1.9.3 error/warning confusion

[noproblemwiththat]

I have a small project type-checked with steep (for which, many thanks!). I've had some warnings which I don't fully understand (see here) but up to 1.8 were only warnings. Today my monthly heartbeat CI picked up 1.9.3 for the first time and it seemed that the warnings had been promoted to errors

2025-01-03 20:12:01.579: ERROR: [Steep 1.9.3] [typecheck:typecheck@3] [background]
 [#typecheck_source(path=lib/rtree.rb)] [#type_check_file(lib/rtree.rb@lib)] [synthesize:(5:1)] 
... [synthesize:(165:5)] [singleton(::RTree) <: instance] `T <: instance` doesn't hold generally, 
but testing it with `singleton(::RTree) <: instance && instance <: singleton(::RTree)` for compatibility
2025-01-03 20:12:02.053: ERROR: [Steep 1.9.3] [typecheck:typecheck@3] [background]
[#typecheck_source(path=lib/rtree.rb)] [#type_check_file(lib/rtree.rb@lib)] [synthesize:(5:1)] 
... [synthesize:(597:5)] [singleton(::RTree::Style) <: instance] `T <: instance` doesn't hold generally, 
but testing it with `singleton(::RTree::Style) <: instance && instance <: singleton(::RTree::Style)` 
for compatibility
F.

lib/rtree.rb:349:12: [warning] Empty array doesn't have type annotation
│ Diagnostic ID: Ruby::UnannotatedEmptyCollection
│
└       ids = []
              ~~
Detected 1 problem from 1 file

as well as generating a new warning UnannotatedEmptyCollection. From a quick read around, the latter can be fixed with a

ids = [] #: Array[Integer]

so I do that, and to my surprise that fixes the error

2025-01-03 20:47:16.179: ERROR: [Steep 1.9.3] ...
...  <: instance && instance <: singleton(::RTree::Style)` for compatibility

No type error detected. 🍵

So the long standing warning, which since 1.9.3 reports ERROR is a warning, the new [warning] (on UnannotatedEmptyCollection) is in fact an error.

All a bit confusing!