Note that go 1.15 is no longer supported, only the two most recent versions are (right now, 1.17 and 1.16).
Anyway the 1.17 compiler crashes on this too.
test.go:37:10: incompatible type: cannot use cw (variable of type *CertWatcher) as CertWatcherIf value
test.go:55:19: incompatible type: cannot use cw (variable of type *CertWatcher) as CertWatcherIf value
Which seems correct to me (so your reproducer does not compile). Possibly already fixed on tip? Do you have a reproducer that does compile?
This seems fixed on tip (so nothing more to do there), and the first version is a program that doesn't compile anyway, so even if it does crash 1.17, I think it may not be worth a backport (to 1.17.2 for example).
After a major version is released (for example go1.17), minor releases are issued (1.17.1, etc...) following this policy:
fixes for security issues, serious problems with no workaround, and documentation fixes are backported to the most recent two release branches, if applicable to that branch.
In my opinion this doesn't qualify for a backport, since it's not a "serious problem without a workaround", given that the code doesn't compile anyway. So it would be fixed in go1.18 when it is released.
Of course if we find a program that should compile (and it crashes the compiler), that would probably qualify for a backport.