rubocops/cask: Don't force on_* blocks for all stanzas

- In the event that there's only one common stanza within the `on_*`
  blocks (eg, `url`) with a generic `version` that doesn't change per-OS,
  let's not force adding `version` to each `on_*` block as well.

Library/Homebrew/rubocops/cask/no_overrides.rb

@@ -17,17 +17,36 @@ module RuboCop
         def on_cask(cask_block)
           return if (cask_stanzas = cask_block.toplevel_stanzas).empty?
           # Skip if there are no `on_*` blocks.
-          return unless cask_stanzas.any? { |s| ON_SYSTEM_METHODS.include?(s.stanza_name) }
+          return unless (on_blocks = cask_stanzas.select { |s| ON_SYSTEM_METHODS.include?(s.stanza_name) }).any?
 
           cask_stanzas.each do |stanza|
-            # TODO: We probably only want to disallow `version`, `url`, and `sha256` stanzas being overridden?
+            # Skip if the stanza is itself an `on_*` block.
             next unless RuboCop::Cask::Constants::STANZA_ORDER.include?(stanza.stanza_name)
             # Skip if the stanza we detect is already in an `on_*` block.
             next if stanza.parent_node.block_type? && ON_SYSTEM_METHODS.include?(stanza.parent_node.method_name)
+            # Skip if the stanza outside of a block is not also in an `on_*` block.
+            next if on_system_stanzas(on_blocks).none?(stanza.stanza_name)
 
             add_offense(stanza.source_range, message: format(MESSAGE, stanza: stanza.stanza_name))
           end
         end
+
+        def on_system_stanzas(on_system)
+          names = []
+          method_nodes = on_system.map(&:method_node)
+          method_nodes.each do |node|
+            next unless node.block_type?
+
+            node.child_nodes.each do |child|
+              child.each_node(:send) do |send_node|
+                next if ON_SYSTEM_METHODS.include?(send_node.method_name)
+
+                names << send_node.method_name
+              end
+            end
+          end
+          names
+        end
       end
     end
   end

Library/Homebrew/test/rubocops/cask/no_overrides_spec.rb

@@ -38,6 +38,27 @@ describe RuboCop::Cop::Cask::NoOverrides do
     include_examples "does not report any offenses"
   end
 
+  context "when there's only one difference between the `on_*` blocks" do
+    let(:source) do
+      <<~CASK
+        cask "foo" do
+          version "1.2.3"
+
+          on_big_sur :or_older do
+            sha256 "bbb"
+            url "https://brew.sh/legacy/foo-2.3.4.dmg"
+          end
+          on_monterey :or_newer do
+            sha256 "aaa"
+            url "https://brew.sh/foo-2.3.4.dmg"
+          end
+        end
+      CASK
+    end
+
+    include_examples "does not report any offenses"
+  end
+
   context "when there are top-level standalone stanzas" do
     let(:source) do
       <<~CASK
@@ -62,15 +83,6 @@ describe RuboCop::Cop::Cask::NoOverrides do
         line:     2,
         column:   2,
         source:   "version '2.3.4'",
-      }, {
-        message:  <<~EOS,
-          Do not use top-level `url` stanza as the default, add an `on_{system}` block instead.
-          Use `:or_older` or `:or_newer` to specify a range of macOS versions.
-        EOS
-        severity: :convention,
-        line:     7,
-        column:   2,
-        source:   "url 'https://brew.sh/foo-2.3.4.dmg'",
       }]
     end