Issue in inferring types in Switch Case for object methods having union mapped types.

[kansalanmol0609]

Bug Report

🔎 Search Terms

Issue in inferring types in Switch Case for object methods having union mapped types.

🕗 Version & Regression Information

Version - All versions upto v5.0.0-dev.20221116

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about mapped types, union types and switch cases.

⏯ Playground Link

Playground link with relevant code

💻 Code

enum Actions{
ACTIONA,
ACTIONB
};

type ActionA = {
  type : Actions.ACTIONA,
  payload: {
    body: string;
    message: object;
  }
}

type ActionB = {
  type : Actions.ACTIONB,
  payload: {
    message: object;
  }
}

type ActionType = ActionA | ActionB;

const actionHandlers: {
  [key in  Actions.ACTIONA | Actions.ACTIONB]: (payload: Extract<ActionType, { type: key }>['payload']) => void
} = {
  [Actions.ACTIONA]: ({body, message}) => console.log(body, message),
  [Actions.ACTIONB]: ({message}) => console.log( message),
}

const performAction = (action: ActionType): void => {
  switch(action.type){
    case Actions.ACTIONA:
    case Actions.ACTIONB:
      return actionHandlers[action.type](action.payload);  //Error here
  }
}

🙁 Actual behavior

Getting this error -

Argument of type '{ body: string; message: object; } | { message: object; }' is not assignable to parameter of type '{ body: string; message: object; }'.
  Property 'body' is missing in type '{ message: object; }' but required in type '{ body: string; message: object; }'.

🙂 Expected behavior

TypeScript compiler should have been able to infer that action.type and action.payload are connected. It looks like it is treating them as independent variables.

[fatcerberus]

Duplicate of #30581, but see #30581 (comment)