[Issue 259] Comparing signed to unsigned does not generate an error

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=259



--- Comment #35 from Andrei Alexandrescu <andrei at erdani.com> 2013-04-07 18:38:23 PDT ---
Great. Here's a solution Walter and I just discussed:

Consider a comparison a < b, a <= b, a > b, or a >= b, in which a and b are
integral types of different signedness. Without loss of generality, let's
consider a is signed and b is unsigned and the comparison is a < b. Then we
have the following cases:

1. If a.sizeof > b.sizeof, then a < b is lowered into a < cast(typeof(a)) b.
Then signed comparison proceeds normally. This is a classic value-based
conversion dating from the C days, and we do it in D as well.

2. Otherwise, if a is determined through Value Range Propagation to be greater
than or equal to zero, then a < b is lowered into cast(U) a < b, where U is the
unsigned variant of typeof(a). Then unsigned comparison proceeds normally.

3. Otherwise, the comparison is in error. The error message may recommend using
the std.traits.unsigned function, which executes a size-informed cast.

Walter, if you agree with this resolution please mark this as "preapproved".

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