[Version 11.0] Feature support for static abstract members in interfaces

[RexJaeschke]

This is Rex's adaptation of the corresponding MS proposal.

Issue

The MS spec contains a section called “Pattern matching.” In there, a change to constant patterns is proposed. However, I cannot find the text below they propose augmenting, anywhere in the spec

A constant pattern tests the value of an expression against a constant value. The constant may be any constant expression, such as a literal, the name of a declared const variable, or an enumeration constant. When the input value is not an open type, the constant expression is implicitly converted to the type of the matched expression; if the type of the input value is not pattern-compatible with the type of the constant expression, the pattern-matching operation is an error. If the constant expression being matched against is a numeric value, the input value is a type that inherits from System.Numerics.INumberBase<T>, and there is no constant conversion from the constant expression to the type of the input value, the pattern-matching operation is an error.

Note that the proposed new text refers to System.Numerics.INumberBase<T>, which I mention elsewhere in the PR as needing attention.

[RexJaeschke]

This is the first mention of the type System.Numerics.INumberBase<T> in the spec. Unless we are going to add all the machinery about that type and how built-in numeric types now have that as a base interface, we'll need some alternative wording here.