trait Checkable extends AnyRef
On pattern matcher checkability:
The spec says that case _: List[Int] should be always issue an unchecked warning:
> Types which are not of one of the forms described above are > also accepted as type patterns. However, such type patterns > will be translated to their erasure (§3.7). The Scala compiler > will issue an “unchecked” warning for these patterns to flag > the possible loss of type-safety.
But the implementation goes a little further to omit warnings based on the static type of the scrutinee. As a trivial example:
def foo(s: Seq[Int]) = s match { case _: List[Int] => }
need not issue this warning.
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 will conform to 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 ∈ X) ⇒ (x ∉ 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.
We evaluate "X with conform to P" by checking X <: P_wild, where
P_wild is the result of substituting wildcard types in place of
pattern type variables. This is intentionally stricter than
(X matchesPattern P), see scala/bug#8597 for motivating test cases.
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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