Compiler bug w.r.t. protocol inheritance and generics?

[xwu (Xiaodi Wu)]

@Slava_Pestov

If the protocol has associated types, static methods or inits, there would be a soundness issue in allowing it to self-conform. If it doesn't have any requirements of that sort, the issue is that the runtime representation doesn't work out.

A protocol without associated types, static methods, or inits can still have semantic requirements that make self-conformance unsound.

Consider a hypothetical marker protocol, HasTwoValues—all conforming types have two possible values. Thus, Bool can conform to HasTwoValues. So can, for example, a hypothetical type ChessPlayer. If both types conform, then the existential type any HasTwoValues now has (at least) four values—therefore, it does not meet the semantic requirement of HasTwoValues.