add JavaScript support for new private fields syntax

Author: erik-krogh

javascript/extractor/src/com/semmle/jcorn/Parser.java

@@ -1646,6 +1646,15 @@ protected Expression parseExprAtom(DestructuringErrors refDestructuringErrors) {
       node = new ThisExpression(new SourceLocation(this.startLoc));
       this.next();
       return this.finishNode(node);
+    } else if (this.type == TokenType.pound) {
+      Position startLoc = this.startLoc;
+      // there is only one case where this is valid, and that is "Ergonomic brand checks for Private Fields", i.e. `#name in obj`. 
+      Identifier id = parseIdent(true);
+      String op = String.valueOf(this.value);
+      if (!op.equals("in")) {
+        this.unexpected(startLoc);
+      }
+      return this.parseExprOp(id, this.start, startLoc, -1, false); 
     } else if (this.type == TokenType.name) {
       Position startLoc = this.startLoc;
       Identifier id = this.parseIdent(this.type != TokenType.name);
@@ -3314,9 +3323,6 @@ protected MemberDefinition<?> parseClassPropertyBody(
       if (pi.kind.equals("set") && node.getValue().hasRest())
         this.raiseRecoverable(params.get(params.size() - 1), "Setter cannot use rest params");
     }
-    if (pi.key instanceof Identifier && ((Identifier)pi.key).getName().startsWith("#")) {
-      raiseRecoverable(pi.key, "Only fields, not methods, can be declared private.");
-    }
     return node;
   }
 

javascript/extractor/src/com/semmle/js/extractor/ASTExtractor.java

@@ -847,8 +847,15 @@ public Label visit(AssignmentExpression nd, Context c) {
     @Override
     public Label visit(BinaryExpression nd, Context c) {
       Label key = super.visit(nd, c);
-      visit(nd.getLeft(), key, 0, true);
+      if (nd.getOperator().equals("in") && nd.getLeft() instanceof Identifier && ((Identifier)nd.getLeft()).getName().startsWith("#")) {
+        // this happens with Ergonomic brand checks for Private Fields (see https://github.com/tc39/proposal-private-fields-in-in). 
+        // it's the only case where private field identifiers are used not as a field.
+        visit(nd.getLeft(), key, 0, IdContext.LABEL, true);
+      } else {
+        visit(nd.getLeft(), key, 0, true);
+      }
       visit(nd.getRight(), key, 1, true);
+      
       extractRegxpFromBinop(nd, c);
       return key;
     }