I know I keep beating a dead horse on this one, but do we know why TypeScript doesn't infer this correctly?

const createMock = (): jest.Mocked<ContextServiceContract> => ({
  setup: jest.fn().mockReturnValue(createSetupContractMock()),
});

It seems strange that we need to create temporary throw-away variables to satisfy some problem with typing here.

I think @restrry knows more. I believe it's a fixable problem in DefinitelyTyped but not sure if it's been fixed yet?

jest doesn't know which function it mocks. if we want to have type safety, we should provide types manually
https://github.com/DefinitelyTyped/DefinitelyTyped/blob/master/types/jest/index.d.ts#L120-L127
There are several options

• define a type manually for every function

type Arguments<T> = T extends (...args: infer U) => any ? U : never;
//...
const mock = {
    start: jest
      .fn<
        ReturnType<typeof createStartContractMock>,
        Arguments<typeof createStartContractMock>
      >()
      .mockReturnValue(createStartContractMock),
}

• or define a result type

const mock: jest.Mocked<ContextServiceContract> = {
    start: jest.fn(),
}
// and now, knowing the result type, jest can check whether a passed value satisfies the type
mock.start.mockImplementation(createStartContractMock);

when you declare a type as:

const createMock = (): jest.Mocked<ContextServiceContract> => ({
  setup: jest.fn().mockReturnValue(createSetupContractMock()),
});

TS uses this declaration https://github.com/DefinitelyTyped/DefinitelyTyped/blob/master/types/jest/index.d.ts#L123
and infers a type as jest.fn<any, any>

Thus, any value passed as mockReturnValue or mockImplementation satisfies this type.

probably there is a better solution I'm not aware of.

never met service owner terminology in our code so far. is it core service?
If no, what if a plugin calls register without pluginName?

.register('ctxFromPluginA', context => ..);

now the result is provided to the all plugins, even if they have no dependency on pluginA.

This will have to be made impossible by the service owner (which can be a Core service or a plugin).

The service owner that exposes this context container's register method should populate the pluginName argument based on the scoped service contract pattern in #40822

should we add a test to check an exception, raised during register phase, bubbles to the caller?

I'm not sure what you mean? Do you mean that we move / duplicate this test to ContextService?

jest doesn't know which function it mocks. if we want to have type safety, we should provide types manually
https://github.com/DefinitelyTyped/DefinitelyTyped/blob/master/types/jest/index.d.ts#L120-L127
There are several options

• define a type manually for every function

type Arguments<T> = T extends (...args: infer U) => any ? U : never;
//...
const mock = {
    start: jest
      .fn<
        ReturnType<typeof createStartContractMock>,
        Arguments<typeof createStartContractMock>
      >()
      .mockReturnValue(createStartContractMock),
}

• or define a result type

const mock: jest.Mocked<ContextServiceContract> = {
    start: jest.fn(),
}
// and now, knowing the result type, jest can check whether a passed value satisfies the type
mock.start.mockImplementation(createStartContractMock);

when you declare a type as:

const createMock = (): jest.Mocked<ContextServiceContract> => ({
  setup: jest.fn().mockReturnValue(createSetupContractMock()),
});

TS uses this declaration https://github.com/DefinitelyTyped/DefinitelyTyped/blob/master/types/jest/index.d.ts#L123
and infers a type as jest.fn<any, any>

Thus, any value passed as mockReturnValue or mockImplementation satisfies this type.

probably there is a better solution I'm not aware of.