useLazyQuery's exec function return type doesn't thread errorPolicy through

[mx-bernhard]

Issue Description

Using modern signatures ApolloClient.QueryResult<TData, TErrorPolicy> correctly narrows based on error policy — e.g. errorPolicy: 'none' maps to { data: TData; error?: never } since GraphQL errors reject the promise instead of resolving with partial data.

useLazyQuery's ExecFunction type hardcodes the default policy and never looks at the errorPolicy passed to the hook:

    type ExecFunction<TData, TVariables extends OperationVariables> = (...args: {} extends TVariables ? [
        options?: useLazyQuery.ExecOptions<TVariables>
    ] : [options: useLazyQuery.ExecOptions<TVariables>]) => ObservableQuery.ResultPromise<ApolloClient.QueryResult<TData /*, missing TErrorPolicy here */>>;

Repro

import type { TypedDocumentNode } from '@apollo/client';
import { useLazyQuery } from '@apollo/client/react';

const [loadOperation] = useLazyQuery({} as TypedDocumentNode<{ someField: boolean }, {}>, { errorPolicy: 'none' });

loadOperation({}).then(({ data }) => {
  data.someField; // TS18048: 'data' is possibly 'undefined'
});

Even though at runtime, with errorPolicy: 'none', data is guaranteed defined on promise resolution (a GraphQL error rejects the promise instead), the type says otherwise. Expected: ExecFunction/ResultTuple should carry the resolved TErrorPolicy (from the hook's Options), so the exec function's promise resolves to the correctly narrowed QueryResult<TData, TErrorPolicy>.

Link to Reproduction

This is a Typescript-only issue

Reproduction Steps

No response

@apollo/client version

4.2.8

[jerelmiller]

Hey @mx-bernhard 👋

I'm working on a fix for #13342 that I think might help here as well (currently my open PR only fixes useQuery to validate the approach, but the same flaw is present in almost every hooks/API that works with default options). I'll make sure this is looked at as part of that. Thanks for reporting!

[atharv-sys32]

Hi @jerelmiller, I'd like to pick this up. Plan: thread the hook-level errorPolicy through the ExecFunction return type so loadOperation({}).then(...) narrows data/error correctly, and add a type test under integration-tests/type-tests/signatures/ following the pattern from #13352. Will open a PR shortly.

[jerelmiller]

@atharv-sys32 actually do you mind holding off on this one a bit? I'm about to change the type signatures quite a bit to fix another issue related to this, and in the process and I might or might not fix this. I'm meeting with the team on Monday to discuss what direction we want to go with updated types so I'd hate for you to waste time on something that might get thrown away or changed drastically right away.

I appreciate the enthusiasm though! If you see something else that looks interesting, let me know!

[atharv-sys32]

Sure @jerelmiller , I'll look into some other issues, we can hold this for now. I'll get back to you in case any other issue resonates with me.