[Proposal] List of allowed types as generic constraint

[Expro]

Generic type constraint in form of
public static T Extension<T>(this T) where T : InterfaceOne, InterfaceTwo
Means that T must implement both interfaces: InterfaceOne and InterfaceTwo.

Currently, there is no way to define overloaded (extension) method with same signature, but different constraints. For example, following code is not valid:
public static T Extension<T>(this T) where T : InterfaceOne
public static T Extension<T>(this T) where T : InterfaceTwo

Proposal: allow for listing of type constraints, where constaint will be met if generic parameter meets any of listed types. For example, if we assume following code:
public static T Extension<T>(this T) where T in InterfaceOne, InterfaceTwo
Would allow for any object implementing only InterfaceOne, only InterfaceOne or both.

Alternative syntax:
public static T Extension<T>(this T) where T from InterfaceOne, InterfaceTwo

[benaadams]

public static T Extension<T>(this T) where T : InterfaceOne
public static T Extension<T>(this T) where T : InterfaceTwo

Would be better (perhaps via garbled names under the covers for the clr)

Otherwise it would potentially lead to runtime type casting in a shared generic which would be inefficient vs a compile time link up.

Related #1628

[ufcpp]

Union types?

[YairHalberstadt]

Would definitely aid on defining a method on eg byte, int16, int32, int64, float, double, decimal etc all at once.
However shapes may solve this issue.

[YairHalberstadt]

Closing as a duplicate of #1371