Discriminated Union Return Type #60467

type Classifier = 'fooString' | 'barNumber';

type ClassifierType<T extends Classifier> = T extends 'fooString'
  ? string
  : number;

type GenericType<T extends Classifier> = {
  classifier: T;
  value: ClassifierType<T>;
};

type GenericTypeMap = {
  fooString: GenericType<'fooString'>;
  barNumber: GenericType<'barNumber'>;
};

// use this to create a type a discriminated union: GenericType<'fooString'> | GenericType<'barNumber'>
// as opposed to a non-discimintated union GenericType<'fooString' | 'barNumber'>
type GenericTypeUnion<T extends Classifier> = {
  [K in T]: GenericTypeMap[K];
}[T];

type ClassifierTypeMap = {
  [K in Classifier]: GenericType<K>['value'];
};

// use this to create a type a discriminated union: ClassifierType<'fooString'> | ClassifierType<'barNumber'>
// as opposed to a non-discimintated union ClassifierType<'fooString' | 'barNumber'>
type ClassifierTypeUnion<T extends Classifier> = {
  [K in T]: ClassifierTypeMap[K];
}[T];

function genericFunction<T extends Classifier>(
  _classifier: T, // need to pass a dummy classifier var to help the typescript compiler, weird but not what this report is about
  input: GenericTypeUnion<T>
): ClassifierTypeUnion<T> {
  if (input.classifier === 'fooString') {
    return input.value[0];
//. ^^^^^^ ERROR: string is not assignable to type never
  } else {
    return input.value + 1;
//. ^^^^^^ ERROR: number is not assignable to type never
  }
}

const val = genericFunction('fooString', {
//.   ^^^ but is correctly inferred as a string here
  classifier: 'fooString',
  value: 'foo',
});

// TS knows that the input type (second arg) is not matching the classifier type (first argument)
// TS has inferred that the return type should be a number based on the classifer type (first argument)
const val = genericFunction('barNumber', {
  classifier: 'fooString',
  value: 'foo',
});

// TS knows that the input type value is not matching the classifier type 
const val = genericFunction3('fooString', {
  classifier: 'fooString',
  value: 9,
});