# TSR0702: Type predicate '… is …' is not proved: when it returns false, '…' can still be '…'.

> A type guard returns false for some values that are T, so callers would narrow them away wrongly. Make the false side exact or use is<T>().

## What it means

Callers narrow a parameter to "not `T`" when a type predicate returns false. In a sound file, wherever the guard returns false, the parameter's flow type must fit that narrowed type. A guard like `x === "a"` for `x is string` returns false for other strings, which callers would then treat as numbers.

## Example

```ts
// When this returns false, x can still be a string, but callers would narrow it to number.
function isString(x: string | number): x is string { // @expect TSR0702
  return x === "a";
}
function measure(v: string | number): number {
  return isString(v) ? v.length : v;
}
export const n: number = measure("b");
```

## How to fix

- Make the check match the predicate exactly: `typeof x === "string"` for `x is string`.
- Or narrow the predicate to what the body checks (`x is "a"`).

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/TSR0702/