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add Predef.Uninitialized for vars that are null only initially #15123

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add Predef.Uninitialized for vars that are null only initially #15123

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16 changes: 16 additions & 0 deletions library/src/scala/runtime/stdLibPatches/Predef.scala
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Original file line number Diff line number Diff line change
@@ -1,5 +1,7 @@
package scala.runtime.stdLibPatches

import scala.annotation.experimental

object Predef:
import compiletime.summonFrom

Expand Down Expand Up @@ -47,4 +49,18 @@ object Predef:
*/
extension [T](x: T | Null) inline def nn: x.type & T =
scala.runtime.Scala3RunTime.nn(x)

/** Marker for `var`s that are initialized to `null`, but cannot be assigned `null` after initialization.
* @example {{{
* var cache: String | Uninitialized = initiallyNull
* def readCache: String =
* if(cache == null) cache = "hello"
* cache
* }}}
*/
@experimental
type Uninitialized <: Null
/** Initializer for `var`s of type `Uninitialized` */
@experimental
val initiallyNull = null.asInstanceOf[Uninitialized]
end Predef