Add annotation for __dir__

[ksss]

Summary

Add %a{implicitly-returns-nil} to Kernel#__dir__ and change its return type from String? to String.

__dir__ returns nil only when the caller has no file path, which in practice means a string eval without a file name:

eval("__dir__")                  # => nil
binding.eval("__dir__")          # => nil
Object.class_eval("__dir__")     # => nil
eval("__dir__", nil, "/tmp/x.rb") # => "/tmp"
ERB.new("<%= __dir__ %>").result # => "."

In ordinary code, i.e. in a file, it is always a String. The same trade-off is already made for Module#name, Exception#backtrace, Array#first, etc.

Why

__dir__ is almost always used to build a path relative to the current file. The path-taking methods do not accept nil, so String? forces every caller to handle a case that does not occur in that usage:

# File.join: (*path) -> String
# File.expand_path: (path file_name, ?path dir_string) -> String
config = File.read(File.join(__dir__, "config.yml"))
Dir.glob(File.join(__dir__, "fixtures", "*.json"))
File.expand_path("../lib", __dir__)

With String? each of these is a type error and needs __dir__ or raise / __dir__.to_s-style workarounds. With this change they type-check as written.

The nil case is still covered by test___dir__ (assert_equal nil, eval('__dir__')), and RBS::Test skips the return-type check for nil thanks to the annotation.

Notes

• __method__ / __callee__ are left as Symbol? on purpose.

🤖 Generated with Claude Code