Packages

•  package root

  The Scala compiler and reflection APIs.

  The Scala compiler and reflection APIs.

  Definition Classes

  root

•  package scala

  Definition Classes

  root

•  package tools

  Definition Classes

  scala

•  package nsc

  Definition Classes

  tools

•  package typechecker

  Definition Classes

  nsc

•  trait Checkable extends AnyRef

  On pattern matcher checkability:

  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 be a P, because it is an X [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 will 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.

  Definition Classes

  typechecker

• Checkability
• InferCheckable

scala.tools.nsc.typechecker.Checkable

Checkability