Warn linux users about using --cask

Library/Homebrew/cli/parser.rb

@@ -341,14 +341,20 @@ module Homebrew
 
         @args_parsed = true
 
-        if !ignore_invalid_options && @args.help?
+        unless ignore_invalid_options
+          if @args.help?
             puts generate_help_text
             exit
           end
 
+          validate_options
+        end
+
         @args
       end
 
+      def validate_options; end
+
       def generate_help_text
         Formatter.format_help_text(@parser.to_s, width: COMMAND_DESC_WIDTH)
                  .gsub(/\n.*?@@HIDDEN@@.*?(?=\n)/, "")
@@ -701,3 +707,5 @@ module Homebrew
     end
   end
 end
+
+require "extend/os/parser"

Library/Homebrew/extend/os/linux/parser.rb

@@ -0,0 +1,20 @@
+# typed: false
+# frozen_string_literal: true
+
+module Homebrew
+  module CLI
+    class Parser
+      undef validate_options
+
+      def validate_options
+        return unless @args.respond_to?(:cask?)
+        return unless @args.cask?
+
+        msg = "Casks are not supported on Linux"
+        raise UsageError, msg unless Homebrew::EnvConfig.developer?
+
+        opoo msg unless @args.quiet?
+      end
+    end
+  end
+end

Library/Homebrew/extend/os/parser.rb

@@ -0,0 +1,4 @@
+# typed: false
+# frozen_string_literal: true
+
+require "extend/os/linux/parser" if OS.linux?

Library/Homebrew/test/cli/parser_spec.rb

@@ -562,4 +562,22 @@ describe Homebrew::CLI::Parser do
       expect { parser.parse(["--not-a-command"]) }.to raise_error(OptionParser::InvalidOption, /--not-a-command/)
     end
   end
+
+  describe "--cask on linux", :needs_linux do
+    subject(:parser) do
+      described_class.new do
+        switch "--cask"
+      end
+    end
+
+    it "throws an error for normal users" do
+      allow(Homebrew::EnvConfig).to receive(:developer?).and_return(false)
+      expect { parser.parse(["--cask"]) }.to raise_error UsageError, /Casks are not supported on Linux/
+    end
+
+    it "only warns developers" do
+      allow(Homebrew::EnvConfig).to receive(:developer?).and_return(true)
+      expect { parser.parse(["--cask"]) }.not_to raise_error
+    end
+  end
 end

Library/Homebrew/test/cmd/--cache_spec.rb

@@ -17,7 +17,7 @@ describe "brew --cache" do
       .and be_a_success
   end
 
-  it "prints the cache files for a given Cask", :integration_test do
+  it "prints the cache files for a given Cask", :integration_test, :needs_macos do
     expect { brew "--cache", cask_path("local-caffeine") }
       .to output(%r{#{HOMEBREW_CACHE}/downloads/[\da-f]{64}--caffeine\.zip}o).to_stdout
       .and output(/Treating #{Regexp.escape(cask_path("local-caffeine"))} as a cask/).to_stderr

Library/Homebrew/test/cmd/home_spec.rb

@@ -34,7 +34,7 @@ describe "brew home" do
       .and be_a_success
   end
 
-  it "opens the homepage for a given Cask", :integration_test do
+  it "opens the homepage for a given Cask", :integration_test, :needs_macos do
     expect { brew "home", local_caffeine_path, "HOMEBREW_BROWSER" => "echo" }
       .to output(/#{local_caffeine_homepage}/).to_stdout
       .and output(/Treating #{Regexp.escape(local_caffeine_path)} as a cask/).to_stderr