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)

Reproduce the TypeScript 2.7 example from the issue or its Playground link, then inspect the class/interface compatibility checking path and compare it with the abstract superclass example. Done means an abstract class can implement an interface without declaring each unimplemented member abstract, while concrete subclasses still satisfy the interface contract; review related issues #19847 and the discussion for constraints.