This bug was introduced in 39dde09, which use Curfn.Func.lsym unconditionally. For function name _, Curfn.Func.lsym is nil. The bug exists because for function name _, we don't generate code but still walk through it.
This was partially fixed in cc90e7a, since when we don't walk through function name _ anymore.
I think the right fix, is making reflectdata.MarkUsedIfaceMethod handle function name _ correctly. Though, It does not affect the functionality of the compiler at tip (because we only call reflectdata.MarkUsedIfaceMethod during walk), it makes shipping a backport to 1.16 more easily.