From 03bca1d8fe4282a0af0735c48ae6c8be36c84394 Mon Sep 17 00:00:00 2001
From: Wesley Wigham <wewigham@microsoft.com>
Date: Tue, 13 Aug 2024 11:55:27 -0700
Subject: [PATCH] Allow trailing commas on lambda type parameter lists (#59612)

---
 src/compiler/emitter.ts                               |  2 +-
 src/testRunner/unittests/printer.ts                   | 11 +++++++++++
 ...ileCorrectly.lambda type parameter lists in tsx.js |  1 +
 3 files changed, 13 insertions(+), 1 deletion(-)
 create mode 100644 tests/baselines/reference/printerApi/printsFileCorrectly.lambda type parameter lists in tsx.js

diff --git a/src/compiler/emitter.ts b/src/compiler/emitter.ts
index 6f68329ba0a..493e60af2b8 100644
--- a/src/compiler/emitter.ts
+++ b/src/compiler/emitter.ts
@@ -4508,7 +4508,7 @@ export function createPrinter(printerOptions: PrinterOptions = {}, handlers: Pri
         if (isFunctionLike(parentNode) && parentNode.typeArguments) { // Quick info uses type arguments in place of type parameters on instantiated signatures
             return emitTypeArguments(parentNode, parentNode.typeArguments);
         }
-        emitList(parentNode, typeParameters, ListFormat.TypeParameters);
+        emitList(parentNode, typeParameters, ListFormat.TypeParameters | (isArrowFunction(parentNode) ? ListFormat.AllowTrailingComma : ListFormat.None));
     }
 
     function emitParameters(parentNode: Node, parameters: NodeArray<ParameterDeclaration>) {
diff --git a/src/testRunner/unittests/printer.ts b/src/testRunner/unittests/printer.ts
index 76f23ab692f..86a7b41d167 100644
--- a/src/testRunner/unittests/printer.ts
+++ b/src/testRunner/unittests/printer.ts
@@ -100,6 +100,17 @@ describe("unittests:: PrinterAPI", () => {
                 ts.ScriptKind.TSX,
             ));
         });
+
+        // https://github.com/microsoft/TypeScript/issues/59587
+        printsCorrectly("lambda type parameter lists in tsx", {}, printer => {
+            return printer.printFile(ts.createSourceFile(
+                "source.tsx",
+                String.raw`export const id = <T,>(id: T): T => id`,
+                ts.ScriptTarget.ESNext,
+                /*setParentNodes*/ undefined,
+                ts.ScriptKind.TSX,
+            ));
+        });
     });
 
     describe("No duplicate ref directives when emiting .d.ts->.d.ts", () => {
diff --git a/tests/baselines/reference/printerApi/printsFileCorrectly.lambda type parameter lists in tsx.js b/tests/baselines/reference/printerApi/printsFileCorrectly.lambda type parameter lists in tsx.js
new file mode 100644
index 00000000000..c28604711e8
--- /dev/null
+++ b/tests/baselines/reference/printerApi/printsFileCorrectly.lambda type parameter lists in tsx.js	
@@ -0,0 +1 @@
+export const id = <T,>(id: T): T => id;