Do pure functions solve the "return const" problems?

[Russell Lewis]

Steven Schveighoffer wrote:
> Having never used pure functions, I'm not sure that this is the case for 
> C++'s pure functions, but I thought pure functions for D would require 
> invariant arguments, no?  Otherwise, how do you guarantee another thread 
> doesn't come along and munge the data while the pure function is running?

Interesting question, I don't really know the answer.  However, it seems 
to me that "pure" is a statement that you are making about the function, 
not necessarily about the caller.  That is, "pure" means "this function 
does not have side-effects, so use it with impunity," whereas 
"invariant" means "the caller must obey rules that the function is 
relying on."

Put another way, saying that a function is "pure" allows the caller to 
make certain optimizations.  Saying that the arguments are invariant 
allows the pure function to optimize internally.

I think that it might be quite possible to have a pure function which 
doesn't require invariant arguments...it would just lose out on internal 
optimization opportunities.