Better error messages for mixins

[ghost]

TypeScript Version: nightly (2.5.0-dev.20170727)

Issue #4890, "Allow class to extend from a generic type parameter", is marked solved. So let's try it out:

function f<T extends new() => any>(C: T) {
	return class extends C {}
}

Huh? We get src/a.ts(2,23): error TS2507: Type 'T' is not a constructor function type..
That's not too intuitive since it worked fine with C: new() => any.

We actually need:

function g<T extends new(...args: any[]) => any>(C: T) {
	return class extends C {}
}

Maybe the error message should mention something about needing ...args: any[]?

[aluanhaddad]

@andy-ms thank you! I didn't realize the signature had to be exactly new (...args: any[]) => any and was trying to use new (...args: {}[]) => any which currently fails with the not a constructor function type error.

[mhegazy]

@DanielRosenwasser recommendations?

[DanielRosenwasser]

Classes may only extend types bounded by constructors with a single rest parameter of type 'any[]' (e.g. 'new (...args: any[]) => {0}').

Related span:

Did you mean for '{0}' to be declared with type 'new (...args: any[]) => {1}'.

[ghost]

Correction: We actually actually need:

function g<T extends new(...args: any[]) => {}>(C: T) {
	return class extends C {}
}

Returning any is unsafe, see #26286

[thejohnfreeman]

People have been getting this wrong for over a year. Maybe TypeScript should add a constructor<T> built-in type?