Abstract classes that implement interfaces shouldn't require method signatures

TypeScript Version: 2.7

Search Terms: abstract class implements interface

Code

interface FooFace {
    foo();
}
abstract class FooClass implements FooFace {
    //         ^^^^^^^^
    // Class 'FooClass' incorrectly implements interface 'FooFace'.
    //   Property 'foo' is missing in type 'FooClass'.
    //
    // unless the following is uncommented:
    //abstract foo();
}

// contrast:
abstract class BarClass {
    abstract bar();
}
abstract class BarSubClass extends BarClass {
    // no `abstract bar();` required
}

Expected behavior: Abstract classes that implement interfaces don’t need to define abstract type definitions to satisfy interface.

Actual behavior: They do.

Playground Link

Related Issues:

• #19847, where typescript fails to see that a superclass does implement a method (contrast this ticket, where typescript fails to see that a class need not implement a method)

[syffs]

2 years later…

this seems like an obvious and basic feature for any object-oriented language: how come it hasn’t been implemented ?

[laughinghan]

Apparently this can be worked-around with class and interface merging:

interface FooFace {
    foo();
}
abstract class FooClass implements FooFace {}
interface FooClass extends FooFace {}

let x: FooClass;
x.foo(); // no error

That seems redundant, though, I still think this should be fixed.

https://github.com/Microsoft/TypeScript/issues/6413#issuecomment-170737032