Segfault in "outlined init with copy"

Apologies if this isn't the right category; I'm not quite sure where to put this.

I've found what appears to be a compiler bug involving opaque types, which causes a segfault at runtime. The code to reproduce it is here:

If I run the code as-is, and enter a valid stack exchange user email and password, I get an error either for a 404 (bug in my own code) or a CAPTCHA (not much I can do about that). However, if I change some of the types in main.swift:

-let httpClient: HTTPClient = HTTPClientImpl()
-let auth: AuthHandler = AuthHandlerImpl(client: httpClient)
+let httpClient: some HTTPClient = HTTPClientImpl()
+let auth: some AuthHandler = AuthHandlerImpl(client: httpClient)

All of a sudden, I get a segfault. According to the debugger, the signal occurs in "outlined init with copy of HTTPClient".

AuthHandlerImpl still stores an any HTTPClient, so I don't see why the declared typings in main.swift should have any effect here. Am I doing something very wrong, or is this an actual bug?

Since it's probably relevant, I'm compiling and running this on an Ubuntu 18.04 VM on a Windows 10 machine (both x86_64).