Support Scala.js unions extra methods via implicit conversion

The companion of `js.|` contains the following widely used implicit
conversion to a value class:

    /** Provides an [[Option]]-like API to [[js.UndefOr]]. */
    implicit def undefOr2ops[A](value: js.UndefOr[A]): js.UndefOrOps[A] =
      new js.UndefOrOps(value)

(where `UndefOr[A]` dealiases to `A | Unit`).

Since we re-interpret Scala.js unions as real unions, this companion is
not in the implicit scope of `A | Unit`, we work around this by
injecting a new `UnitOps` with the implicit conversion we want in the
implicit scope of `Unit` (we could have directly injected the object
`js.|`, but that contains other implicits we do not need).

This finally lets us compile and run the sjsJUnitTests after the
previous two commits.

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

@@ -59,6 +59,8 @@ 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

@@ -2,6 +2,7 @@ package dotty.tools
 package dotc
 package typer
 
+import backend.sjs.JSDefinitions
 import core._
 import ast.{Trees, TreeTypeMap, untpd, tpd, DesugarEnums}
 import util.Spans._
@@ -634,6 +635,14 @@ trait ImplicitRunInfo:
               else pre.member(sym.name.toTermName)
                 .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/scalajs/js/internal/UnitOps.scala

@@ -0,0 +1,8 @@
+package scala.scalajs.js.internal
+
+import scala.scalajs.js
+
+/** 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)

project/Build.scala

@@ -698,7 +698,7 @@ object Build {
     settings(
       libraryDependencies +=
         ("org.scala-js" %% "scalajs-library" % scalaJSVersion).withDottyCompat(scalaVersion.value),
-      Compile / unmanagedSourceDirectories :=
+      Compile / unmanagedSourceDirectories ++=
         (`scala3-library-bootstrapped` / Compile / unmanagedSourceDirectories).value,
 
       // Configure the source maps to point to GitHub for releases