[3.7.0-beta] Child class cannot override base member function with arrow function

[MLoughry]

TypeScript Version: 3.7.0-dev.20190927 (works on 3.7.0-dev.20190926)

Search Terms:
TS2424, member function

Code

class BaseClass {
    foo() {
        return null;
    }
}

class ChildClass extends BaseClass {
    foo = () => null;
}

Expected behavior:
In 3.6.3 and <=3.7.0-dev.20190926, this code would compile without error

Actual behavior:

error TS2424: Class 'BaseClass' defines instance member function 'foo', but extended class 'ChildClass' defines it as instance member property.

8   foo = () => null;
    ~~~

Since our code has to compile down to ES5 (to support IE11), we need to perform some sort of method binding on methods that reference this. With this change, it means we would need to make all possibly-overridable member functions arrow functions

Playground Link: http://www.typescriptlang.org/play/index.html?ts=3.7-Beta#code/MYGwhgzhAEBCkFMDC4rQN4Cho+gMwHsCAKASg21yoCcEAXAV2oDtpmGQQBuSnAX0wDMoSDCQALAJYgAJilHQEADzoJmMmPAjJUMLFUIFoAXmhkTAPjYdugoA

Related Issues:

[sandersn]

I looked at the change from TS 3.0 again and found that it was supposed to remove only this error but actually removed it as well as the opposite:

class B {
  foo = () => null
}
class C extends B {
  foo() {
    return null
  }
}

I restored both errors in #33509 while adding new errors -- I noticed dead code and confused an intern's first check-in with a broken change.

I'll take this error (property-overrides-method) back out, but leave in the error for method-overrides-property.

[MLoughry]

Thanks for the quick fix, @sandersn

[ulrichb]

Hmm, this was released in TS@next ~ 3 days ago, but since typescript@3.7.0-dev.20191018 the same error appeared again :/