# TSR1205: Overload '…' is not satisfied by its implementation: called through it, the body can return '…', which is not assignable to '…'.

> Called through one of its overloads, a function's body can return a value that overload's return type doesn't allow. Fix the branch or the overload.

## What it means

TypeScript checks an overloaded function's body only against its implementation signature, not against each overload, so an overload can promise a return type the body never produces for its arguments. In sound files the body is checked once per overload signature: with the parameters typed as that overload declares them, every `return` the body can reach, and a reachable end as `undefined`, must be assignable to the overload's return type. `{0}` is the overload, `{1}` what the body returns through it, and `{2}` the overload's return type. It's reported at the `return`.

Branches the overload can't reach are skipped: if a parameter narrows to `never` on a path (a `typeof x === "string"` test under an overload that takes a `number`), returns on that path aren't checked against it. Overloads without an implementation (interfaces, `.d.ts`) aren't checked.

## Example

```ts
// Called through the second overload, the body returns the number it was given, not a string.
function first(value: string): string;
function first(value: number): string;
function first(value: string | number): string | number {
  return value; // @expect TSR1205
}
export const n: string = first(7);
```

## How to fix

- Fix the branch so it returns the overload's type for that overload's arguments (here `String(value)`).
- Or correct the overload's return type if the body is right (`function first(value: number): number`).
- Or narrow the parameter before returning, so each overload only reaches the branch meant for it.

This code replaced TSR1203, which banned overload implementations in sound files; TSR1203 is retired. Sound mode is defined in [design record 0007](https://github.com/Webb-Ventures/typescript-rust/blob/main/docs/design/0007-sound-mode.md) (accepted 2026-10-15).

Source: https://ts-r.dev/errors/TSR1205/