Type inference issue in subclasses

[tommytroylin]

TypeScript Version: 2.2.2

Code

class A<T> {
  foo: T;
}

class B extends A<'bar'> {
  foo = 'bar';
}

workaround:

class A<T> {
  foo: T;
}

class B extends A<'bar'> {
  foo = 'bar' as 'bar';
}

Expected behavior:
No Error;
Actual behavior:
Type string is not assignable to type 'bar';

I don't know if this behavior is intended.
I think type inference should be more accurate in this case to make programmers not to write as sometype once again.

[aluanhaddad]

The problem is that foo "overrides" (not really but in OO speak) the base property and is allowed to declare a less specific type.

Improved workaround:

class A<T> {
  foo: T;
}

class B extends A<'bar'> {
  readonly foo = 'bar';
}

Note there have been dozens of issues about inheriting exact signatures, and @sandersn worked very hard on multiple potential implementations. See #6118 and #10570 where this is discussed in detail.

Personally, the compile time checked immutability in the above workaround is a nice side-effect at least.