Reject annotations on ivars defined in base class (#9502)

spec/compiler/semantic/annotation_spec.cr

@@ -730,7 +730,7 @@ describe "Semantic: annotation" do
           end
 
           def foo
-            {% if @type.instance_vars.first.annotations(Foo) %}
+            {% if @type.instance_vars.first.annotation(Foo) %}
               1
             {% else %}
               'a'
@@ -918,6 +918,23 @@ describe "Semantic: annotation" do
     end
   end
 
+  it "errors when annotate instance variable in subclass" do
+    assert_error %(
+      annotation Foo
+      end
+
+      class Base
+        @x : Nil
+      end
+
+      class Child < Base
+        @[Foo]
+        @x : Nil
+      end
+      ),
+      "can't annotate @x in Child because it was first defined in Base"
+  end
+
   it "errors if wanting to add type inside annotation (1) (#8614)" do
     assert_error %(
       annotation Ann

src/compiler/crystal/semantic/type_declaration_processor.cr

@@ -266,6 +266,11 @@ struct Crystal::TypeDeclarationProcessor
       unless supervar.type.same?(type_decl.type)
         raise TypeException.new("instance variable '#{name}' of #{supervar.owner}, with #{owner} < #{supervar.owner}, is already declared as #{supervar.type} (trying to re-declare as #{type_decl.type})", type_decl.location)
       end
+
+      # Reject annotations to existing instance var
+      type_decl.annotations.try &.each do |_, ann|
+        ann.raise "can't annotate #{name} in #{owner} because it was first defined in #{supervar.owner}"
+      end
     else
       declare_meta_type_var(owner.instance_vars, owner, name, type_decl, instance_var: true, check_nilable: !owner.module?)
       remove_error owner, name