Compiler crash, conditional conformance and standard library protocols

[GreatApe (Gustaf Kugelberg)]

It seems like there wasn’t a formal rule against this, at least in 2018, since Jordan then proposed this as a new rule:

@jrose

It is an error in Swift 5 (warning in Swift 4 mode) to declare a retroactive conformance if both the protocol and the conforming type are from "resilient" libraries or system frameworks.