Revert "do not patch the implicit scope of scala.Unit in Scala.js" (#24475)

I'll unfortunately revert this change until we somehow have an actual
platform dependent TASTy in the stdlib.

Reverts #24405
Closes #24467

Author: hamzaremmal

compiler/src/dotty/tools/backend/sjs/JSDefinitions.scala

@@ -69,6 +69,8 @@ final class JSDefinitions()(using Context) {
     @threadUnsafe lazy val PseudoUnion_fromTypeConstructorR = PseudoUnionModule.requiredMethodRef("fromTypeConstructor")
     def PseudoUnion_fromTypeConstructor(using Context) = PseudoUnion_fromTypeConstructorR.symbol
 
+  @threadUnsafe lazy val UnionOpsModuleRef = requiredModuleRef("scala.scalajs.js.internal.UnitOps")
+
   @threadUnsafe lazy val JSArrayType: TypeRef = requiredClassRef("scala.scalajs.js.Array")
   def JSArrayClass(using Context) = JSArrayType.symbol.asClass
   @threadUnsafe lazy val JSDynamicType: TypeRef = requiredClassRef("scala.scalajs.js.Dynamic")

compiler/src/dotty/tools/dotc/typer/Implicits.scala

@@ -738,6 +738,13 @@ trait ImplicitRunInfo:
                 .suchThat(companion => companion.is(Module) && companion.owner == sym.owner)
                 .symbol)
 
+            // The companion of `js.|` defines an implicit conversions from
+            // `A | Unit` to `js.UndefOrOps[A]`. To keep this conversion in scope
+            // in Scala 3, where we re-interpret `js.|` as a real union, we inject
+            // it in the scope of `Unit`.
+            if t.isRef(defn.UnitClass) && ctx.settings.scalajs.value then
+              companions += JSDefinitions.jsdefn.UnionOpsModuleRef
+
             if sym.isClass then
               for p <- t.parents do companions ++= iscopeRefs(p)
             else

library-js/src/scala/Unit.scala

@@ -2,7 +2,7 @@ package scala.scalajs.js.internal
 
 import scala.scalajs.js
 
-@deprecated(message = "The implicit conversion was moved to the companion object of `scala.Unit` in Scala.js", since = "3.8.0")
+/** Under -scalajs, this object is part of the implicit scope of `scala.Unit` */
 object UnitOps:
   implicit def unitOrOps[A](x: A | Unit): js.UndefOrOps[A] =
     new js.UndefOrOps(x)