], and B[Int] <:< A[Int].
* `g` requires no warning because X=A[Int], P=B[Int], XR=B[Int], and B[Int] <:< B[Int].
* XR=B[Int] because a value of type A[Int] which is tested to be a B can
* only be a B[Int], due to the definition of B (B[T] extends A[T].)
*
* This is something like asSeenFrom, only rather than asking what a type looks
* like from the point of view of one of its base classes, we ask what it looks
* like from the point of view of one of its subclasses.
*/
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