scala3-compiler-bootstrapped/dotty.tools.dotc.transform/TypeTestsCasts

TypeTestsCasts

object TypeTestsCasts

This transform normalizes type tests and type casts, also replacing type tests with singleton argument type with reference equality check Any remaining type tests

  • use the object methods $isInstanceOf and $asInstanceOf
  • have a reference type as receiver
  • can be translated directly to machine instructions

Unfortunately this phase ended up being not Y-checkable unless types are erased. A cast to an ConstantType(3) or x.type cannot be rewritten before erasure. That's why TypeTestsCasts is called from Erasure.

class Object
trait Matchable
class Any

Value members

Concrete methods

def checkable(X: Type, P: Type, span: Span)(using Context): Boolean

Whether (x:X).isInstanceOf[P] can be checked at runtime?

Whether (x:X).isInstanceOf[P] can be checked at runtime?

First do the following substitution: (a) replace T @unchecked and pattern binder types (e.g., _$1) in P with WildcardType

Then check:

  1. if X <:< P, TRUE
  2. if P is a singleton type, TRUE
  3. if P refers to an abstract type member or type parameter, FALSE
  4. if P = Array[T], checkable(E, T) where E is the element type of X, defaults to Any.
  5. if P is pre.F[Ts] and pre.F refers to a class which is not Array: (a) replace Ts with fresh type variables Xs (b) constrain Xs with pre.F[Xs] <:< X (c) maximize pre.F[Xs] and check pre.F[Xs] <:< P
  6. if P = T1 | T2 or P = T1 & T2, checkable(X, T1) && checkable(X, T2).
  7. if P is a refinement type, FALSE
  8. otherwise, TRUE