runtime: Top-level extend T::Sig is not supported

[AaronC81]

Input

→ View on sorbet.run

# typed: strong
require 'sorbet-runtime'
extend T::Sig

sig { params(a: Numeric, b: Numeric).returns Numeric }
def division(a, b)
    a / b
end

puts division(4, 2)

Observed output

No errors! Great job.

The issue is not with the typechecking itself, but instead with sorbet-runtime. Executing this file results in:

Traceback (most recent call last):
        3: from main.rb:5:in `<main>'
        2: from /var/lib/gems/2.5.0/gems/sorbet-runtime-0.4.4254/lib/types/sig.rb:24:in `sig'
        1: from /var/lib/gems/2.5.0/gems/sorbet-runtime-0.4.4254/lib/types/private/methods/_methods.rb:16:in `declare_sig'
/var/lib/gems/2.5.0/gems/sorbet-runtime-0.4.4254/lib/types/private/methods/_methods.rb:307:in `install_hooks': undefined method `singleton_class?' for main:Object (NoMethodError)
Did you mean?  singleton_class

(Note that line 5 is the sig line.)

I am using "Ruby 2.5.1p57 (2018-03-29 revision 63029) [x86_64-linux-gnu]", on Ubuntu 18.04. The behaviour does not change whether I execute the file using Bundler (bundle exec ruby main.rb) or just with the standard Ruby interpreter directly (ruby main.rb).

Expected behavior

I would expect that the file executes successfully and prints 2.

[AaronC81]

It appears that the issue only arises when the method definitions are in the top-level of the file (i.e. not inside a module). This works fine:

# typed: strong
require 'sorbet-runtime'

module A
    extend T::Sig

    sig { params(a: Numeric, b: Numeric).returns Numeric }
    def self.division(a, b)
        a / b
    end
end

puts A.division(4, 2)

[jez]

This is a duplicate of #199 but I like the examples and extra information in this issue, so I'm closing the other one.

[jez]

Fixed in #3491