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feat: introduce a static cast to scala.compiletime #24482

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feat: introduce a static cast to scala.compiletime #24482

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12 changes: 12 additions & 0 deletions library/src/scala/compiletime/package.scala
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Expand Up @@ -193,3 +193,15 @@ def byName[T](x: => T): T = x
*/
extension [T](x: T)
transparent inline def asMatchable: x.type & Matchable = x.asInstanceOf[x.type & Matchable]

extension (inline self: Any)
/**
* TODO: Write the scaladoc here and how it works
* ???
* @param self ???
* @param T ???
* @return ???
*/
transparent inline def as[T]: T = inline self match
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@hamzaremmal hamzaremmal Nov 19, 2025 •
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The name is not set in stone. I like as since it is short, to the point and it is not available unless we import it from the scala.compiletime. A suggestion during the LAMP meeting was to call it inlineAs.

case self: T => self
case _ => error("Cannot statically prove the correctness of the cast")
4 changes: 4 additions & 0 deletions tests/neg/static-cast.check
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@@ -0,0 +1,4 @@
-- Error: tests/neg/static-cast.scala:11:18 ----------------------------------------------------------------------------
11 |val _ = singletons[B, (A.A1.type, A.A2.type, A.A3.type)] // error
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
| Cannot statically prove the correctness of the cast
11 changes: 11 additions & 0 deletions tests/neg/static-cast.scala
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@@ -0,0 +1,11 @@
import scala.compiletime.*

trait B
enum A { case A1, A2, A3 }

private inline def singletons[T, Elem <: Tuple]: Seq[T] =
inline erasedValue[Elem] match
case _: EmptyTuple => Seq.empty
case _: (h *: t) => valueOf[h](using summonInline).as[T] +: singletons[T, t]

val _ = singletons[B, (A.A1.type, A.A2.type, A.A3.type)] // error
10 changes: 10 additions & 0 deletions tests/pos/static-cast.scala
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@@ -0,0 +1,10 @@
import scala.compiletime.*

enum A { case A1, A2, A3 }

private inline def singletons[T, Elem <: Tuple]: Seq[T] =
inline erasedValue[Elem] match
case _: EmptyTuple => Seq.empty
case _: (h *: t) => valueOf[h](using summonInline).as[T] +: singletons[T, t]
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@hamzaremmal hamzaremmal Nov 19, 2025

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After #23773, we should be able to write valueOf[h].as[T]


val x = singletons[A, (A.A1.type, A.A2.type, A.A3.type)]
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