Revert "Revert "Add class variable validation""

lib/steep/diagnostic/signature.rb

@@ -264,6 +264,24 @@ def header_line
         end
       end
 
+      class ClassVariableDuplicationError < Base
+        attr_reader :class_name
+        attr_reader :other_class_name
+        attr_reader :variable_name
+
+        def initialize(class_name:, other_class_name:, variable_name:, location:)
+          super(location: location)
+
+          @class_name = class_name
+          @other_class_name = other_class_name
+          @variable_name = variable_name
+        end
+
+        def header_line
+          "Class variable definition `#{variable_name}` in `#{class_name}` may be overtaken by `#{other_class_name}`"
+        end
+      end
+
       class InstanceVariableTypeError < Base
         attr_reader :name
         attr_reader :variable

lib/steep/signature/validator.rb

@@ -319,6 +319,25 @@ def validate_one_class(name)
                 end
               end
 
+              definition.class_variables.each do |name, var|
+                if var.declared_in == definition.type_name
+                  if (parent = var.parent_variable) && var.declared_in != parent.declared_in
+                    class_var = definition.entry.decls.flat_map {|decl| decl.decl.members }.find do |member|
+                      member.is_a?(RBS::AST::Members::ClassVariable) && member.name == name
+                    end
+
+                    if class_var
+                      @errors << Diagnostic::Signature::ClassVariableDuplicationError.new(
+                        class_name: definition.type_name,
+                        other_class_name: parent.declared_in,
+                        variable_name: name,
+                        location: class_var.location[:name]
+                      )
+                    end
+                  end
+                end
+              end
+
               ancestors = builder.ancestor_builder.one_singleton_ancestors(name)
               mixin_constraints(definition, ancestors.extended_modules, immediate_self_types: ancestors.self_types).each do |relation, ancestor|
                 checker.check(

test/validation_test.rb

@@ -597,6 +597,42 @@ class D
     end
   end
 
+  def test_validate_class_variables
+    with_checker <<-EOF do |checker|
+class A
+  @@foo: Integer
+end
+
+class B < A
+  @@foo: Integer?
+end
+
+class C < B
+end
+    EOF
+      Validator.new(checker: checker).tap do |validator|
+        validator.validate_one_class(TypeName("::A"))
+
+        assert_predicate validator, :no_error?
+      end
+
+      Validator.new(checker: checker).tap do |validator|
+        validator.validate_one_class(TypeName("::C"))
+
+        assert_predicate validator, :no_error?
+      end
+
+      Validator.new(checker: checker).tap do |validator|
+        validator.validate_one_class(TypeName("::B"))
+
+        assert_predicate validator, :has_error?
+        assert_any!(validator.each_error, size: 1) do |error|
+          assert_instance_of Diagnostic::Signature::ClassVariableDuplicationError, error
+        end
+      end
+    end
+  end
+
   def test_validate_type_application
     with_checker <<-EOF do |checker|
 class Foo[X < Numeric]