Print trailing comma after a single TS generic in arrow fns (#14113)

packages/babel-generator/src/generators/typescript.ts

@@ -12,9 +12,13 @@ export function TSTypeAnnotation(this: Printer, node: t.TSTypeAnnotation) {
 export function TSTypeParameterInstantiation(
   this: Printer,
   node: t.TSTypeParameterInstantiation,
+  parent: t.Node,
 ): void {
   this.token("<");
   this.printList(node.params, node, {});
+  if (parent.type === "ArrowFunctionExpression" && node.params.length === 1) {
+    this.token(",");
+  }
   this.token(">");
 }
 

packages/babel-generator/test/fixtures/typescript/arrow-function-async-generic/output.js

@@ -1 +1 @@
-async <T>(a: T): T => a;
+async <T,>(a: T): T => a;

packages/babel-generator/test/fixtures/typescript/arrow-function-generic-tsx-trailing-comma/input.js

@@ -0,0 +1,2 @@
+// Verify typescript doesn't change anything inside type parameter declaration
+const foo = <T,>(a: T): T => a;

packages/babel-generator/test/fixtures/typescript/arrow-function-generic-tsx-trailing-comma/options.json

@@ -0,0 +1,3 @@
+{
+  "plugins": ["jsx", "typescript"]
+}

packages/babel-generator/test/fixtures/typescript/arrow-function-generic-tsx-trailing-comma/output.js

@@ -0,0 +1,2 @@
+// Verify typescript doesn't change anything inside type parameter declaration
+const foo = <T,>(a: T): T => a;

packages/babel-generator/test/fixtures/typescript/arrow-function-generic-tsx/output.js

@@ -1,2 +1,2 @@
 // Same as `generic`. Verify that JSX doesn't change things.
-<T>(a: T): T => a;
+<T,>(a: T): T => a;

packages/babel-generator/test/fixtures/typescript/arrow-function-generic/output.js

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

packages/babel-plugin-syntax-typescript/test/fixtures/disallow-jsx-ambiguity/type-parameter-unambiguous/output.js

@@ -1,3 +1,3 @@
-<T>() => 1;
+<T,>() => 1;
 
-<T>(x) => 1;
+<T,>(x) => 1;

packages/babel-plugin-transform-parameters/test/fixtures/regression/11344-typescript/output.js

@@ -4,6 +4,6 @@ function a() {
 }
 
 function d() {
-  let e = arguments.length > 0 && arguments[0] !== undefined ? arguments[0] : <T>() => {};
+  let e = arguments.length > 0 && arguments[0] !== undefined ? arguments[0] : <T,>() => {};
   let T;
 }