Types fulfill an interface, but interfaces do not

[zachrip]

TypeScript Version: 3.7.x-dev.201xxxxx

Search Terms:
Constrain type to a specific set of types, serializable values, nested types
Code

This version does not work:

export type BaseTypes<P> = string | number | boolean | null | P;
export interface ICacheableObject<P> extends Record<string, Cacheable<P>> {}

export type Cacheable<P = never> =
  | BaseTypes<P>
  | BaseTypes<P>[]
  | ICacheableObject<P>
  | ICacheableObject<P>[];

interface SimpleUserGroup {
  group_id: string;
}

// no extra classes, just basic primitives/arrays/objects
interface UserInfo {
  user_id: string;
  groups: SimpleUserGroup[];
}

// the objective is for this to be allowed:
const data: Cacheable = {
  user_id: 'test',
  groups: [],
} as UserInfo;


class Foo { test = 'abc' }
// but not this
const otherData: Cacheable = {
  foo: new Foo()
};

This version does work (using types instead of interfaces):

export type BaseTypes<P> = string | number | boolean | null | P;
export interface ICacheableObject<P> extends Record<string, Cacheable<P>> {}

export type Cacheable<P = never> =
  | BaseTypes<P>
  | BaseTypes<P>[]
  | ICacheableObject<P>
  | ICacheableObject<P>[];

type SimpleUserGroup {
  group_id: string;
}

// no extra classes, just basic primitives/arrays/objects
type UserInfo {
  user_id: string;
  groups: SimpleUserGroup[];
}

// the objective is for this to be allowed:
const data: Cacheable = {
  user_id: 'test',
  groups: [],
} as UserInfo;


class Foo { test = 'abc' }
// but not this
const otherData: Cacheable = {
  foo: new Foo()
};

Expected behavior:
I expect that UserInfo can be a valid value when compared to Cacheable as it only contains basic strings/arrays. The intended use case is to create a type safe serializer.

Actual behavior:
When using interfaces, typescript wants UserInfo to have an index or to be a type instead of interface.

Playground Link:
Not working: https://www.typescriptlang.org/play?#code/KYDwDg9gTgLgBDAnmYcBCBDAzsAKs4LAHgAUA+OAXjixigEsA7AczgB85GBXAWwCNgUdnD4QIAG2AZGw7uPHCSAbgCwAKFCRYcJjEEAzDAGNUASQDCxgBZS+kgPJ8AVsCMxSFUHsYATLHAAlV2gfIloGFgAaOEsjGww7YA8KAG8AX3V1TWh4JBQY61tJUipOYAA3QQpKdThhTBx8FGJyWvrsPAIWsgBtAF02jgtChIdnV3dWtTqh2PjExxc3D37VNXVdA2NUAGV6HjBJAFUcKABxKAguMDgUtuZL64B9eh8ALho6JmY1jPW1AD0AM4EDgXigGDgRnE2BwWGiTi4tBE2HoRjgYAYPHoMHolSwAIwUAhiAJEHGbiwG0YeighhMcBOglMjH0oLu0zgSMEL3enwiP3ujzAWA+ewOx1OFyuYFW6j+6iBCBscHJS1xlR0-jZQhgVno-hgoIEcAw8ggAHdgO91EYIIxkT4MDAMB85kVUNQOXVuVBeR8AOR6WgByJCmWiuD9MNqNKm-xMqAstlrTJqaGwuAAMTEtwQhHg1ADCSMAbgfyVfC48EYEFy+qp6ftyLrNigABFna6CnEPaVvXA2RAPoxgBbs2IABQASnlSiAA

Working: https://www.typescriptlang.org/play?#code/KYDwDg9gTgLgBDAnmYcBCBDAzsAKs4LAHgAUA+OAXjixigEsA7AczgB85GBXAWwCNgUdnD4QIAG2AZGw7uPHCSAbgCwAKFCRYcJjEEAzDAGNUASQDCxgBZS+kgPJ8AVsCMxSFUHsYATLHAAlV2gfIloGFgAaOEsjGww7YA8KAG8AX3V1TWh4JBQY61tJUipOYAA3QQpKdThhTBx8FGJyWvrsPAIWsgBtAF02jgtChIdnV3dWtTqh2PjExxc3D37VNXU81ABleh4wSQBVHCgAcSgILjA4FLbmc8uAfXofAC4aOiZmNYz1tQB6P6cCBwLxQDBwIzibA4LDRJxcWgibD0IxwMAMHj0GD0SpYP4YKBgxB4iDjNxYDYEOBHQSmRj6YE3aZwBGCJ6vd4RL63e5gLBvHZ7Q7HM4XMCrdQ-dQAhA2OCkpbYyo6fwMoQwKz0fwwYECOAYeQQADuwFe6iMEEYiJ8GBgGDecyKqGoTLqrKg7LeAHI9LQvZEeWL+XB+gG1Gl9f4aVA6Qy1pk1JDoXAAGJia4IQjwahehJGL1wH4yvhceCMCC5TUUxOWxEVmxQAAitvtBTiTtKrrgDIgb0YwCNqbEAAoAJSSpRAA
Related Issues:

[RyanCavanaugh]

Type aliases can implicitly fulfill an index signature (which is being declared at the Record<string> usage), but not interfaces (since they're subject to declaration merging)

[typescript-bot]

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

[DanielRosenwasser]

Design notes at #7059

[jcalz]

Crosslinking #15300