Implement interface codefix is offered when the implemented interface type is invalid

[danyalahmed1995]

🔎 Search Terms

implement interface codefix invalid type argument unresolved type symbol fixClassIncorrectlyImplementsInterface

🕗 Version & Regression Information

This is the behavior in the current repository state.

I found existing fourslash tests that appear to document this as undesirable behavior, but the tests currently still expect the codefix to be available:

• tests/cases/fourslash/codeFixClassImplementInterfaceTypeParamInstantiateError.ts
• tests/cases/fourslash/codeFixClassImplementInterfaceUndeclaredSymbol.ts

⏯ Playground Link

N/A: this is a language service codefix issue, reproduced through existing fourslash/codefix tests.

💻 Code

interface I<T extends string> {
  x: T;
}

class C implements I<number> { }

A second repro:

interface I {
  x: T;
}

class C implements I { }

🙁 Actual behavior

class C implements I {
x: T;
}

The resulting code still has semantic errors because the implemented interface reference or member type was already invalid.

🙂 Expected behavior

The `Implement interface` codefix should not be offered when the implemented interface reference or required member types contain semantic errors, such as an invalid type argument or an unresolved type symbol.

### Additional information about the issue

Nearby fourslash tests already contain comments indicating these cases should use `verify.codeFixAvailable([])`:

- `tests/cases/fourslash/codeFixClassImplementInterfaceTypeParamInstantiateError.ts`
- `tests/cases/fourslash/codeFixClassImplementInterfaceUndeclaredSymbol.ts`

The likely code path is:

- `src/services/codefixes/fixClassIncorrectlyImplementsInterface.ts`

The fix appears to be created from `checker.getTypeAtLocation(implementedTypeNode)` without rejecting implemented type nodes that have related semantic errors.

Focused test commands used locally:

```sh
npx.cmd hereby runtests --tests=tests/cases/fourslash/codeFixClassImplementInterfaceTypeParamInstantiateError.ts --light=false
npx.cmd hereby runtests --tests=tests/cases/fourslash/codeFixClassImplementInterfaceUndeclaredSymbol.ts --light=false