3.4.* introduces a regression on the `merge` policy

Intended outcome: The 3.4.* versions introduce a regression on the merge policy

Actual outcome: The merge custom policy should receive correct parameters

How to reproduce the issue:

Here a unit test:

import { gql, InMemoryCache, useQuery } from '@apollo/client'
import { MockedProvider } from '@apollo/client/testing'
import { render, screen, waitFor } from '@testing-library/react'
import faker from 'faker'

const cache = new InMemoryCache({
  typePolicies: {
    Country: {
      fields: {
        cities: {
          keyArgs: ['size'],
          merge(existing, incoming, { args }) {
            console.log(existing, incoming, { args })
            if (!args) return incoming

            const items = existing ? existing.slice(0) : []

            const offset = args.offset ?? 0
            for (let i = 0; i < incoming.length; ++i) {
              items[offset + i] = incoming[i]
            }

            return items
          },
        },
      },
    },
    CityInfo: {
      merge: true,
    },
  },
})

const GET_COUNTRIES = gql`
  query GetCountries {
    countries {
      id
      ...WithSmallCities
      ...WithAirQuality
    }
  }

  fragment WithSmallCities on Country {
    biggestCity {
      id
    }
    smallCities: cities(size: SMALL) {
      id
    }
  }

  fragment WithAirQuality on Country {
    biggestCity {
      id
      info {
        airQuality
      }
    }
  }
`

function Test() {
  const { data } = useQuery(GET_COUNTRIES)
  return <div data-testid="countryNumber">{data?.countries.length}</div>
}

it('should render without error', async () => {
  const countries = [
    {
      __typename: 'Country',
      id: faker.datatype.uuid(),
      biggestCity: {
        __typename: 'City',
        id: faker.datatype.uuid(),
        info: {
          __typename: 'CityInfo',
          airQuality: faker.datatype.number(),
        },
      },
      smallCities: [
        {
          __typename: 'City',
          id: faker.datatype.uuid(),
        },
      ],
    },
  ]

  const mock = { request: { query: GET_COUNTRIES }, result: { data: { countries } } }

  render(
    <MockedProvider cache={cache} mocks={[mock]} resolvers={{}}>
      <Test />
    </MockedProvider>,
  )

  await waitFor(() => {
    const element = screen.getByTestId('countryNumber')
    expect(element).toHaveTextContent(countries.length.toString())
  })
})

If you try to run this test, you will have this error Uncaught [TypeError: existing.slice is not a function], since we receive these arguments into the merge function: { __ref: 'City:a44cfd2e-c50f-4fcf-924d-d8e275c8df07' } as existing { id: 'a44cfd2e-c50f-4fcf-924d-d8e275c8df07', __typename: 'City', info: { __typename: 'CityInfo', airQuality: 20807 } } as incoming

We should have: undefined as existing { __ref: 'City:a44cfd2e-c50f-4fcf-924d-d8e275c8df07' } as incoming

FYI If we delete CityInfo: { merge: true } from the cache configuration, it works.

Versions It works with 3.3.21 but not with the new 3.4.* versions

[benjamn]

@Grmiade (and @anark) Thanks for the reproduction! This should be fixed in @apollo/client@3.4.7 (just published).

[Grmiade]

It also works on our side. Thanks a lot for the reactivity 💪