Why can `any Shape` not conform to `Shape`

[jamieQ (Jamie)]

i found this quote from @xwu in a recent discussion regarding the motivation for why protocol existentials in general do not self-conform helpful:

@xwu

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.

i've also found this document regarding some of the differences between 'nominal' and 'non-nominal' types clarifying.