[traverse] Allow skipping nodes inserted with .replaceWith() (#9777)

* Avoid regression

packages/babel-traverse/src/path/context.js

@@ -67,7 +67,12 @@ export function visit(): boolean {
     return false;
   }
 
-  if (this.call("enter") || this.shouldSkip) {
+  // Note: We need to check "this.shouldSkip" twice because
+  // the visitor can set it to true. Usually .shouldSkip is false
+  // before calling the enter visitor, but it can be true in case of
+  // a requeued node (e.g. by .replaceWith()) that is then marked
+  // with .skip().
+  if (this.shouldSkip || this.call("enter") || this.shouldSkip) {
     this.debug("Skip...");
     return this.shouldStop;
   }
@@ -233,6 +238,14 @@ export function setKey(key) {
 export function requeue(pathToQueue = this) {
   if (pathToQueue.removed) return;
 
+  // TODO: Uncomment in Babel 8. If a path is skipped, and then replaced with a
+  // new one, the new one shouldn't probably be skipped.
+  // Note that this currently causes an infinite loop because of
+  // packages/babel-plugin-transform-block-scoping/src/tdz.js#L52-L59
+  // (b5b8055cc00756f94bf71deb45f288738520ee3c)
+  //
+  // pathToQueue.shouldSkip = false;
+
   // TODO(loganfsmyth): This should be switched back to queue in parent contexts
   // automatically once #2892 and #4135 have been resolved. See #4140.
   // let contexts = this._getQueueContexts();

packages/babel-traverse/test/traverse.js

@@ -1,6 +1,7 @@
 import cloneDeep from "lodash/cloneDeep";
 import traverse from "../lib";
 import { parse } from "@babel/parser";
+import * as t from "@babel/types";
 
 describe("traverse", function() {
   const code = `
@@ -174,4 +175,45 @@ describe("traverse", function() {
       expect(p).not.toBe(scopes[i]);
     });
   });
+
+  describe("path.skip()", function() {
+    it("replaced paths can be skipped", function() {
+      const ast = parse("id");
+
+      let skipped;
+      traverse(ast, {
+        noScope: true,
+        Identifier(path) {
+          path.replaceWith(t.numericLiteral(0));
+          path.skip();
+          skipped = true;
+        },
+        NumericLiteral() {
+          skipped = false;
+        },
+      });
+
+      expect(skipped).toBe(true);
+    });
+
+    // Skipped: see the comment in the `NodePath.requque` method.
+    it.skip("skipped and requeued paths should be visited", function() {
+      const ast = parse("id");
+
+      let visited = false;
+      traverse(ast, {
+        noScope: true,
+        Identifier(path) {
+          path.replaceWith(t.numericLiteral(0));
+          path.skip();
+          path.requeue();
+        },
+        NumericLiteral() {
+          visited = true;
+        },
+      });
+
+      expect(visited).toBe(true);
+    });
+  });
 });