Revert "Fix: check if param is assignable when parsing arrow return type annotation" (#12198)

This reverts commit 84987a00e6.

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

@@ -1453,7 +1453,7 @@ export default class ExpressionParser extends LValParser {
     if (
       canBeArrow &&
       this.shouldParseArrow() &&
-      (arrowNode = this.parseArrow(arrowNode, exprList))
+      (arrowNode = this.parseArrow(arrowNode))
     ) {
       if (!this.isAwaitAllowed() && !this.state.maybeInAsyncArrowHead) {
         this.state.awaitPos = oldAwaitPos;
@@ -1509,10 +1509,7 @@ export default class ExpressionParser extends LValParser {
     return !this.canInsertSemicolon();
   }
 
-  parseArrow(
-    node: N.ArrowFunctionExpression,
-    exprList: N.Node[], // eslint-disable-line no-unused-vars
-  ): ?N.ArrowFunctionExpression {
+  parseArrow(node: N.ArrowFunctionExpression): ?N.ArrowFunctionExpression {
     if (this.eat(tt.arrow)) {
       return node;
     }

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

@@ -51,60 +51,10 @@ export default class LValParser extends NodeUtils {
   +parseDecorator: () => Decorator;
   */
 
-  /**
-   * Check if a node can be converted to a binding identifier or binding pattern.
-   * https://tc39.es/ecma262/#prod-BindingIdentifier
-   * https://tc39.es/ecma262/#prod-BindingPattern
-   * Note that although a mebmer expression can serve as a LHS in the init of for loop,
-   * i.e. `for (a.b of []);`, it is not a binding pattern
-   *
-   * @param {Node} node
-   * @returns {boolean}
-   * @memberof LValParser
-   */
-  isAssignable(node: Node): boolean {
-    switch (node.type) {
-      case "Identifier":
-      case "ObjectPattern":
-      case "ArrayPattern":
-      case "AssignmentPattern":
-      case "RestElement":
-        return true;
-
-      case "ObjectExpression": {
-        const last = node.properties.length - 1;
-        return node.properties.every((prop, i) => {
-          return (
-            prop.type !== "ObjectMethod" &&
-            (i === last || prop.type === "SpreadElement") &&
-            this.isAssignable(prop)
-          );
-        });
-      }
-
-      case "ObjectProperty":
-        return this.isAssignable(node.value);
-
-      case "SpreadElement":
-        return this.isAssignable(node.argument);
-
-      case "ArrayExpression":
-        return node.elements.every(element => this.isAssignable(element));
-
-      case "AssignmentExpression":
-        return node.operator === "=";
-
-      case "ParenthesizedExpression":
-        return this.isAssignable(node.expression);
-
-      default:
-        return false;
-    }
-  }
-
   // Convert existing expression atom to assignable pattern
   // if possible.
-  // When this one is updated, please check if `isAssignable` also needs to be updated.
+  // NOTE: There is a corresponding "isAssignable" method in flow.js.
+  // When this one is updated, please check if also that one needs to be updated.
 
   toAssignable(node: Node): Node {
     let parenthesized = undefined;

packages/babel-parser/src/plugins/flow.js

@@ -1943,7 +1943,7 @@ export default (superClass: Class<Parser>): Class<Parser> =>
       }
 
       return partition(arrows, node =>
-        node.params.every(param => this.isAssignable(param)),
+        node.params.every(param => this.isAssignable(param, true)),
       );
     }
 
@@ -2146,12 +2146,47 @@ export default (superClass: Class<Parser>): Class<Parser> =>
       }
     }
 
