Scala.js: Add support for the JS operator **.

This is a forward port of the compiler changes in the Scala.js commit
scala-js/scala-js@5bfa254

It is now possible to use `@JSOperator` on a method named `**` in
a JS type. In that case, it will translate to the JavaScript `**`
operator. This is only valid when emitting ES 2016 or later, as it
is not valid ES 2015 code.

Without an explicit `@JSOperator` annotation, such a method still
defaults to being a method call, to preserve backward
compatibility.

Author: sjrd

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

@@ -85,6 +85,8 @@ final class JSDefinitions()(using Context) {
   def JSGlobalScopeAnnot(using Context) = JSGlobalScopeAnnotType.symbol.asClass
   @threadUnsafe lazy val JSNameAnnotType: TypeRef = requiredClassRef("scala.scalajs.js.annotation.JSName")
   def JSNameAnnot(using Context) = JSNameAnnotType.symbol.asClass
+  @threadUnsafe lazy val JSOperatorAnnotType: TypeRef = requiredClassRef("scala.scalajs.js.annotation.JSOperator")
+  def JSOperatorAnnot(using Context) = JSOperatorAnnotType.symbol.asClass
   @threadUnsafe lazy val JSFullNameAnnotType: TypeRef = requiredClassRef("scala.scalajs.js.annotation.JSFullName")
   def JSFullNameAnnot(using Context) = JSFullNameAnnotType.symbol.asClass
   @threadUnsafe lazy val JSBracketAccessAnnotType: TypeRef = requiredClassRef("scala.scalajs.js.annotation.JSBracketAccess")

compiler/src/dotty/tools/dotc/transform/sjs/JSSymUtils.scala

@@ -73,10 +73,16 @@ object JSSymUtils {
           lazy val pc = sym.info.paramNamess.map(_.size).sum
 
           sym.name match {
-            case nme.apply                             => Call
-            case JSUnaryOpMethodName(code) if pc == 0  => UnaryOp(code)
-            case JSBinaryOpMethodName(code) if pc == 1 => BinaryOp(code)
-            case _                                     => default
+            case nme.apply =>
+              Call
+            case JSUnaryOpMethodName(code, defaultsToOp)
+                if (defaultsToOp || sym.hasAnnotation(jsdefn.JSOperatorAnnot)) && pc == 0 =>
+              UnaryOp(code)
+            case JSBinaryOpMethodName(code, defaultsToOp)
+                if (defaultsToOp || sym.hasAnnotation(jsdefn.JSOperatorAnnot)) && pc == 1 =>
+              BinaryOp(code)
+            case _ =>
+              default
           }
         } else {
           default
@@ -232,41 +238,43 @@ object JSSymUtils {
 
   private object JSUnaryOpMethodName {
     private val map = Map(
-      nme.UNARY_+ -> js.JSUnaryOp.+,
-      nme.UNARY_- -> js.JSUnaryOp.-,
-      nme.UNARY_~ -> js.JSUnaryOp.~,
-      nme.UNARY_! -> js.JSUnaryOp.!
+      nme.UNARY_+ -> (js.JSUnaryOp.+, true),
+      nme.UNARY_- -> (js.JSUnaryOp.-, true),
+      nme.UNARY_~ -> (js.JSUnaryOp.~, true),
+      nme.UNARY_! -> (js.JSUnaryOp.!, true),
     )
 
-    def unapply(name: TermName): Option[js.JSUnaryOp.Code] =
+    def unapply(name: TermName): Option[(js.JSUnaryOp.Code, Boolean)] =
       map.get(name)
   }
 
   private object JSBinaryOpMethodName {
     private val map = Map(
-      nme.ADD -> js.JSBinaryOp.+,
-      nme.SUB -> js.JSBinaryOp.-,
-      nme.MUL -> js.JSBinaryOp.*,
-      nme.DIV -> js.JSBinaryOp./,
-      nme.MOD -> js.JSBinaryOp.%,
-
-      nme.LSL -> js.JSBinaryOp.<<,
-      nme.ASR -> js.JSBinaryOp.>>,
-      nme.LSR -> js.JSBinaryOp.>>>,
-      nme.OR  -> js.JSBinaryOp.|,
-      nme.AND -> js.JSBinaryOp.&,
-      nme.XOR -> js.JSBinaryOp.^,
-
-      nme.LT -> js.JSBinaryOp.<,
-      nme.LE -> js.JSBinaryOp.<=,
-      nme.GT -> js.JSBinaryOp.>,
-      nme.GE -> js.JSBinaryOp.>=,
-
-      nme.ZAND -> js.JSBinaryOp.&&,
-      nme.ZOR  -> js.JSBinaryOp.||
+      nme.ADD -> (js.JSBinaryOp.+, true),
+      nme.SUB -> (js.JSBinaryOp.-, true),
+      nme.MUL -> (js.JSBinaryOp.*, true),
+      nme.DIV -> (js.JSBinaryOp./, true),
+      nme.MOD -> (js.JSBinaryOp.%, true),
+
+      nme.LSL -> (js.JSBinaryOp.<<, true),
+      nme.ASR -> (js.JSBinaryOp.>>, true),
+      nme.LSR -> (js.JSBinaryOp.>>>, true),
+      nme.OR  -> (js.JSBinaryOp.|, true),
+      nme.AND -> (js.JSBinaryOp.&, true),
+      nme.XOR -> (js.JSBinaryOp.^, true),
+
+      nme.LT -> (js.JSBinaryOp.<, true),
+      nme.LE -> (js.JSBinaryOp.<=, true),
+      nme.GT -> (js.JSBinaryOp.>, true),
+      nme.GE -> (js.JSBinaryOp.>=, true),
+
+      nme.ZAND -> (js.JSBinaryOp.&&, true),
+      nme.ZOR  -> (js.JSBinaryOp.||, true),
+
+      termName("**") -> (js.JSBinaryOp.**, false),
     )
 
-    def unapply(name: TermName): Option[js.JSBinaryOp.Code] =
+    def unapply(name: TermName): Option[(js.JSBinaryOp.Code, Boolean)] =
       map.get(name)
   }
 }