defaultOptions.mutate.fetchPolicy is ignored by useMutation

[awinograd]

Intended outcome:

Setting defaultOptions.mutate.fetchPolicy in client instantiation should result in the option being used by useMutation when useMutation doesn't override the value.

Actual outcome:

useMutation passes { fetchPolicy: undefined } which ends up overriding the default fetchPolicy when defaultOptions are spread with override options.

Bad object merging:

apollo-client/src/ApolloClient.ts

Lines 345 to 348 in 0340c48

	options = {
	...this.defaultOptions.mutate,
	...options,
	} as MutationOptions<T, TVariables>;

Called with undefined fetchPolicy from:

apollo-client/src/react/data/MutationData.ts

Lines 101 to 112 in a38a2ca

	return this.refreshClient().client.mutate({
	mutation,
	optimisticResponse,
	refetchQueries:
	mutateOptions.refetchQueries || this.getOptions().refetchQueries,
	awaitRefetchQueries,
	update,
	context: mutationContext,
	fetchPolicy,
	variables: mutateVariables,
	...mutateOptions
	});

Related issues / fixes:

#3256 (dup of this issue, but it was closed to due inactivity)
trojanowski/react-apollo-hooks#77 (related fix for same bug in react-apollo-hooks)
#3717

How to reproduce the issue:

1. Instantiate a client with defaultOptions.mutate.fetchPolicy set to no-cache.
2. Use useMutation without specifying any options
3. Note that default fetchPolicy is not respected.

Versions

  System:
    OS: macOS 10.15.3
  Binaries:
    Node: 12.14.0 - ~/.nodenv/versions/12.14.0/bin/node
    Yarn: 1.17.3 - ~/programming/oui-aviva/node_modules/.bin/yarn
    npm: 6.13.4 - ~/.nodenv/versions/12.14.0/bin/npm
  Browsers:
    Chrome: 80.0.3987.116
    Firefox: 72.0.1
    Safari: 13.0.5
  npmPackages:
    @apollo/react-hooks: 3.1.3 => 3.1.3 
    apollo: ^2.21.0 => 2.21.2 
    apollo-cache-inmemory: ^1.6.3 => 1.6.3 
    apollo-cache-persist: ^0.1.1 => 0.1.1 
    apollo-client: ^2.6.8 => 2.6.8 
    apollo-link-error: ^1.1.11 => 1.1.11 
    apollo-link-http: ^1.5.15 => 1.5.15 
    apollo-link-retry: ^2.2.15 => 2.2.15 
    apollo-link-state: ^0.4.2 => 0.4.2 
    apollo-upload-client: ^11.0.0 => 11.0.0 

[Dimous]

@benjamn @hwillson please, please, fix/assign someone to fix this longstanding issue 🙏

[Titozzz]

related to #6561 #6677 probably

[benjamn]

Fix incoming (sorry for the long wait!): #6715

[awinograd]

Thanks for the fix @benjamn !

[benjamn]

Following up: we just published @apollo/client@3.1.0 to npm!