On Thursday, 17 March 2016 at 17:09:46 UTC, Steven Schveighoffer
wrote:
Converting unsigned to signed or vice versa (of the same size
type) is safe. No information is lost.
Saying that "no information is lost" in such a case, is like
saying that if I encrypt my hard drive and then throw away the
password, "no information is lost". Technically this is true: the
bit count is the same as it was before.
In practice, though, the knowledge of how information is encoded
is essential to actually using it.
In the same way, using `cast(ulong)` to pass `-1L` to a function
that expects a `ulong` results in a de-facto loss of information,
because that `-1L` can no longer distinguished from `ulong.max`,
despite the fundamental semantic difference between the two.
VRP on steroids would be nice, but I don't think it's as
trivial to solve.
D's current VRP is actually surprisingly anemic: it doesn't even
understand integer comparisons, or the range restrictions implied
by the predicate when a certain branch of an `if` statement is
taken.
Lionello Lunesu made a PR a while back that adds these two
features, and it makes the compiler feel a lot smarter. (The PR
was not accepted at the time, but I have since revived it:
https://github.com/D-Programming-Language/dmd/pull/5229)
