Check if param is assignable when parsing arrow return type (#11992)

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

@@ -1444,7 +1444,7 @@ export default class ExpressionParser extends LValParser {
     if (
       canBeArrow &&
       this.shouldParseArrow() &&
-      (arrowNode = this.parseArrow(arrowNode))
+      (arrowNode = this.parseArrow(arrowNode, exprList))
     ) {
       if (!this.isAwaitAllowed() && !this.state.maybeInAsyncArrowHead) {
         this.state.awaitPos = oldAwaitPos;
@@ -1500,7 +1500,10 @@ export default class ExpressionParser extends LValParser {
     return !this.canInsertSemicolon();
   }
 
-  parseArrow(node: N.ArrowFunctionExpression): ?N.ArrowFunctionExpression {
+  parseArrow(
+    node: N.ArrowFunctionExpression,
+    exprList: N.Node[], // eslint-disable-line no-unused-vars
+  ): ?N.ArrowFunctionExpression {
     if (this.eat(tt.arrow)) {
       return node;
     }