typescript version 7 complains about unused generic type even though version 6 does not

🔎 Search Terms

"TS6196", "is declared but never used", "unused generic", "noUnusedParameters"

🕗 Version & Regression Information

• This changed between versions 6.0.3 and 7.0.2

⏯ Playground Link

https://www.typescriptlang.org/play/?#code/C4TwDgpgBAShBmAeAKgPigXigbwFAEgA3AQwBsBXCALimQBpcBfXXecgOwGNgBLAe3ZROAJwjFgEOEnoBVKBAAeE9gBMAzlEJ8eK1AAoSFapu0qAlDSkpUAblYdu-QSLESraA2Uo1kF2Ams7Ni5eASFRcUkAj0NvWj93dDx8UWByYUFkoi8IBnxGO2ZcTgE1YChReABGTHDXKPg9MzsS9jU+UggAOlI+AHM9Sqrm4tLyyoAmWpdIq3ZyAFsAIwhhOhMdfRHW9s6e-sGECe2xioQAZmmItwD55dX9KvOTto7u3oHK57sgA

💻 Code

type Ref<T> = {
	value: T,
}

function createRef<T,U extends void>(value: void): Ref<T>;
function createRef<T>(value: T): Ref<T>;
function createRef<T>(value: T): Ref<T> {
	return {
		value,
	};
}

const ref1 = createRef();
console.log(ref1);
const ref2 = createRef<number, void>();
console.log(ref2);
const ref3 = createRef<number>(13);
console.log(ref3);

🙁 Actual behavior

The typescript compiler version 7 fails with following error message:

code.ts:5:22 - error TS6196: 'U' is declared but never used.

5 function createRef<T,U extends void>(value: void): Ref<T>;
                       ~~~~~~~~~~~~~~


Found 1 error in code.ts:5

The typescript compiler version 6 succeeds without any errors.

🙂 Expected behavior

The typescript compiler version 7 should behave exactly like version 6 i.e. not fail.

Additional information about the issue

This error only occurs with noUnusedParameters set to true.

Reproduce the error from the linked Playground using noUnusedParameters and the shown createRef overloads, then trace the compiler’s handling of unused generic type parameters in overload declarations. Add a regression test covering the version-7 behavior and verify that the example no longer reports TS6196 while other unused-parameter diagnostics remain correct.