Added LDM notes for February 8th, 2021
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meetings/2021/LDM-2021-02-08.md
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# C# Language Design Meeting for Feb 8th, 2021
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## Agenda
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1. Virtual statics in interfaces
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1. [Syntax Clashes](#syntax-clashes)
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2. [Self-applicability as a constraint](#self-applicability-as-a-constraint)
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3. [Relaxed operator operand types](#relaxed-operator-operand-types)
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4. [Constructors](#constructor)
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## Quote(s) of the Day
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- "If you need to kick yourself I see that [redacted] has a foot in the chat you can kick yourself with"
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- "Are we at the point where we derail your meeting with other proposals?"
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- "It's the classic, noble art of retconning"
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## Discussion
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Today, we want to take a look at a few top-level design decisions for virtual statics in interfaces that will drive further design and implementation
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decisions for this feature.
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### Syntax Clashes
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C# 8 brought 2 new features to interfaces: default members, and non-virtual static members. This sets up a clash between static virtual members and
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static non-virtual members. In pre-C# 8, `interface` members were always virtual and abstract. C# 8 blurred the line here: private and static members
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in interfaces are non-virtual. For private members, this makes perfect sense, as the concept of a virtual private method is an oxymoron: if you can't
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see it, you can't override it. For statics, it's a bit more unfortunate because we now have inconsistency in the default-virtualness of members in
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the interface. We have a few options for addressing this inconsistency:
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1. Accept life as it is. We'd require `abstract` and/or `virtual` on static members to mark them as such. There is some benefit here: `static` has
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meant something for 21 years in C#, and that is not `virtual`. Requiring a modifier means this does not change.
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2. Break the world. Make static members in interfaces mean static virtual by default. This gets us consistency, but breaks consumers in some fashion
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and/or introduces multiple dialects of C# to support.
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3. We could introduce a bifurcation in interfaces with a `virtual` modifier on the `interface` declaration itself. When marked with `virtual`, `static`
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members of the interface are automatically `virtual` by default. This is very not C#-like: we require `static` on all members of `static class`es, why
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would `virtual interface`s be any different here? And how would you define the existing non-`virtual` members in such an interface?
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Options 2 and 3 also have a question of how they will apply to class members. Due to the size of the changes required we may have to split virtual
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static members, shipping with just interfaces in the first iteration and adding class types at a later point. However, we need to make sure that we
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design the language side of these changes together, so when class virtual statics are added they don't feel like an afterthought. The second and third
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proposals would likely need to have the first proposal for the class version of the feature anyway. While it would be consistent with instance members,
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neither of them would totally eliminate the needs to apply the modifiers to interface members.
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#### Conclusion
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We will require `abstract` or `virtual` be applied to a virtual static member. We will also look at allowing these modifiers for intstance interface
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methods, even though they are redundant, much like we allow `public` on members today.
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### Self-applicability as a constraint
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`abstract` static members introduce an interesting scenario in which an interface is no longer valid as a substitute for a type parameter constrained
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to that interface type. Consider this scenario:
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```cs
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interface IHasStatics
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{
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abstract static int GetValue(); // No implementation of GetValue here
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}
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class C : IHasStatics
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{
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static int GetValue() => 0;
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}
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void UseStatics<T>() where T : IHasStatics
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{
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int v = T.GetValue();
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}
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UseStatics<C>(); // C satisfies constraint
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UseStatics<IHasStatics>(); // Error: IHasStatics doesn't satisfy constraint
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```
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The main question here is what do we do about this? We have 2 paths:
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1. Forbid interfaces with an abstract static from satisfying a constraint on itself.
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2. Forbid access to static virtuals with a type parameter unless you have an additional constraint like `concrete`.
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Option 2 seems weird here. Why would a user have constrained to a type that implements an interface, rather than just taking the interface, unless
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they wanted to use these methods? Yes, adding an `abstract static` method to an interface where one does not exist today would be a breaking change
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for consumers, but that's nothing new: that's why we added DIMs in the first place, and it would continue to be possible to avoid the break by providing
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a default method body for the virtual method, instead of making it abstract.
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#### Conclusion
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We choose option 1.
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### Relaxed operator operand types
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Today, C# requires that at least one of the operand-types of a user-defined operator be the current type. This breaks down with user-defined virtual
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operators in interfaces, however, as the type in the operator won't be the current type, it will be some type derived from the current type. Here,
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we naturally look at self types as a possible option. We are concerned with the amount of work that self-types will require, however, and aren't sure
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that we want to tie the shipping of virtual statics to the need for self types (and any other associated-type feature). We also need to make sure that
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we relax operators enough, and define BCL-native interfaces in a way, such that asymmetric types are representable. For example, a matrix type would
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want to be able to add a matrix and a numeric type, and return a matrix. Or `byte`'s `+` operator, which does not return a `byte` today. Given that,
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we think it is alright to ship this feature and define a set of operator interfaces without the self type, as we would likely be forced to not use it
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in the general interfaces anyway to keep them flexible enough for all our use cases.
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#### Conclusion
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We're ok with relaxing the constraints as much as we need to here. We won't block on self types being in the language.
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### Constructors
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Finally, we looked at allowing or disallowing constructors as virtual in interfaces. This is an interesting area: either derived types would be required
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to provide a constructor that matches the interface, or derived types would be allowed to not implement interfaces that their base types do. The feature
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itself is a parallel to regular static methods; in order to properly describe it in terms of static methods, you'd need to have a self type, which is
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what makes it hard to describe in today's C# terms in the first place. Adding this also brings in the question of what do we do about `new()`? This may
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be an area where we should prefer a structural approach over a nominal approach, or add a form of type classes to the language: if we were to effectively
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deprecate `new()`, that would mean that every type that has a parameterless constructor would need to implement an `IHasConstructor` interface instead.
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And we would need to have infinite variations of that interface, and add them to every type in the BCL. This would be a serious issue, both in terms of
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sheer surface area required and in terms of effect on type loads and runtime performance penalties for thousands and thousands of new types.
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#### Conclusion
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We will not have virtual constructors for now. We think that if we add type classes (and allow implementing a type class on a type the user doesn't own),
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that will be a better place for them. If we want to improve the `new()` constraint in the mean time, we can look at a more structural form, and users
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can work around the lack of additional constraints for now by using a static virtual.
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@ -43,15 +43,20 @@ First class native integer support - https://github.com/dotnet/csharplang/issues
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- Global usings
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- Global usings
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- *Triage championed features and milestones*
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- *Triage championed features and milestones*
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## Feb 8, 2021
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- Statics in interfaces
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# C# Language Design Notes for 2021
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# C# Language Design Notes for 2021
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Overview of meetings and agendas for 2021
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Overview of meetings and agendas for 2021
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## Feb 8, 2021
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[C# Language Design Notes for February 8th, 2021](https://github.com/dotnet/csharplang/blob/master/meetings/2021/LDM-2021-02-08.md)
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1. Virtual statics in interfaces
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1. Syntax Clashes
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2. Self-applicability as a constraint
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3. Relaxed operator operand types
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4. Constructors
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## Feb 3, 2021
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## Feb 3, 2021
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[C# Language Design Notes for February 3rd, 2021](https://github.com/dotnet/csharplang/blob/master/meetings/2021/LDM-2021-02-03.md)
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[C# Language Design Notes for February 3rd, 2021](https://github.com/dotnet/csharplang/blob/master/meetings/2021/LDM-2021-02-03.md)
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