TypeScript: Issue a custom error message when trying to assign constraint type to generic type parameter

I find this error message undecipherable. I have lots of this type of error message:

‘A’ is assignable to the constraint of type ‘B’, but ‘B’ could be instantiated with a different subtype of constraint ‘{}’.

The last placeholder is always, the {}. I have no idea what this means or what the fix is.

Is it relevant to include some sort of “you should consider doing X, Y or Z to solve this problem” ?

It could speed up the process of finding a solution for new comers, as it is not really easy to understand, and there are not many available resources online,

I managed to found this : https://stackoverflow.com/a/56701587/6790372

Could you at least modify your first post in this issue, to explain why each case does not work, and what could be done instead ?

@RyanCavanaugh function fn1<T extends string>(x: T): T { return "hello world!"; } How to resolve this error? Change to return "hello world" as T?

My 2cents on this matter is that TS seems to punish you for using default params. Makes sense to me for this to be a warning instead.

@RyanCavanaugh says:

People are regularly confused why code like this doesn’t work:

function fn1<T extends string>(x: T): T {
 return "hello world!";
}

function fn2<T extends HTMLElement>(obj: T): T[] {
 return [obj, new HTMLDialogElement()];
}

function fn3<T extends boolean>(obj: T = false) {
}

So, did confused reduce after https://github.com/microsoft/TypeScript/pull/30394? I highly doubt it. Please describe this case in the documentation. At the moment you can find a sea of questions about this.

@moccaplusplus I agree that there is a fine balance between being informative and concise. IMO the extra information is helpful; as you can see I’ve been collating issues that trigger this error, so it’s something people definitely come across.

Generics are a hard subject, subtyping is a hard subject, and variance is a hard subject—this error is a combination of all three. I think spelling out the problem in a more concrete way is a definite improvement.