Steep does not support %a{implicitly-returns-nil} annotation

[tk0miya]

Since RBS-3.6, some core methods have been annotated as %a{implicitly-returns-nil} and do not return nil in type definitions (see ruby/rbs#1921).

However Steep does not support the annotation. So it emits unexpected warnings to users.

For example, Steep emits UnreachableBranch for the following code:

item = [1, 2, 3].first

if item
  puts "Item is #{item}"
else
  puts "Item is nil"
end

Result:

$ bundle exec steep check --severity=information
# Type checking files:

.........................................................................................................................................................................................................................................................................................F.

app/app.rb:5:0: [information] The branch is unreachable
│ Diagnostic ID: Ruby::UnreachableBranch
│
└ else
  ~~~~

Detected 1 problem from 1 file

Steepfile:

D = Steep::Diagnostic

target :app do
  signature "sig"
  check "app"

  configure_code_diagnostics(D::Ruby.strict)
end

Environment:

• Steep 1.7.1 or 1.8.0
• RBS 3.6.1

The warning is not emitted with RBS-3.5.3.

$ bundle exec steep check --severity=information
# Type checking files:

..........................................................................................................................................................................................................................................................................................

No type error detected. 🫖

[nickjer]

Looks like this is related to the PR: #1258

[soutaro]

Yes. I think this is done with #1258.

[tk0miya]

Thanks!