DIP-1000 scope analysis doesn't kick in for small-size-optimized GC-string

[Per Nordlöw]

I'm now satisfied with my SSOString at

https://github.com/nordlow/phobos-next/blob/master/src/sso_string.d

However, I'm having problems within making lifetime analysis via 
scope kick in. For instance, the function `f` at

https://github.com/nordlow/phobos-next/blob/master/src/sso_string.d#L219

shouldn't compile, but it does.

Have I missed something or is this yet another corner-case that 
requires a new bugzilla issue?

Walter?

Andrei?