Inheritance of purity

[Walter Bright]

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?