`Instant::checked_add_duration` claims to check for overflow, doesn't

[Dridus]

use of wrapping_add where the bases are equal, here: https://github.com/korken89/fugit/blob/master/src/instant.rs#L171 and similarly if bases are different, here: https://github.com/korken89/fugit/blob/master/src/instant.rs#L181

claim that it checks for overflow, here: https://github.com/korken89/fugit/blob/master/src/instant.rs#L156

evcxr transcript showing that it wraps:

>> :dep fugit = "0.3.6"
>> fugit::Instant::<u32, 1, 1000>::from_ticks(0xffff_ffff) + fugit::Duration::<u32, 1, 1000>::from_ticks(10)
Instant { ticks: 9 }

[korken89]

Hi, this is intended behavior. fugit's Instant represents a wrap-able time of MAX/2 -1 span (think iXX semantics). The overflow check that it does is checking whether the change of base in checked_add_duration would cause overflows.

[Dridus]

it was pretty surprising to me that the implementation of Add wrapped, as usually I have to reach for wrapping_add or similar to get that effect rather than panics, but Add doesn't make that behavior explicit in its docs, it's just the default it seems for most numeric types.

that said, if the current behavior is intended, may I suggest expanding the doc comment to make it clear what kind of overflow is being checked for?