[Vuejs]-Why can't these generic type be inferred?

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This is because you’re using P: string for your mutations and getters, so we don’t know what the specific keys are, and therefore their types can be inferred.

You should use extra generic parameters for P here – for the MutationKeys and GetterKeys, which can then be inferred correctly:

interface Options<
  State,
  MutationKeys extends string,
  G extends Getters<State, string>
> {
  state: State;
  getters: G;
  mutations: {
    [P in MutationKeys]: (state: State, getters: G) => void;
  };
}

type Getters<State, Keys extends string> = {
  [key in Keys]: (state: State) => any;
};

function createStore<
  State,
  MutationKeys extends string,
  GetterKeys extends string
>(options: Options<State, MutationKeys, Getters<State, GetterKeys>>) {
  return options;
}

createStore({
  state: {
    count: 0
  },
  getters: {
    isOdd: (state) => state.count % 2 === 1
  },
  mutations: {
    incrementIfOdd(state, getters) {
      if (getters.isOdd(state)) {
        ++state.count;
      }
    }
  }
});

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