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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Checkable.scala
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Type Members

  1. trait InferCheckable extends AnyRef

Value Members

  1. final def !=(arg0: Any): Boolean
    Definition Classes
    AnyRef → Any
  2. final def ##(): Int
    Definition Classes
    AnyRef → Any
  3. def +(other: String): String
    Implicit
    This member is added by an implicit conversion from Checkable to any2stringadd[Checkable] performed by method any2stringadd in scala.Predef.
    Definition Classes
    any2stringadd
  4. def ->[B](y: B): (Checkable, B)
    Implicit
    This member is added by an implicit conversion from Checkable to ArrowAssoc[Checkable] performed by method ArrowAssoc in scala.Predef.
    Definition Classes
    ArrowAssoc
    Annotations
    @inline()
  5. final def ==(arg0: Any): Boolean
    Definition Classes
    AnyRef → Any
  6. final def asInstanceOf[T0]: T0
    Definition Classes
    Any
  7. def clone(): AnyRef
    Attributes
    protected[java.lang]
    Definition Classes
    AnyRef
    Annotations
    @native() @throws( ... )
  8. def ensuring(cond: (Checkable) ⇒ Boolean, msg: ⇒ Any): Checkable
    Implicit
    This member is added by an implicit conversion from Checkable to Ensuring[Checkable] performed by method Ensuring in scala.Predef.
    Definition Classes
    Ensuring
  9. def ensuring(cond: (Checkable) ⇒ Boolean): Checkable
    Implicit
    This member is added by an implicit conversion from Checkable to Ensuring[Checkable] performed by method Ensuring in scala.Predef.
    Definition Classes
    Ensuring
  10. def ensuring(cond: Boolean, msg: ⇒ Any): Checkable
    Implicit
    This member is added by an implicit conversion from Checkable to Ensuring[Checkable] performed by method Ensuring in scala.Predef.
    Definition Classes
    Ensuring
  11. def ensuring(cond: Boolean): Checkable
    Implicit
    This member is added by an implicit conversion from Checkable to Ensuring[Checkable] performed by method Ensuring in scala.Predef.
    Definition Classes
    Ensuring
  12. final def eq(arg0: AnyRef): Boolean
    Definition Classes
    AnyRef
  13. def equals(arg0: Any): Boolean
    Definition Classes
    AnyRef → Any
  14. def finalize(): Unit
    Attributes
    protected[java.lang]
    Definition Classes
    AnyRef
    Annotations
    @throws( classOf[java.lang.Throwable] )
  15. def formatted(fmtstr: String): String
    Implicit
    This member is added by an implicit conversion from Checkable to StringFormat[Checkable] performed by method StringFormat in scala.Predef.
    Definition Classes
    StringFormat
    Annotations
    @inline()
  16. final def getClass(): Class[_]
    Definition Classes
    AnyRef → Any
    Annotations
    @native()
  17. def hashCode(): Int
    Definition Classes
    AnyRef → Any
    Annotations
    @native()
  18. final def isInstanceOf[T0]: Boolean
    Definition Classes
    Any
  19. final def ne(arg0: AnyRef): Boolean
    Definition Classes
    AnyRef
  20. final def notify(): Unit
    Definition Classes
    AnyRef
    Annotations
    @native()
  21. final def notifyAll(): Unit
    Definition Classes
    AnyRef
    Annotations
    @native()
  22. def propagateKnownTypes(from: Global.Type, to: Global.Symbol): Global.Type

    The applied type of class 'to' after inferring anything possible from the knowledge that 'to' must also be of the type given in 'from'.

  23. final def synchronized[T0](arg0: ⇒ T0): T0
    Definition Classes
    AnyRef
  24. def toString(): String
    Definition Classes
    AnyRef → Any
  25. final def wait(): Unit
    Definition Classes
    AnyRef
    Annotations
    @throws( ... )
  26. final def wait(arg0: Long, arg1: Int): Unit
    Definition Classes
    AnyRef
    Annotations
    @throws( ... )
  27. final def wait(arg0: Long): Unit
    Definition Classes
    AnyRef
    Annotations
    @native() @throws( ... )
  28. def [B](y: B): (Checkable, B)
    Implicit
    This member is added by an implicit conversion from Checkable to ArrowAssoc[Checkable] performed by method ArrowAssoc in scala.Predef.
    Definition Classes
    ArrowAssoc

Inherited from AnyRef

Inherited from Any

Inherited by implicit conversion any2stringadd from Checkable to any2stringadd[Checkable]

Inherited by implicit conversion StringFormat from Checkable to StringFormat[Checkable]

Inherited by implicit conversion Ensuring from Checkable to Ensuring[Checkable]

Inherited by implicit conversion ArrowAssoc from Checkable to ArrowAssoc[Checkable]

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