`createMuiTheme` return type doesn't take into account merged objects

• The issue is present in the latest release.
• I have searched the issues of this repository and believe that this is not a duplicate.

Thank you so much for this project – it is amazing!

Current Behavior 😯

I would like to add custom fields to the Theme object as described in the docs. However I struggle to get this to typecheck. The current Typescript definition for createMuiTheme is

export default function createMuiTheme(options?: ThemeOptions, ...args: object[]): Theme;

createMuiTheme returns a Theme object, rather than the type of deep merging options and args together.

Expected Behavior 🤔

The return type of createMuiTheme would reflect that args was merged in. For example the following wouldn’t fail (today it says Theme doesn’t have field foo).

createMuiTheme({}, {foo: "bar"}).foo

Proposed Solution

One possible solution would be to change the return type of createMuiTheme to

export default function createMuiTheme<T>(
  options?: ThemeOptions,
  ...args: Array<Partial<T>>
): Theme & T;

[eps1lon]

Customization is explained in our typescript guide. Does this work for you?

Changing the signature of createMuiTheme would be incomplete since it wouldn’t affect the type you read from context.

[blasterpistol]

@mcobzarenco it can works if you use this typed theme object directly, but not when it goes from the context