[phobos] is*Range + Qualified Types

[David Simcha]

  I just wanted to note that this problem is a heck of a lot worse than 
it looks now, and IMHO really requires changes to IFTI to deal with at 
all reasonably.  Also note that it's not even a problem specific to 
ranges, as things as simple as pow() were broken and required some ugly 
kludges to fix, due to similar issues.

I've discovered that huge amounts of code in Phobos are relying on Bug 
3534 (http://d.puremagic.com/issues/show_bug.cgi?id=3534) to work, since 
this allows calling popFront()/popBack() on const/immutable arrays.  I 
tried to change popFront() and popBack() to explicitly prevent them from 
being called on const/immutable arrays, thinking/hoping that not too 
much code would be relying on this bug and that I could fix whatever 
does.  It ended up having ripple effects all over Phobos, breaking 
modules from std.string to std.algorithm to std.xml, so I gave up for now.

I realize D isn't getting tail const, so a perfect general solution is 
out of the question.  However, I do think that a lot of important cases 
(arrays and stuff without mutable indirection) can be solved by making 
IFTI implicitly instantiate with Unqual!T when passed a T, iff T is 
implicitly convertible to Unqual!T.  For example:

struct S {
     int[] arr;
     int i;
}

immutable str = "abc";
immutable num = 1;
immutable S s;

void doStuff(T1, T2, T3)(T1 t1, T2 t2, T3 t3) {}

doStuff(str, num, s);

This would call doStuff!(immutable(char)[], int, immutable(S)). 
immutable(char[]) can implicitly convert to immutable(char)[], so the 
implicit conversion is done. immutable(int) can implicitly convert to 
int, so the conversion is done. immutable(S) cannot implicitly convert 
to S, so the conversion is not done.