[culturedsystems]

I initially interpreted "unaccounted-for null values may cause compile-time warnings, but not compile-time errors" as meaning "in some cases, an unaccounted-for null value might not cause a compile-time error", but in the context of the rest of the spec, I think it actually means "unaccounted-for null values are not permitted to cause compile-time errors, only warnings", which seems like a bad idea to me. I can see why allowing implicit conversion from unannotated "Object" to "Object!" is a reasonable compromise to work with existing code, but I don't see why conversion from "Object?" to "Object!" would not cause a compile-time error.

Worse, permitting this conversion at compile time means developers will ignore the warning, so we'll have actual codebases which include these conversions. Any later change to enforce nullability checking at compile time will then have a significant backwards compatibility cost.