Add new Sorbet/ForbidExtendTSig cop

config/default.yml

@@ -156,6 +156,12 @@ Sorbet/OneAncestorPerLine:
   Enabled: false
   VersionAdded: '0.6.0'
 
+Sorbet/ForbidExtendTSig:
+  Description: 'Forbid the usage of redundant `extend T::Sig`. For use in applications that monkey patch it into `Module` directly, where it is useful to reduce noise.'
+  Enabled: false
+  Safe: false
+  VersionAdded: '<<next>>'
+
 Sorbet/SignatureBuildOrder:
   Description: >-
                   Enforces the order of parts in a signature.

lib/rubocop/cop/sorbet/redundant_extend_t_sig.rb

@@ -0,0 +1,49 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Sorbet
+      # Forbids the use of redundant `extend T::Sig`. Meant for use in
+      # applications that monkey patch `Module` to `extend T::Sig` globally,
+      # which would make it redundant.
+      #
+      # @safety
+      #   This cop cannot know if the host application has monkey patched
+      #   `Module` to `extend T::Sig`, or if the offense is itself the
+      #   monkey-patch.
+      #
+      # @example
+      #   # bad
+      #   class Greeter
+      #     extend T::Sig
+      #     sig { void }
+      #     def say_hello = puts "hello"
+      #   end
+      #
+      #   # good
+      #   class Greeter
+      #     sig { void }
+      #     def say_hello = puts "hello"
+      #   end
+      #
+      class RedundantExtendTSig < Base
+        extend AutoCorrector
+
+        MSG = "Do not `extend T::Sig` when it is already extended into all modules."
+        RESTRICT_ON_SEND = [:extend].freeze
+
+        def_node_matcher :extend_t_sig?, <<~PATTERN
+          (send _ :extend (const (const {nil? | cbase} :T) :Sig))
+        PATTERN
+
+        def on_send(node)
+          return unless extend_t_sig?(node)
+
+          add_offense(node) do |corrector|
+            corrector.remove(node)
+          end
+        end
+      end
+    end
+  end
+end

lib/rubocop/cop/sorbet_cops.rb

@@ -8,6 +8,7 @@
 require_relative "sorbet/callback_conditionals_binding"
 require_relative "sorbet/forbid_t_unsafe"
 require_relative "sorbet/forbid_t_untyped"
+require_relative "sorbet/redundant_extend_t_sig"
 require_relative "sorbet/type_alias_name"
 
 require_relative "sorbet/rbi/forbid_extend_t_sig_helpers_in_shims"

spec/rubocop/cop/sorbet/redundant_extend_t_sig_spec.rb

@@ -0,0 +1,81 @@
+# frozen_string_literal: true
+
+RSpec.describe(RuboCop::Cop::Sorbet::RedundantExtendTSig, :config) do
+  let(:config) { RuboCop::Config.new }
+  let(:message) { "Do not `extend T::Sig` when it is already extended into all modules." }
+
+  shared_examples "block form" do |label:, header:| # rubocop:disable Lint/UnusedBlockArgument
+    it 'registers an offense when using `extend T::Sig` #{label}' do
+      expect_offense(<<~RUBY)
+        #{header}
+          extend T::Sig
+          ^^^^^^^^^^^^^ #{message}
+        end
+      RUBY
+
+      expect_correction(<<~RUBY)
+        #{header}
+          #{trailing_whitespace}
+        end
+      RUBY
+    end
+
+    private
+
+    def trailing_whitespace
+      ""
+    end
+  end
+
+  include_examples "block form", label: "in a module", header: "module M"
+  include_examples "block form", label: "in a class", header: "class C"
+  include_examples "block form", label: "in an anonymous module", header: "Module.new do"
+  include_examples "block form", label: "in an anonymous class", header: "Class.new do"
+  include_examples "block form", label: "in `self`'s singleton class", header: "class << self"
+  include_examples "block form", label: "in an arbitrary singleton class", header: "class << object"
+  include_examples "block form", label: "in a module with other contents", header: <<~RUBY.chomp
+    module M
+      extend SomethingElse
+  RUBY
+
+  it "registers an offense when using `extend T::Sig` on its own" do
+    expect_offense(<<~RUBY)
+      extend T::Sig
+      ^^^^^^^^^^^^^ #{message}
+    RUBY
+
+    expect_correction(a_blank_line)
+  end
+
+  it "registers an offense when using `extend ::T::Sig` (fully qualified)" do
+    expect_offense(<<~RUBY)
+      extend ::T::Sig
+      ^^^^^^^^^^^^^^^ #{message}
+    RUBY
+
+    expect_correction(a_blank_line)
+  end
+
+  it "registers an offense when using `extend T::Sig` with an explicit receiver" do
+    expect_offense(<<~RUBY)
+      some_module.extend T::Sig
+      ^^^^^^^^^^^^^^^^^^^^^^^^^ #{message}
+    RUBY
+
+    expect_correction(a_blank_line)
+  end
+
+  it "does not register an offense when extending other modules in the T namespace" do
+    expect_no_offenses(<<~RUBY)
+      module M
+        extend T::Helpers
+      end
+    RUBY
+  end
+
+  private
+
+  def a_blank_line
+    "\n"
+  end
+end