Subtracting thermodynamic temperatures should produce a temperature interval

[ryanolf-tb]

I have an application where I need to take a difference of thermodynamic temperatures, and my expectation would be that this produces a temperature interval. At present, this operation is not allowed.

[iliekturtles]

An additional Sub impl should be added and it looked trivial so I tried and get the following error. TLDR: without specialization the default Sub impl conflicts with the desired impl and there isn't a way to add a negative bound to the new impl that is the opposite of D::Kind: $crate::marker::$AddSubTrait,.

I'll keep thinking on ways to work around the issue without specialization.

   Compiling uom v0.33.0 (/home/mjboutin/uom)
error[E0119]: conflicting implementations of trait `std::ops::Sub<si::Quantity<(dyn si::Dimension<M = typenum::Z0, Kind = (dyn si::marker::TemperatureKind + 'static), N = typenum::Z0, L = typenum::Z0, I = typenum::Z0, T = typenum::Z0, J = typenum::Z0, Th = typenum::PInt<typenum::UInt<typenum::UTerm, typenum::B1>>> + 'static), _, _>>` for type `si::Quantity<(dyn si::Dimension<M = typenum::Z0, Kind = (dyn si::marker::TemperatureKind + 'static), N = typenum::Z0, L = typenum::Z0, I = typenum::Z0, T = typenum::Z0, J = typenum::Z0, Th = typenum::PInt<typenum::UInt<typenum::UTerm, typenum::B1>>> + 'static), _, _>`
   --> src/system.rs:395:17
    |
395 | /                 impl<D, Ul, Ur, V> $crate::lib::ops::$AddSubTrait<Quantity<D, Ur, V>>
396 | |                     for Quantity<D, Ul, V>
397 | |                 where
398 | |                     D: Dimension + ?Sized,
...   |
413 | |                     }
414 | |                 }}
    | |_________________^ conflicting implementation for `si::Quantity<(dyn si::Dimension<M = typenum::Z0, Kind = (dyn si::marker::TemperatureKind + 'static), N = typenum::Z0, L = typenum::Z0, I = typenum::Z0, T = typenum::Z0, J = typenum::Z0, Th = typenum::PInt<typenum::UInt<typenum::UTerm, typenum::B1>>> + 'static), _, _>`
    |
   ::: src/si/thermodynamic_temperature.rs:177:1
    |
177 | / impl<Ul, Ur, V> crate::lib::ops::Sub<ThermodynamicTemperature<Ur, V>>
178 | |     for ThermodynamicTemperature<Ul, V>
179 | | where
180 | |     Ul: super::Units<V> + ?Sized,
...   |
193 | |     }
194 | | }
    | |_- first implementation here
    |
   ::: src/si/mod.rs:10:1
    |
10  | / system! {
11  | |     /// [International System of Quantities](https://jcgm.bipm.org/vim/en/1.6.html) (ISQ).
12  | |     ///
13  | |     /// ## Generic Parameters
...   |
112 | |     }
113 | | }
    | |_- in this macro invocation
    |
    = note: this error originates in the macro `impl_ops` (in Nightly builds, run with -Z macro-backtrace for more info)

For more information about this error, try `rustc --explain E0119`.
error: could not compile `uom` due to previous error

[ryanolf-tb]

I'm a bit confused about how the default Sub impl conflicts when it seems the compiler doesn't have an implementation for Sub for thermodynamic temperatures.

[iliekturtles]

The error in my previous comment was from an attempt to add a new Sub impl for ThermodynamicTemperature - TemperatureInterval. This new impl conflicts with the default Sub impl.

uom/src/system.rs

Lines 395 to 414 in 974449f

	impl<D, Ul, Ur, V> $crate::lib::ops::$AddSubTrait<Quantity<D, Ur, V>>
	for Quantity<D, Ul, V>
	where
	D: Dimension + ?Sized,
	D::Kind: $crate::marker::$AddSubTrait,
	Ul: Units<V> + ?Sized,
	Ur: Units<V> + ?Sized,
	V: $crate::num::Num + $crate::Conversion<V>,
	{
	type Output = Quantity<D, Ul, V>;
	#[inline(always)]
	fn $addsub_fun(self, rhs: Quantity<D, Ur, V>) -> Self::Output {
	Quantity {
	dimension: $crate::lib::marker::PhantomData,
	units: $crate::lib::marker::PhantomData,
	value: self.value $addsub_op change_base::<D, Ul, Ur, V>(&rhs.value),
	}
	}
	}}

[ryanolf-tb]

Why doesn't the default Sub impl work? There is some Rust subtlety here that I'm not seeing. Sorry! It seems odd that there's a conflicting implementation but that the compiler can't actually use that implementation for the operation on the given types.

[iliekturtles]

The default impl is for any Quantity - Quantity where the quantity's dimension's kind supports add/sub (line 399). ThermodynamicTemperature and TemperatureInterval are just type aliases for Quantity so they are covered by this default impl. However, because ThermodynamicTemperature's kind does not support add/sub the default impl does not take effect. A new specialized impl can't be added because the type aliases boil down to the same types in the default impl. If there was some way to add a negative trait bound then this could be used to make the two disjoint.