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Dealias type of type constructor in typedAppliedTypeTree #24522

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Merged
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Nov 25, 2025
+22 −1
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Dealias type of type constructor in typedAppliedTypeTree #24522

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2 changes: 1 addition & 1 deletion compiler/src/dotty/tools/dotc/typer/Typer.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -2680,7 +2680,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
val tpt1 = withoutMode(Mode.Pattern):
typed(tree.tpt, AnyTypeConstructorProto)

val tparams = tpt1.tpe.typeParams
val tparams = tpt1.tpe.dealiasKeepAnnotsAndOpaques.typeParams
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@hamzaremmal hamzaremmal Nov 25, 2025

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I've been playing with some examples. In all of them, typeParams was able to return the actual typeParams but the top level case (the issue this tries to solve) without the dealiasing. Which means that in the contract of typeParams, it should be able to return the actual params without dealiasing at all. I think it's still worth investigating and fixing such that we don't need the dealiasing here.

if tpt1.tpe.isError then
val args1 = tree.args.mapconserve(typedType(_))
assignType(cpy.AppliedTypeTree(tree)(tpt1, args1), tpt1, args1)
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14 changes: 14 additions & 0 deletions tests/pos/i23752.scala
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@@ -0,0 +1,14 @@
trait MyOutput[T]
final type MyOutputAlias1[T] = MyOutput[T]
final type MyOutputAlias2 = MyOutput

trait MyType
object MyType {
val value1: MyOutput[MyType] = ???
val value2: MyOutputAlias1[MyType] = ???
val value3: MyOutputAlias2[MyType] = ??? // was COMPILATION ERROR
}

val value1: MyOutput[MyType] = ???
val value2: MyOutputAlias1[MyType] = ???
val value3: MyOutputAlias2[MyType] = ???
7 changes: 7 additions & 0 deletions tests/pos/i23752min.scala
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@@ -0,0 +1,7 @@
trait MyOutput[T]
type MyOutputAlias2 = MyOutput

object MyType:
val v: MyOutputAlias2[String] = ??? // was COMPILATION ERROR

@main def Test = ()
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