useMutation types: variables are optional

Describe the bug

in TypeScript, the mutation function created by the useMutation hook can always be called without passing arguments, because the type TVariables is set to optional:

https://github.com/tannerlinsley/react-query/blob/00b9e968a80ca7c51fd50479aee852bb41bf9bc6/src/react/useMutation.ts#L132

To Reproduce

I made a codeSandbox example: https://codesandbox.io/s/async-currying-wepni

Please look at the comment at line 54

I should be forced to call that function like this:

mutatePostTodo(text)

but I can also call it like this:

mutatePostTodo()

Expected behavior

The type of variables that you need to pass to that function should be equal to the type that the actual mutation function accepts, in other words, it should be variables: TVariables rather than variables?: TVariables

Desktop (please complete the following information):

• OS: MacOS Catalina
• Browser: Chrome
• Version: 85

Additional context

All the usages of variables in that function are accessed with the bang operator, for example:

https://github.com/tannerlinsley/react-query/blob/93db5b61246dcd381d699054de48fca685bef73b/src/react/useMutation.ts#L146

or https://github.com/tannerlinsley/react-query/blob/00b9e968a80ca7c51fd50479aee852bb41bf9bc6/src/react/useMutation.ts#L174

So I don’t really see why it should be optional to begin with 🤷 .

Happy to make a PR if you approve of this 😃

[boschni]

The typing issue has been fixed in V3

[tannerlinsley]

I would be okay implementing this in v2 as non-breaking, but I’ll leave that up to @boschni since he knows more about the type implications. Otherwise, expect it in v3.