fix: reject abstract singleton methods on interfaces

lib/type_toolkit/dsl.rb

@@ -26,6 +26,12 @@ def abstract(method_name)
         MSG
       end
 
+      if is_singleton_method && is_a?(TypeToolkit::Interface)
+        singleton_class.remove_method(method_name)
+        raise TypeError,
+          "Interfaces can't declare abstract singleton methods. Define instance methods instead."
+      end
+
       # The `method_owner` is the class whose method table stores the abstract method.
       #
       # Example:

lib/type_toolkit/interface.rb

@@ -18,6 +18,18 @@ def make_interface!(mod)
       mod.extend(TypeToolkit::DSL)
       mod.extend(TypeToolkit::MethodDefRecorder)
       mod.extend(TypeToolkit::HasAbstractMethods)
+      mod.singleton_class.extend(TypeToolkit::InterfaceSingletonClass)
+    end
+  end
+
+  module InterfaceSingletonClass
+    #: (Symbol) -> bot
+    def abstract(method_name)
+      #: self as Module[top]
+
+      remove_method(method_name) if method_defined?(method_name) || private_method_defined?(method_name)
+      raise TypeError,
+        "Interfaces can't declare abstract singleton methods. Define instance methods instead."
     end
   end
 

spec/interface_spec.rb

@@ -64,6 +64,38 @@ def m2 = "PartiallyInheritsItsImpl#m2"
           end
         end
 
+        it "raises when declaring an abstract singleton method" do
+          mod = Module.new
+
+          error = assert_raises(TypeError) do
+            mod.module_eval do
+              interface!
+
+              abstract def self.the_method_name = assert_never_called!
+            end
+          end
+
+          assert_match("Interfaces can't declare abstract singleton methods.", error.message)
+          refute_respond_to mod, :the_method_name
+        end
+
+        it "raises when declaring an abstract method on the interface singleton class" do
+          mod = Module.new
+
+          error = assert_raises(TypeError) do
+            mod.module_eval do
+              interface!
+
+              class << self
+                abstract def the_method_name = assert_never_called!
+              end
+            end
+          end
+
+          assert_match("Interfaces can't declare abstract singleton methods.", error.message)
+          refute_respond_to mod, :the_method_name
+        end
+
         it "raises when it sees a different method name than what MethodDefRecorder recorded" do
           error = assert_raises(RuntimeError) do
             Module.new do