Don't disallow adding a DIM to an interface

[gewarren]

Fixes #42748.
Also reorganizes interface keyword doc as it didn't seem to flow properly.

cc @jskeet.

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Internal previews

📄 File	🔗 Preview link
docs/core/compatibility/library-change-rules.md	Change rules for compatibility
docs/csharp/language-reference/keywords/interface.md	:::no-loc text="interface"::: (C# Reference)
docs/csharp/whats-new/csharp-version-history.md	The history of C#

[gewarren]

This list got orphaned at some point.

[jskeet]

Thanks so much for this!

[BillWagner]

Thanks @gewarren

This looks good, with a few small changes.

[BillWagner]

This is no longer true. Let's change it:

Suggested change

	Interface members without an implementation can't include an access modifier. Members with a default implementation can include any access modifier.
	Interface members without an implementation are `public` by default. Members with an implementation are `private` by default. Interface members can include any access modifier.

[BillWagner]

add clarifying comment:

Suggested change

	- Explicit access modifiers (the default access is [`public`](access-modifiers.md)).
	- Explicit access modifiers (the default access for abstract methods is [`public`](access-modifiers.md)).

[BillWagner]

contraction:

Suggested change

Interfaces cannot contain instance state. While static fields are now permitted, instance fields aren't permitted in interfaces. [Instance auto-properties](../../programming-guide/classes-and-structs/auto-implemented-properties.md) aren't supported in interfaces, as they would implicitly declare a hidden field. This rule has a subtle effect on property declarations. In an interface declaration, the following code doesn't declare an automatically implemented property as it does in a `class` or `struct`. Instead, it declares a property that doesn't have a default implementation but must be implemented in any type that implements the interface:

Interfaces can't contain instance state. While static fields are now permitted, instance fields aren't permitted in interfaces. [Instance auto-properties](../../programming-guide/classes-and-structs/auto-implemented-properties.md) aren't supported in interfaces, as they would implicitly declare a hidden field. This rule has a subtle effect on property declarations. In an interface declaration, the following code doesn't declare an automatically implemented property as it does in a `class` or `struct`. Instead, it declares a property that doesn't have a default implementation but must be implemented in any type that implements the interface:

[BillWagner]

Adding one more condition:

Suggested change

If you [provide an implementation](../../csharp/advanced-topics/interface-implementation/default-interface-methods-versions.md), adding a new member to an existing interface won't necessarily result in compile failures in downstream assemblies. However, not all languages support DIMs. Also, in some scenarios, the runtime can't decide which default interface member to invoke. For these reasons, use judgment when adding a member to an existing interface.

If you [provide an implementation](../../csharp/advanced-topics/interface-implementation/default-interface-methods-versions.md), adding a new member to an existing interface won't necessarily result in compile failures in downstream assemblies. However, not all languages support DIMs. Also, in some scenarios, the runtime can't decide which default interface member to invoke. In some scenarios, interfaces are implemented by `ref struct` types. Because `ref struct` types can't be boxed, they can't be converted to interface types. Therefore, `ref struct` types must provide an implicit implementation for every interface member. They can't make use of the default implementation provided by the interface. For these reasons, use judgment when adding a member to an existing interface.