Conditional types in constructor parameters cannot infer type from generics

[kaczor6418]

Bug Report

🔎 Search Terms

conditional types, generics

🕗 Version & Regression Information

This is the behavior in every version I tried, and I reviewed the FAQ for entries about 4.6.4 (also beta channel)

⏯ Playground Link

Conditional types in constructor parameters cannot infer type from generics

💻 Code

I want to create an abstract class that will store some common logic and type will be inferred based on the generic passed to this abstract class.

Simple example

type StringOrNuber = string | number;

type ConditionalType<T> = T extends StringOrNuber ? { value: T } : T;

interface HandleBaseClass<T> {
  value: T
}

abstract class BaseClass<T> implements HandleBaseClass<T> {
  test: ConditionalType<T>;

  constructor(value: ConditionalType<T>) {
    this.test = value;
  }

  abstract get value(): T;

  abstract set value(newValue: T);
}

class ClassA<T extends StringOrNuber> extends BaseClass<T> {
  constructor(initialValue: T) {
    super({value: initialValue});
  }

  get value() {
    return this.test.value;
  }

  set value(newValue: T) {
    this.test.value = newValue;
  }
}

class ClassB<T extends Record<PropertyKey, unknown>> extends BaseClass<T> {
  constructor(initialValue: T) {
    super(initialValue);
  }

  get value() {
    return this.test;
  }

  set value(newValue: T) {
    this.test = newValue;
  }
}

Playground Link: Provided

In the above example I have a ConditionalType<T> that can be an object with value key only if we pass a number or string value, for all other cases the type should be the same as given type. Let's consider abstract class BaseClass<T> that takes one parameter in the constructor, which will be a type of { value: T } or T. We have created two more specific class ClassA and ClassB.

ClassA

ClassA extends BaseClass and adds restriction to its generic that it can be only sth that extends StringOrNuber type (string | number) so the BaseClass knows that T can be only a string or number so type of constructor parameter and test field should be { value: StringOrNumber }. This partly works. If I try to access this.test from ClassA then TS says the type of this.test is { value: number; } | { value: string; } what is good, but if I try to pass value that can be only a string or number wrapped in an object I am getting an error. What should not happen, I think (correct me if I am wrong)

abstract class BaseClass<T> implements HandleBaseClass<T> {
  test: ConditionalType<T>;

  constructor(value: ConditionalType<T>) {
    this.test = value;
  }
  // ...
}

class ClassA<T extends StringOrNuber> extends BaseClass<T> {
  constructor(initialValue: T) {
    super({value: initialValue});
  }
  // ...
}

Argument of type '{ value: StringOrNuber; }' is not assignable to parameter of type 'ConditionalType'

ClassB

ClassB extends BaseClass and adds restriction to its generic that it can be only sth that extends Record<PropertyKey, unknown> type (basically any object) so the BaseClass knows that T can NOT be a string or number so type of constructor parameter and test field should be T. This partly works. If I try to access this.test from ClassB then TS says the type of this.test is T what is good, but if I try to pass value that can be only an object I am getting an error. What should not happen, I think (correct me if I am wrong).

abstract class BaseClass<T> implements HandleBaseClass<T> {
  test: ConditionalType<T>;

  constructor(value: ConditionalType<T>) {
    this.test = value;
  }
  // ...
}

class ClassB<T extends Record<PropertyKey, unknown>> extends BaseClass<T> {
  constructor(initialValue: T) {
    super(initialValue);
  }
  // ...
}

Argument of type 'T' is not assignable to parameter of type 'ConditionalType'.
Type 'Record<PropertyKey, unknown>' is not assignable to type 'ConditionalType'.

If we use this ConditionalType<T> like that:

type ConditionalType<T> = T extends StringOrNuber ? { value: T } : T;

const x: ConditionalType<number> = {
  value: 12
}
const y: ConditionalType<Person> = {
  firstName: 'Jan',
  lastName: 'Kowlaski',
  age: 21
}

Then there is no any issues and types are inferred as expected.

To solve the above issue, I can use Union type and initialize test separately in ClassA and ClassB, but I would like to do it only once and do not repeat this operation in every class.

🙁 Actual behavior

Class constructor parameters do not infer generic type to use it in ConditionalType

🙂 Expected behavior

Class constructor parameters should infer generic type and use it in ConditionalType

[whzx5byb]

The constraint provided for ClassA is not applied to the conditional type in BaseClass, even they are literally the same. See discussions in #31096 and #32591.

[kaczor6418]

The constraint provided for ClassA is not applied to the conditional type in BaseClass, even they are literally the same. See discussions in #31096 and #32591.

@whzx5byb So, this is impossible right now because of TypeScript limitations and this can be more like feature request than bug ?

[whzx5byb]

@kaczor6418 I think so. At the moment you can use type assertion super({value: initialValue} as ConditionalType<T>) as a workaround.

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.