Inheritance of purity

[Timon Gehr]

On 02/26/2012 06:39 PM, Jason House wrote:
> On Friday, 17 February 2012 at 02:49:40 UTC, Walter Bright wrote:
>> Given:
>>
>> class A { void foo() { } }
>> class B : A { override pure void foo() { } }
>>
>> This works great, because B.foo is covariant with A.foo, meaning it
>> can "tighten", or place more restrictions, on foo. But:
>>
>> class A { pure void foo() { } }
>> class B : A { override void foo() { } }
>>
>> fails, because B.foo tries to loosen the requirements, and so is not
>> covariant.
>>
>> Where this gets annoying is when the qualifiers on the base class
>> function have to be repeated on all its overrides. I ran headlong into
>> this when experimenting with making the member functions of class
>> Object pure.
>>
>> So it occurred to me that an overriding function could *inherit* the
>> qualifiers from the overridden function. The qualifiers of the
>> overriding function would be the "tightest" of its explicit qualifiers
>> and its overridden function qualifiers. It turns out that most
>> functions are naturally pure, so this greatly eases things and
>> eliminates annoying typing.
>>
>> I want do to this for @safe, pure, nothrow, and even const.
>>
>> I think it is semantically sound, as well. The overriding function
>> body will be semantically checked against this tightest set of
>> qualifiers.
>>
>> What do you think?
>
> I'm still not convinced about this apply to const. Consider this example:
>
> Initial code:
> class A{
> void foo(int) const;
> void foo(float) const;
> }
> class B{
> alias A.foo foo;
> override void foo(int);
> }
>
>
> Revision to class A:
> class A{
> void foo(int);
> void foo(int) const;
> void foo(float);
> void foo(float) const;
> }
>
> When the user recompiles, there will be no errors or warnings. All uses
> of foo(int) through a const B will revert to using the base class's
> implementation.

This is by far not the only hijacking scenario enabled by using alias 
for merging in the parent's overload set. re-implementing the method and 
calling super is the only safe way to warrant hijacking protection.