I'm not sure why this is a counter-example: the implicit declaration
would only be for the _missing_ methods - those which must be declared
for C to be used.

On 15/08/2020 13:00, Andrey Semashev via Std-Discussion wrote:
> On 2020-08-15 12:01, Eyal Rozenberg via Std-Discussion wrote:
>> 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.
>
> A counter-example:
>
> class A {
> virtual int foo() = 0;
> };
>
> class B : A {
> int foo() override;
> };
>
> class C final : B {
> };
>
> C* p = new C();
> p->foo();
>
> If the declaration of C::foo was implicit, the compiler would have
> mistakenly generated a call to C::foo even if it doesn't exist.
> --
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