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Make opaque types decomposable #24068

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Make opaque types decomposable #24068

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e00caba
Make opaque types decomposable
nox213 Sep 25, 2025
4224938
address review
nox213 Oct 3, 2025
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2 changes: 0 additions & 2 deletions compiler/src/dotty/tools/dotc/transform/patmat/Space.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -717,14 +717,12 @@ object SpaceEngine {

extension (tp: Type)
def isDecomposableToChildren(using Context): Boolean =
val sym = tp.typeSymbol // e.g. Foo[List[Int]] = type Foo (i19275)
val cls = tp.classSymbol // e.g. Foo[List[Int]] = class List
tp.hasSimpleKind // can't decompose higher-kinded types
&& cls.is(Sealed)
&& cls.isOneOf(AbstractOrTrait) // ignore sealed non-abstract classes
&& !cls.hasAnonymousChild // can't name anonymous classes as counter-examples
&& cls.children.nonEmpty // can't decompose without children
&& !sym.isOpaqueAlias // can't instantiate subclasses to conform to an opaque type (i19275)

val ListOfNoType = List(NoType)
val ListOfTypNoType = ListOfNoType.map(Typ(_, decomposed = true))
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27 changes: 27 additions & 0 deletions tests/pos/i22513.scala
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Original file line number Diff line number Diff line change
@@ -0,0 +1,27 @@
opaque type R[T] <: T = T

object Test {
enum E:
case A(a: Int)

val v: R[E] = ???
v match
case E.A(_) =>
}

sealed trait Foo

case class FooA() extends Foo
case class FooB() extends Foo

object O {
opaque type OpaqueFoo <: Foo = Foo
def fooB(): OpaqueFoo = FooB()
}

@main def main =
val p: O.OpaqueFoo = O.fooB()

p match
case _: FooA => println("fooA")
case _: FooB => println("fooB")
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