Wrong type assertion in classes using "this" keyword

[nicolasmelo1]

Bug Report

🔎 Search Terms

this, assertion error, generics, type inference, automatic intersection

🕗 Version & Regression Information

Looks like it's above version 3.3.3333

⏯ Playground Link

Playground link with relevant code

💻 Code

// -------- BASE CLASSES ---------------
class Model {
  fields!: ModelFields;
  options!: ModelOptions<this>;
}

// ------- BUG ------------------------
class User extends Model {
  fields = {
    a: FieldTypes.BOOL
  }
//                        V - ⛔️ HERE IS THE BUG!!!! Why? and how to solve it?
  options: ModelOptions<this> = {
    indexes: [{
      unique: true,
      fields: ['a'] // <- ✅ This works as expected though
    }]
  }
}

// ------------ TYPES ----------------
enum FieldTypes {
  CHAR = 'CHAR',
  BOOL = 'BOOL',
  INT = 'INT'
}

type ModelIndex<M extends Model> = {
  unique: true;
  fields: (keyof M["fields"])[];
}

type OrderingOfModelOptions<M extends Model> = keyof M["fields"]|
  keyof { [F in keyof M["fields"] as F extends string ? `-${F}` : never] : 1}

type ModelOptions<M extends Model> = {
  indexes?: ModelIndex<M>[];
  ordering?: OrderingOfModelOptions<M>[];
}

type ModelFields = {
  [field: string]: FieldTypes
}

🙁 Actual behavior

Although i'm passing the correct arguments, it just don't work. It just don't work because the type ModelIndexes is an object, which is really strange to me.

• I've tried changing the name of the fields to something else like attributes but no success, so it does not seem that the problem is the name.
• I saw that on ModelIndexes it looks like M and Model are having an automatic intersection like M & Model, but it needs some further investigation.
• I've tried doing conditional like M extends Model ? (keyof M["fields"])[] : never; but no success, the condition actually works but the problem then will be something like Type '"a"[]' is not assignable to type 'this extends Model ? (keyof this["fields"])[] : never

🙂 Expected behavior

in this example indexes: { union: true, fields: ['a']} should work because 'a' is defined in the User model fields object and indexes: { union: true, fields: ['b']} should not work because 'b' it's not defined inside of fields in User model

The expected behavior is to just work as 3.3.3 worked, so it accepts a union of all of the keys of the object not as all of the above versions works.

[MartinJohns]

Your property fields has the type { a: FieldTypes }, but your generic type ModelOptions<> requires the type argument to extend Model. But the type of this is { fields: { a: FieldTypes } }, and it does not extend Model (which is { fields: ModelFields; options: ModelOptions<this> }).

When you type your property fields correctly it works:

class User extends Model {
  fields: ModelFields = {
    a: FieldTypes.BOOL
  }
  // ...
}

The type of re-defined properties is not inherited. This would require #10570.

[nicolasmelo1]

Whoa, nobody was being able to solve this, yeah, it works as expected now, and now i understood why this error happens.

Thanks a lot for the help and explanation, closing the issue now.