upgrade: raise error if developer and there are cyclic dependencies

Library/Homebrew/exceptions.rb

@@ -758,3 +758,13 @@ class ShebangDetectionError < RuntimeError
     super "Cannot detect #{type} shebang: #{reason}."
   end
 end
+
+# Raised when one or more formulae have cyclic dependencies.
+class CyclicDependencyError < RuntimeError
+  def initialize(strongly_connected_components)
+    super <<~EOS
+      The following packages contain cyclic dependencies:
+        #{strongly_connected_components.select { |packages| packages.count > 1 }.map(&:to_sentence).join("\n  ")}
+    EOS
+  end
+end

Library/Homebrew/upgrade.rb

@@ -47,8 +47,7 @@ module Homebrew
       begin
         formulae_to_install = dependency_graph.tsort & formulae_to_install
       rescue TSort::Cyclic
-        # Failed to sort formulae topologically because there are cyclic
+        raise CyclicDependencyError, dependency_graph.strongly_connected_components if Homebrew::EnvConfig.developer?
-        # dependencies. Let FormulaInstaller handle it.
       end
 
       formula_installers = formulae_to_install.map do |formula|