Fully transitive const is not necessary

[Walter Bright]

If you do away with transitive const, you cannot have invariant either. 
Const allows you to reuse code that works on invariants to work with 
mutables, too.

Logical const just utterly breaks the whole system. Every scheme we've 
looked at for logical const winds up in some way breaking invariants. If 
invariants are broken, the advantages of them all just go away. I 
suspect that logical const is like perpetual motion - if you think 
you've solved it, you've just made a mistake somewhere <g>. I also 
suspect that the failure of logical const validates the D scheme of 
invariants as being correct - there shouldn't be any holes in it.

You're right that invariant by itself is not enough to specify a pure 
function, a pure function also cannot read or write global state. But 
invariant is still a necessary condition, it's just not sufficient.

Peoples' troubles with const seem to stem from attempting to defeat it 
in one way or another. While defeating const is legal and trivial in 
C++, D tries to close those doors.