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

[RyanCavanaugh]

TypeScript Version: master

Search Terms: generic constrained type parameter error

Code

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) {
}

Actual behavior: Unelaborated error like "Can't assign false to T"

Expected behavior:

If the source type is assignable to the constraint of a type parameter, we should issue a more specific error that attempts to explain the situation:

🚲 🏠

Type "false" is not assignable to type "T"
  "false" is assignable to the constraint type "boolean", but "T" could be a different more-specific type during a call

Playground Link: Link

[jack-williams]

Very much agree with the suggestion. To throw my unicycle into the arena:

Type "false" is not assignable to type "T"
  "false" is assignable to the constraint of type "T", but not all possible instantiations of type "T".

[DanielRosenwasser]

Agree with @jack-williams since this can happen in a class too. I think the following combines the best of both:

'{0}' is assignable to the constraint of type '{1}', but '{1}' could be instantiated with a different type.

Also, when not assignable, or when there's an implicit constraint, but the RHS is not instantiable, I could see an elaboraron like

Concrete types are never assignable to uninstantiated types.

Or something like that.

[RyanCavanaugh]

I think the problem with could be instantiated with a different type is that the user reaction would probably be "You mean the constraint doesn't do anything?".

What about:

'{0}' is assignable to the constraint of type '{1}', but '{0}' could be instantiated with a different subtype of `{1}`.

[jack-williams]

That's a good point @RyanCavanaugh. I'm not sure if I've substituted in the placeholders correctly; is this right?

'false' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of 'boolean'.

In that instance I would probably make it explicit that boolean is the constraint, as I feel it pops out of nowhere abit. So:

'false' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint 'boolean'.

[RyanCavanaugh]

'false' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint 'boolean'.

❤️ !

[a-tarasyuk]

@DanielRosenwasser Seems, that the error needs to add in the method isRelatedTo(, after the condition else if (isComparingJsxAttributes && target.flags & TypeFlags.Intersection) {. Is there any method to determinate generic definition? I've found isGenericObjectType(target) - Is it right direction for this?

[DanielRosenwasser]

Since you're just specializing the error, target.flags & TypeFlags.TypeVariable is probably a good first approximation. I haven't been deep in the codebase in a while, but others reviewing might be able to help you iterate on that.

[jack-williams]

@a-tarasyuk I think structuredTypeRelatedTois probably where you want to look.

[kunlongxu]

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