-    isAssignable(node: N.Node): boolean {
+    isAssignable(node: N.Node, isBinding?: boolean): boolean {
       switch (node.type) {
+        case "Identifier":
+        case "ObjectPattern":
+        case "ArrayPattern":
+        case "AssignmentPattern":
+          return true;
+
+        case "ObjectExpression": {
+          const last = node.properties.length - 1;
+          return node.properties.every((prop, i) => {
+            return (
+              prop.type !== "ObjectMethod" &&
+              (i === last || prop.type === "SpreadElement") &&
+              this.isAssignable(prop)
+            );
+          });
+        }
+
+        case "ObjectProperty":
+          return this.isAssignable(node.value);
+
+        case "SpreadElement":
+          return this.isAssignable(node.argument);
+
+        case "ArrayExpression":
+          return node.elements.every(element => this.isAssignable(element));
+
+        case "AssignmentExpression":
+          return node.operator === "=";
+
+        case "ParenthesizedExpression":
         case "TypeCastExpression":
           return this.isAssignable(node.expression);
+
+        case "MemberExpression":
+        case "OptionalMemberExpression":
+          return !isBinding;
+
         default:
-          return super.isAssignable(node);
+          return false;
       }
     }
 
@@ -2737,10 +2772,7 @@ export default (superClass: Class<Parser>): Class<Parser> =>
     }
 
     // handle return types for arrow functions
-    parseArrow(
-      node: N.ArrowFunctionExpression,
-      exprList: N.Node[],
-    ): ?N.ArrowFunctionExpression {
+    parseArrow(node: N.ArrowFunctionExpression): ?N.ArrowFunctionExpression {
       if (this.match(tt.colon)) {
         const result = this.tryParse(() => {
           const oldNoAnonFunctionType = this.state.noAnonFunctionType;
@@ -2774,7 +2806,7 @@ export default (superClass: Class<Parser>): Class<Parser> =>
           : null;
       }
 
-      return super.parseArrow(node, exprList);
+      return super.parseArrow(node);
     }
 
     shouldParseArrow(): boolean {
@@ -2945,8 +2977,7 @@ export default (superClass: Class<Parser>): Class<Parser> =>
     ): ?N.ArrowFunctionExpression {
       const node = this.startNodeAt(startPos, startLoc);
       this.parseFunctionParams(node);
-      // set exprList to `[]` as the parameters has been validated in `parseFunctionParams`
-      if (!this.parseArrow(node, [])) return;
+      if (!this.parseArrow(node)) return;
       return this.parseArrowExpression(
         node,
         /* params */ undefined,

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

@@ -2540,10 +2540,7 @@ export default (superClass: Class<Parser>): Class<Parser> =>
       }
     }
 
-    parseArrow(
-      node: N.ArrowFunctionExpression,
-      exprList,
-    ): ?N.ArrowFunctionExpression {
+    parseArrow(node: N.ArrowFunctionExpression): ?N.ArrowFunctionExpression {
       if (this.match(tt.colon)) {
         // This is different from how the TS parser does it.
         // TS uses lookahead. The Babel Parser parses it as a parenthesized expression and converts.
@@ -2553,10 +2550,6 @@ export default (superClass: Class<Parser>): Class<Parser> =>
             tt.colon,
           );
           if (this.canInsertSemicolon() || !this.match(tt.arrow)) abort();
-          // check if the exprList is assignable because `: TSType` can be part of conditional expression
-          // i.e. we can only know `: v` is not a return type by checking that `sum(v)` can not be a pattern.
-          // 0 ? v => (sum(v)) : v => 0
-          if (exprList.some(param => !this.isAssignable(param))) abort();
           return returnType;
         });
 
@@ -2568,7 +2561,7 @@ export default (superClass: Class<Parser>): Class<Parser> =>
         }
       }
 
-      return super.parseArrow(node, exprList);
+      return super.parseArrow(node);
     }
 
     // Allow type annotations inside of a parameter list.
@@ -2587,18 +2580,6 @@ export default (superClass: Class<Parser>): Class<Parser> =>
       return param;
     }
 
-    isAssignable(node: N.Node): boolean {
-      switch (node.type) {
-        case "TSAsExpression":
-        case "TSNonNullExpression":
-        case "TSTypeAssertion":
-        case "TSTypeCastExpression":
-          return this.isAssignable(node.expression);
-        default:
-          return super.isAssignable(node);
-      }
-    }
-
     toAssignable(node: N.Node): N.Node {
       switch (node.type) {
         case "TSTypeCastExpression":