TypeScript: conditional default type error

Do you want to request a feature or report a bug? Bug

What is the current behavior?

Type '{ type: BooleanConstructor; default(this: any): boolean; }' is not assignable to type 'SchemaDefinitionProperty<boolean>'.
  Type '{ type: BooleanConstructor; default(this: any): boolean; }' is not assignable to type 'false'.ts(2322)
index.ts(17, 3): The expected type comes from property 'isPet' which is declared here on type '{ name?: SchemaDefinitionProperty<string>; isPet?: SchemaDefinitionProperty<boolean>; }'

If the current behavior is a bug, please provide the steps to reproduce.

import mongoose from 'mongoose';
const { Schema } = mongoose;

async function run() {
  await mongoose.connect('mongodb://localhost:27017/test');

  const animalSchema = new Schema<IAnimal>({
    name: String,
    isPet: { type: Boolean, default() { return this.name === 'foo' } }
  });

  const Animal = mongoose.model<IAnimal>('Animal', animalSchema);
}

interface IAnimal {
  name?: string;
  isPet?: boolean;
}

{
  "compilerOptions": {
    "target": "ES2020",
    "module": "ESNext",
    "moduleResolution": "node",
    "esModuleInterop": true,
    "forceConsistentCasingInFileNames": true,
    "skipLibCheck": true,
  }
}

What is the expected behavior? No type errors

What are the versions of Node.js, Mongoose and MongoDB you are using? Note that “latest” is not a version. Node.js: 16.3.1 Mongoose: 6.3.4 MongoDB: 5.0.5

[AbdelrahmanHafez]

I can reproduce the issue in one of the projects that I use TS on.

[vkarpov15]

I took a look, I have a fix but I have not the slightest idea why this issue happens or why my fix works. The issue is that the below TS:

const t: SchemaTypeOptions<boolean> = {
    type: Boolean,
    default() {
      return this.name === 'Hafez';
    }
  };

gets the below error:

Type (this: any) => boolean is not assignable to type boolean | ((this: any, doc: any) => false) | ((this: any, doc: any) => true) | undefined.
  Type (this: any) => boolean is not assignable to type (this: any, doc: any) => false.
    Type boolean is not assignable to type false. 

Why TypeScript thinks the function type is (this: any, doc: any) => false is beyond me. Removing the T extends Schema.Types.Mixed conditional works, as does the fix I put in.