fix: async arrow functions should not be allowed after binary… (#11284)

* Forbid async arrow functions after binary operator.

This commit makes Babel throw an error when parsing
code like "3 + async() => 2".

* Make atPossibleAsync more accurate

* Change atPossibleAsync to atPossibleAsyncArrow

Add an extra test to atPossibleAsync and refactor it to
atPossibleAsyncArrow. This also fixes a bug in the Typescript plugin,
so a new test has been added.

* Add test for async arrow after unary operator

packages/babel-parser/src/parser/expression.js

@@ -556,7 +556,7 @@ export default class ExpressionParser extends LValParser {
   ): N.Expression {
     const state = {
       optionalChainMember: false,
-      maybeAsyncArrow: this.atPossibleAsync(base),
+      maybeAsyncArrow: this.atPossibleAsyncArrow(base),
       stop: false,
     };
     do {
@@ -748,13 +748,15 @@ export default class ExpressionParser extends LValParser {
     return this.finishNode(node, "TaggedTemplateExpression");
   }
 
-  atPossibleAsync(base: N.Expression): boolean {
+  atPossibleAsyncArrow(base: N.Expression): boolean {
     return (
       base.type === "Identifier" &&
       base.name === "async" &&
       this.state.lastTokEnd === base.end &&
       !this.canInsertSemicolon() &&
-      this.input.slice(base.start, base.end) === "async"
+      // check there are no escape sequences, such as \u{61}sync
+      base.end - base.start === 5 &&
+      base.start === this.state.potentialArrowAt
     );
   }
 

packages/babel-parser/src/plugins/typescript/index.js

@@ -1738,7 +1738,7 @@ export default (superClass: Class<Parser>): Class<Parser> =>
         // There are number of things we are going to "maybe" parse, like type arguments on
         // tagged template expressions. If any of them fail, walk it back and continue.
         const result = this.tsTryParseAndCatch(() => {
-          if (!noCalls && this.atPossibleAsync(base)) {
+          if (!noCalls && this.atPossibleAsyncArrow(base)) {
             // Almost certainly this is a generic async function `async <T>() => ...
             // But it might be a call with a type argument `async<T>();`
             const asyncArrowFn = this.tsTryParseGenericAsyncArrowFunction(

packages/babel-parser/test/fixtures/es2016/simple-parameter-list/async-arrow-function-after-binary-operator/input.js

@@ -0,0 +1 @@
+3 + async() => 2

packages/babel-parser/test/fixtures/es2016/simple-parameter-list/async-arrow-function-after-binary-operator/options.json

@@ -0,0 +1,3 @@
+{
+  "throws": "Unexpected token, expected \";\" (1:12)"
+}

packages/babel-parser/test/fixtures/es2016/simple-parameter-list/async-arrow-function-after-unary-operator/input.js

@@ -0,0 +1 @@
+delete async () => 3;

packages/babel-parser/test/fixtures/es2016/simple-parameter-list/async-arrow-function-after-unary-operator/options.json

@@ -0,0 +1,3 @@
+{
+  "throws": "Unexpected token, expected \";\" (1:16)"
+}

packages/babel-parser/test/fixtures/typescript/arrow-function/async-arrow-function-after-binary-operator/input.ts

@@ -0,0 +1 @@
+4 + async<number>() => 2 

packages/babel-parser/test/fixtures/typescript/arrow-function/async-arrow-function-after-binary-operator/options.json

@@ -0,0 +1,5 @@
+{
+  "sourceType": "module",
+  "plugins": ["typescript"],
+  "throws": "Unexpected token, expected \";\" (1:20)"
+}