Raise an error if a class definition inherits a module

lib/rbs/definition_builder/ancestor_builder.rb

@@ -211,6 +211,9 @@ def one_instance_ancestors(type_name)
             end
 
             NoSuperclassFoundError.check!(super_name, env: env, location: primary.decl.location)
+            if super_class
+              InheritModuleError.check!(super_class, env: env)
+            end
 
             ancestors = OneAncestors.class_instance(
               type_name: type_name,
@@ -268,6 +271,9 @@ def one_singleton_ancestors(type_name)
             end
 
             NoSuperclassFoundError.check!(super_name, env: env, location: primary.decl.location)
+            if super_class
+              InheritModuleError.check!(super_class, env: env)
+            end
 
             ancestors = OneAncestors.singleton(
               type_name: type_name,

lib/rbs/errors.rb

@@ -148,7 +148,27 @@ def initialize(type_name:, location:)
     end
 
     def self.check!(type_name, env:, location:)
-      env.class_decls.key?(type_name) or raise new(type_name: type_name, location: location)
+      if decl = env.class_decls[type_name]
+        return
+      end
+
+      raise new(type_name: type_name, location: location)
+    end
+  end
+
+  class InheritModuleError < DefinitionError
+    attr_reader :super_decl
+
+    def initialize(super_decl)
+      @super_decl = super_decl
+
+      super "#{Location.to_string(super_decl.location)}: Cannot inherit a module: #{super_decl.name}"
+    end
+
+    def self.check!(super_decl, env:)
+      return if env.class_decls[super_decl.name].is_a?(Environment::ClassEntry)
+
+      raise new(super_decl)
     end
   end
 

sig/errors.rbs

@@ -197,6 +197,8 @@ module RBS
     def location: () -> Location[untyped, untyped]?
   end
 
+  # MixinClassError is raised if a include/prepend/extend has a class (not a module) to mix-in
+  #
   class MixinClassError < DefinitionError
     type member = AST::Members::Include | AST::Members::Prepend | AST::Members::Extend
 
@@ -212,6 +214,18 @@ module RBS
     def mixin_name: () -> String
   end
 
+  # InheritModuleError is raised if a class definition inherits a module (not a class)
+  #
+  class InheritModuleError < DefinitionError
+    attr_reader super_decl: AST::Declarations::Class::Super
+
+    def initialize: (AST::Declarations::Class::Super) -> void
+
+    def location: () -> Location[untyped, untyped]?
+
+    def self.check!: (AST::Declarations::Class::Super, env: Environment) -> void
+  end
+
   class RecursiveTypeAliasError < BaseError
     attr_reader alias_names: Array[TypeName]
     attr_reader location: Location[untyped, untyped]?

test/rbs/definition_builder_test.rb

@@ -2243,4 +2243,23 @@ class Foo
         end
       end
   end
+
+  def test_class_definition_inheriting_module
+    SignatureManager.new do |manager|
+      manager.files.merge!(Pathname("foo.rbs") => <<~EOF)
+        module Mod
+        end
+
+        class Foo < Mod
+        end
+      EOF
+
+      manager.build do |env|
+        builder = DefinitionBuilder.new(env: env)
+
+        assert_raises(RBS::InheritModuleError) { builder.build_instance(type_name("::Foo")) }
+        assert_raises(RBS::InheritModuleError) { builder.build_singleton(type_name("::Foo")) }
+      end
+    end
+  end
 end