Consider the following code:
```
class A {
     virtual int foo() = 0;
};

class B final : A {
     int foo() override; // (*)
};

int B::foo() { return 123; }
```
... which fails to compile if we remove the declaration (*).

Since `B` is a final class, it must necessarily have all methods defined
immediately or by inheritance. Specifically, we must define all `B`
methods which are not defined for its (single) base class `A`.

Why, then, must `B` declare `foo()` explicitly? Why is the implicit need
for the definition not sufficient to allow this code to compile? I mean,
I assume the standard requires an explicit declaration here, but what is
the rationale for this requirement?

An obvious counter-rationale is DRY: If class A has N virtual methods
and M final subclasses, we'll have to write M*N redundant declarations.
And these redundant declarations also visually "mask", to some extent,
the declarations of those methods which are specific to B.