Disallow <T>(a => b) when parsing Flow (#13645)

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

@@ -2842,17 +2842,17 @@ export default (superClass: Class<Parser>): Class<Parser> =>
             },
           );
 
+          // <T>(() => {});
           // <T>(() => {}: any);
-          if (
+          if (arrowExpression.extra?.parenthesized) abort();
-            arrowExpression.type !== "ArrowFunctionExpression" &&
-            arrowExpression.extra?.parenthesized
-          ) {
-            abort();
-          }
 
           // The above can return a TypeCastExpression when the arrow
           // expression is not wrapped in parens. See also `this.parseParenItem`.
+          // (<T>() => {}: any);
           const expr = this.maybeUnwrapTypeCastExpression(arrowExpression);
+
+          if (expr.type !== "ArrowFunctionExpression") abort();
+
           expr.typeParameters = typeParameters;
           this.resetStartLocationFromNode(expr, typeParameters);
 

packages/babel-parser/test/fixtures/flow/type-generics/wrong-arrow-parens-no-jsx/input.js

@@ -0,0 +1 @@
+<T>(a => b);

packages/babel-parser/test/fixtures/flow/type-generics/wrong-arrow-parens-no-jsx/options.json

@@ -0,0 +1,5 @@
+{
+  "sourceType": "module",
+  "plugins": ["flow"],
+  "throws": "Expected an arrow function after this type parameter declaration. (1:0)"
+}

packages/babel-parser/test/fixtures/flow/type-generics/wrong-arrow-parens/input.js

@@ -0,0 +1 @@
+<T>(a => b);

packages/babel-parser/test/fixtures/flow/type-generics/wrong-arrow-parens/options.json

@@ -0,0 +1,3 @@
+{
+  "throws": "Unterminated JSX contents. (1:3)"
+}