Can't seem to type my functions correctly (using discriminator models) about mongoose HOT 3 CLOSED

The main difference here is that the in the first instance, the generic type is defined at method level - each time you call testFunction, you can pass in a different type (one that is valid, so it has to extend IBase, which it does in your scenario so everything works well).

In TestClass2, the generic parameter is defined at class level, which is where the issue lies. Once you create an instance of TestClass2, the generic type is fixed for that instance. So if you were to replace const testClass2 = new TestClass2(); with const testClass2 = new TestClass2<IDiscriminator>(); then you would see the type error on the call of the function with BaseModel, and the second one would be fine.

So basically, you have more flexibility when defining the generic parameter at method level. 👍

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The issue is that, while X extends IBase, there's no guarantee that mongoose.Model<X> extends mongoose.Model<IBase> in the following code.

class TestClass2<X extends IBase> {
  public testFunction(M: mongoose.Model<X>) {
    console.log(M);
  }
}

I'd recommend @valeriuflorescu16 's approach: define the generic at the method level, not the class level. TypeScript doesn't have a way for us to tell the compiler that mongoose.Model<IDiscriminator> extends mongoose.Model<IBase>.

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