Generic checking error

[tinganho]

I think #16368 broke this code for me:

class Model {}

class Collection<M extends Model> {
    m: M;
}

class User extends Model {
}

class Users extends Collection<User> {
}

type CollectionClass = typeof Collection;
let a: CollectionClass = Users; // Error 'User' is not assignable to 'M' 

If I provide the --noStrictGenericChecks flag, it compiles without any complaints. Can I correct this without using the flag?

[mhegazy]

#16368 is about generic signatures, and i do not see any generic signatures in the code sample in the OP. so please share some context on what is failing, what is the error that is reported, and why do you believe the error is incorrect.

Before #16368, when generic signatures where compared, all generic parameters were erased to any then the comparison is done. #16368 adds a correct and more strict comparison by instantiating the source in the generic type arguments of the target before doing the check.

[tinganho]

Sorry forgot to include the errors. I updated the OP.

User extends Model, and M is constrained to be at least Model. I think it is weird why User is not assignable to M?

Here is a playground example:
http://www.typescriptlang.org/play/index.html#src=class%20Model%20%7B%7D%0A%0Aclass%20Collection%3CM%20extends%20Model%3E%20%7B%0A%20%20%20%20m%3A%20M%3B%0A%7D%0A%0Aclass%20User%20extends%20Model%20%7B%0A%7D%0A%0Aclass%20Users%20extends%20Collection%3CUser%3E%20%7B%0A%7D%0A%0Atype%20CollectionClass%20%3D%20typeof%20Collection%3B%0Alet%20a%3A%20CollectionClass%20%3D%20Users%3B

#16368 introduced the compilation flag --noStrictGenericChecks. So I would assume it introduced the breaking change. Because when I compile with that flag, no errors occurs.

[mhegazy]

here is a simpler repro:

//type CollectionClass = typeof Collection;
type CollectionClass = { new <M extends Model>(): Collection<M> };
type UserCollectionClass = { new(): Collection<User> };

declare let a: CollectionClass;
declare let u: UserCollectionClass;

a = u;  // User is not assignable to M

the check here is telling you that you are replacing a generic function (constructor function for Collection<M>) with one that is more specific one (constructor function for UserCollection ) which is not allowed.

[tinganho]

OK I see, thanks for the explanation.