Compiler: fixed bug regarding generic type, module inclusion and inst…

…ance vars. Fixes #3635

spec/compiler/semantic/instance_var_spec.cr

@@ -4538,4 +4538,26 @@ describe "Semantic: instance var" do
       Foo.new
       )) { types["Foo"] }
   end
+
+  it "doesn't error if inheriting generic instance (#3635)" do
+    assert_type(%(
+      module Core(T)
+        @a : Bool
+      end
+
+      class Base(T)
+        include Core(Int32)
+
+        @a = true
+      end
+
+      class Foo < Base(String)
+        def a
+          @a
+        end
+      end
+
+      Foo.new.a
+      )) { bool }
+  end
 end

src/compiler/crystal/semantic/type_declaration_processor.cr

@@ -236,6 +236,10 @@ struct Crystal::TypeDeclarationProcessor
   end
 
   private def process_owner_instance_var_declaration(owner, name, type_decl)
+    # Generic instances already have their instance vars
+    # set from uninstantiated generic types
+    return if owner.is_a?(GenericInstanceType)
+
     # Check if a superclass already defined this variable
     supervar = owner.lookup_instance_var?(name)
 
@@ -313,6 +317,10 @@ struct Crystal::TypeDeclarationProcessor
   end
 
   private def process_owner_guessed_instance_var_declaration(owner, name, type_info)
+    # Generic instances already have their instance vars
+    # set from uninstantiated generic types
+    return if owner.is_a?(GenericInstanceType)
+
     case owner
     when NonGenericClassType
       # If a superclass already defines this variable we ignore