Fix setters; add more tests

Author: CamWass

src/com/google/javascript/jscomp/OptimizeArgumentsArray.java

@@ -150,11 +150,8 @@ public void visit(NodeTraversal traversal, Node node, Node parent) {
    */
   private void tryReplaceArguments(Node scopeRoot) {
     Node scopeRootParent = scopeRoot.getParent();
-    // Nothing to do for getters and setters:
-    // - Getters cannot have any params.
-    // - Setters can only have one param, and it is required to be present, so we cannot synthesize
-    //   any new params.
-    if (scopeRootParent.isGetterDef() || scopeRootParent.isSetterDef()) {
+    // Getters cannot have any params, so there are none to replace or synthesize.
+    if (scopeRootParent.isGetterDef()) {
       return;
     }
 
@@ -170,8 +167,14 @@ private void tryReplaceArguments(Node scopeRoot) {
       return;
     }
 
-    ImmutableSortedMap<Integer, String> argNames =
-        assembleParamNames(parametersList, highestIndex + 1);
+    int maxCount = highestIndex + 1;
+    // Setters can only have one param and it is required to be present, so we cannot synthesize any
+    // new params, but we can still replace references to the first param.
+    if (scopeRootParent.isSetterDef()) {
+      maxCount = 1;
+    }
+
+    ImmutableSortedMap<Integer, String> argNames = assembleParamNames(parametersList, maxCount);
     changeMethodSignature(argNames, parametersList);
     changeBody(argNames);
   }

test/com/google/javascript/jscomp/OptimizeArgumentsArrayTest.java

@@ -355,15 +355,30 @@ public void testGlobalArgumentsReferences() {
 
   @Test
   public void testGettersCannotHaveAnyParams() {
-    // Getters cannot have any parameters; synthesizing one would be an error.
+    // Getters cannot have any parameters; synthesizing them would be an error.
     testSame("class Foo { get prop() { arguments[0] } }");
     testSame("const a = { get prop() { arguments[0] } }");
+
+    // Ensure references in nested functions are still eligible.
+    test(
+        "class Foo { get prop() { function f(  ) { arguments[0] } } }", //
+        "class Foo { get prop() { function f(p0) {           p0 } } }");
   }
 
   @Test
   public void testSettersCanOnlyHaveOneParam() {
     // Setters can only have one parameter; synthesizing any more would be an error.
     testSame("class Foo { set prop(x) { arguments[1] } }");
     testSame("const a = { set prop(x) { arguments[1] } }");
+
+    // We can still replace references to the first param.
+    test(
+        "class Foo { set prop(x) { arguments[0]; arguments[1] } }", //
+        "class Foo { set prop(x) {            x; arguments[1] } }");
+
+    // Ensure references in nested functions are still eligible.
+    test(
+        "class Foo { set prop(x) { function f(  ) { arguments[0] } } }", //
+        "class Foo { set prop(x) { function f(p0) {           p0 } } }");
   }
 }