mod operator should narrow integer type

[shawnl]

This should compile (except for the other error...):

pub fn main() void {
var c: u32 = 1235;
        return switch (c % 4) {
        0 => 0,
        1 => 1,
        2 => 2,
        3 => 3,
        };
}

But instead:

/home/shawn/projects/shared/switch.zig:3:16: error: switch must handle all possibilities
        return switch (c % 4) {
               ^

[andrewrk]

This is planned. Related: #509

[shawnl]

I don't expect it to work without an else => unreachable when the mod is not a power of 2.

[shawnl]

Actually, after reading #509 I think this is a bad idea. I wasn't thinking about when the second argument to the mod function isn't known compile-time (which means you can't narrow the type).

Oh, now I see it only applies in very few cases, where value-range-propagation can work. Yeah that is OK.

[andrewrk]

This is still planned.