diff --git a/Library/Homebrew/cask/dsl/depends_on.rb b/Library/Homebrew/cask/dsl/depends_on.rb
index 7a5756f9c0..4e08bb8932 100644
--- a/Library/Homebrew/cask/dsl/depends_on.rb
+++ b/Library/Homebrew/cask/dsl/depends_on.rb
@@ -60,9 +60,9 @@ module Cask
           elsif MacOSVersion::SYMBOLS.key?(args.first)
             MacOSRequirement.new([args.first], comparator: "==")
           elsif (md = /^\s*(?<comparator><|>|[=<>]=)\s*:(?<version>\S+)\s*$/.match(first_arg))
-            MacOSRequirement.new([T.must(md[:version]).to_sym], comparator: md[:comparator])
+            MacOSRequirement.new([T.must(md[:version]).to_sym], comparator: T.must(md[:comparator]))
           elsif (md = /^\s*(?<comparator><|>|[=<>]=)\s*(?<version>\S+)\s*$/.match(first_arg))
-            MacOSRequirement.new([md[:version]], comparator: md[:comparator])
+            MacOSRequirement.new([md[:version]], comparator: T.must(md[:comparator]))
           # This is not duplicate of the first case: see `args.first` and a different comparator.
           else # rubocop:disable Lint/DuplicateBranch
             MacOSRequirement.new([args.first], comparator: "==")
diff --git a/Library/Homebrew/requirements/macos_requirement.rb b/Library/Homebrew/requirements/macos_requirement.rb
index 9b0be0d8d3..4624349739 100644
--- a/Library/Homebrew/requirements/macos_requirement.rb
+++ b/Library/Homebrew/requirements/macos_requirement.rb
@@ -7,14 +7,17 @@ require "requirement"
 class MacOSRequirement < Requirement
   fatal true
 
-  attr_reader :comparator, :version
+  sig { returns(String) }
+  attr_reader :comparator
+
+  attr_reader :version
 
   # TODO: when Yosemite is removed here, keep these around as empty arrays so we
   # can keep the deprecation/disabling code the same.
-  DISABLED_MACOS_VERSIONS = [
+  DISABLED_MACOS_VERSIONS = T.let([
     :yosemite,
-  ].freeze
-  DEPRECATED_MACOS_VERSIONS = [].freeze
+  ].freeze, T::Array[Symbol])
+  DEPRECATED_MACOS_VERSIONS = T.let([].freeze, T::Array[Symbol])
 
   def initialize(tags = [], comparator: ">=")
     @version = begin
@@ -44,10 +47,11 @@ class MacOSRequirement < Requirement
       MacOSVersion.new(HOMEBREW_MACOS_OLDEST_ALLOWED) if comparator == ">="
     end
 
-    @comparator = comparator
+    @comparator = T.let(comparator, String)
     super(tags.drop(1))
   end
 
+  sig { returns(T::Boolean) }
   def version_specified?
     @version.present?
   end
@@ -61,6 +65,7 @@ class MacOSRequirement < Requirement
     false
   end
 
+  sig { returns(MacOSVersion) }
   def minimum_version
     return MacOSVersion.new(HOMEBREW_MACOS_OLDEST_ALLOWED) if @comparator == "<=" || !version_specified?
     return @version.min if @version.respond_to?(:to_ary)
@@ -68,6 +73,7 @@ class MacOSRequirement < Requirement
     @version
   end
 
+  sig { returns(MacOSVersion) }
   def maximum_version
     return MacOSVersion.new(HOMEBREW_MACOS_NEWEST_UNSUPPORTED) if @comparator == ">=" || !version_specified?
     return @version.max if @version.respond_to?(:to_ary)
@@ -75,6 +81,7 @@ class MacOSRequirement < Requirement
     @version
   end
 
+  sig { params(other: MacOSVersion).returns(T::Boolean) }
   def allows?(other)
     return true unless version_specified?
 
@@ -98,6 +105,7 @@ class MacOSRequirement < Requirement
     end
   end
 
+  sig { params(type: Symbol).returns(String) }
   def message(type: :formula)
     return "macOS is required for this software." unless version_specified?
 
@@ -113,6 +121,8 @@ class MacOSRequirement < Requirement
         EOS
       when :cask
         "This cask does not run on macOS versions newer than #{@version.pretty_name}."
+      else
+        "This does not run on macOS versions newer than #{@version.pretty_name}."
       end
     else
       if @version.respond_to?(:to_ary)
@@ -129,6 +139,7 @@ class MacOSRequirement < Requirement
   end
   alias eql? ==
 
+  sig { returns(Integer) }
   def hash
     [super, comparator, version].hash
   end
@@ -151,6 +162,7 @@ class MacOSRequirement < Requirement
     end
   end
 
+  sig { params(options: T.untyped).returns(String) }
   def to_json(options)
     comp = @comparator.to_s
     return { comp => @version.map(&:to_s) }.to_json(options) if @version.is_a?(Array)