Fix dlang#10783: Datetime: Make adding a Duration to Date commutative

[Inkrementator]

The types Date, DateTime and TimeOfDay already implement adding and subtracting a Duration to them.

This makes the addition of a Duration commutative via implementing opBinaryRight.

I chose opBinaryRight over Duration.opBinary, since Duration is in druntime, which shouldn't depend on phobos.

Only addition is implemented here. DateTime and Date already principally support negative Dates, i.e.

Date(0, 1, 1) - days(7) // 0001-Dec-25

works and can be interpreted as Before Christ (BC) dates. On the other hand opUnary isn't implemented (yet), which seems like a precondition for making subtraction commutative in a way that makes sense via the transformation:

days(7) - Date(0,1,1)
=> -Date(0,1,1) + days(7)

and even then the semantics of subtracting a Date from a Duration is debatable.

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[Inkrementator]

Github hyperlink: #10783

[jmdavis]

Personally, I don't think that it makes any sense for adding a Duration to a Date to be commutative. Conceptually, the Duration is being added to the Date. The Date is not being added to the Duration. So, I'm against this change.

But even if we decide that we want this change, it's going to need to be made to SysTime as well for consistency.

[Inkrementator]

Conceptually, the Duration is being added to the Date. The Date is not being added to the Duration.

I agree. Otoh, the result of adding a Date to a Duration is obvious and the complexity overhead of supporting it is minimal.

The upsides are similarly limited. Not running into a (quick to fix) error when you accidentally mix up the order of Date and Duration is nice to have. Someone new to the language might not even instantly understand the compiler error:

none of the overloads of template `core.time.Duration.opBinary` are callable using argument types `!("+")(DateTime)`

Having quickly written code "just work" is valuable for the use case of using D instead of a scripting language.

[jmdavis]

I completely disagree with making duration + date work, because it makes no sense conceptually, and the programmer should know what they're adding together, making it easy to figure out what they did wrong if they get the order wrong for some reason. So, I don't see any problem with it being an error. And I have never seen this issue come up in the 15 years or so that std.datetime has existed. So, I don't buy that this is a problem that much of anyone has been running into or that it's something that needs to be solved.

Having quickly written code "just work" is valuable for the use case of using D instead of a scripting language.

This feels to me like an arguing that a misspelling for a function should be added as an alias just because someone might misspell the function when writing code quickly. Just because code is in a script doesn't make it special, and it's plenty fast to write the code correctly in the first place.

[Herringway]

I completely disagree with making duration + date work, because it makes no sense conceptually, and the programmer should know what they're adding together, making it easy to figure out what they did wrong if they get the order wrong for some reason. So, I don't see any problem with it being an error. And I have never seen this issue come up in the 15 years or so that std.datetime has existed. So, I don't buy that this is a problem that much of anyone has been running into or that it's something that needs to be solved.

It is an inconsistency without purpose. That should not be tolerated.

Having quickly written code "just work" is valuable for the use case of using D instead of a scripting language.

This feels to me like an arguing that a misspelling for a function should be added as an alias just because someone might misspell the function when writing code quickly. Just because code is in a script doesn't make it special, and it's plenty fast to write the code correctly in the first place.

As a Canadian occasionally writing colour-related code, I've dealt with this issue a few times...