What language proposals would benefit from CLR changes?

[gafter]

If we were to plan for one language feature that requires a revision of the CLR, then we might as well do as many of them at the same time as make sense. What changes would we consider for C# 7 and VB 15 that would benefit from CLR support? See #166 for related discussion. This is a pared-down list for us to select from.

1. Virtual extension methods (see Java-like virtual extension methods/default interface implementations, aka traits or (ruby-like) mixins #73, Proposal: Virtual extension methods #258)
2. Generic constraints about static methods or constructors (see also (Proposal) Concepts/Traits (enhanced generic constraints) #129, #154).
3. Array slicing (see Proposal: Slicing #120)
4. Language support for tuples (see Proposal: Language support for Tuples #347) and function types (no issue yet) might benefit from CLR unification across assemblies.
5. Traits (see [Proposal] Add support for traits and/or mixins #60) aka structural interfaces (see #154) or mixins (no issue yet)

Those that we would likely not do in this timeframe include

1. Make void a first-class type with one value; see Suggestion: void as a first-class citizen #234 (Suggested by @ashmind) (Given how long the CLR has been around, it is probably too late for APIs to benefit, as those that would benefit most have already been written)
2. Covariance and contravariance for classes (see Allow type parameters to be declared co/contra variant on classes. #171)
3. Allow |, &, and ~ operators on a type parameter with the enum constraint (see Proposal: support an enum constraint on generic type parameters #262)
4. Intersection types (Proposal: Implicit Interfaces #2146, though likely not that syntax) and/or union types.
5. Support generic indexers (see [Proposal] Generic indexers #523)
6. Higher-kinded polymorphism (see Higher Kinded Polymorphism / Generics on Generics #2212)

[sharwell]

I noticed you omitted the generic indexers request. Was that intentional, or perhaps it is missing because I didn't submit a complete proposal?

[ashmind]

Language support for tuples might benefit from CLR unification across assemblies; see #347.

Just to add, other uses are function types and ASP.NET's [AssemblyNeutral].
Would it be useful for you if I outlined a design suggestion? And if I do, should it go here or to https://github.com/dotnet/coreclr ?

[AdamSpeight2008]

A de-facto Option(Of T).

[gafter]

@sharwell Can you please point me to the indexers issue you're talking about?

@ashmind Language suggestions belong here. CLR suggestions belong there. I'll add function types to the list of language features that would benefit from unification.

@AdamSpeight2008 I don't know what you're suggesting that would benefit from CLR support.

[mikedn]

#262 might also need runtime support. Enum constraints are supported in CLR but to be really useful it would be nice to also support bitwise operations for flags. & and | work but ~ sometimes requires a conversion that cannot be expressed in IL.

[gafter]

@mikedn Added to the list (as something we probably would not do).