Ranges longer than size_t.max

[Peter Alexander]

There was a discussion a while ago about the type returned by 
.length for ranges. I believe the conclusion was that it should 
always be size_t.

My question now is what to do with really large ranges? For 
example, if we have a range of permutations of the elements in a 
set, when the set is larger than 21 the number of permutations is 
21 factorial, which is over 2^64. As far as I see, there are 
three options:

1. Allow ranges to return BigInt for length.
2. Allow ranges like this but assert when .length overflows.
3. Allow ranges like this and just allow .length to overflow 
dangerously.
4. Do not allow ranges like this.

Option 1 solves the problem, but significantly complicates Phobos 
development for the rare case.

Option 2 works, and is practical, although runtime asserts are 
undesirable.

Option 3 is current Phobos practice (presumably because overflow 
is unlikely). See example below. This may be acceptable 
currently, but perhaps less so when overflow is more likely.
--------------------------------
auto a = iota(size_t.max / 2 + 1);	
auto b = chain(a, a);
writeln(a.length); // 9223372036854775808
writeln(b.length); // 0
--------------------------------

Option 4 works, but it would be a shame to lose these ranges.

Thoughts?