patch 9.0.1181: class inheritance and typing insufficiently tested

Problem:    Class inheritance and typing insufficiently tested.
Solution:   Add more tests.  Implement missing behavior.

src/testdir/test_vim9_class.vim

@@ -419,6 +419,44 @@ def Test_class_object_compare()
   endfor
 enddef
 
+def Test_object_type()
+  var lines =<< trim END
+      vim9script
+
+      class One
+        this.one = 1
+      endclass
+      class Two
+        this.two = 2
+      endclass
+      class TwoMore extends Two
+        this.more = 9
+      endclass
+
+      var o: One = One.new()
+      var t: Two = Two.new()
+      var m: TwoMore = TwoMore.new()
+      var tm: Two = TwoMore.new()
+
+      t = m
+  END
+  v9.CheckScriptSuccess(lines)
+
+  lines =<< trim END
+      vim9script
+
+      class One
+        this.one = 1
+      endclass
+      class Two
+        this.two = 2
+      endclass
+
+      var o: One = Two.new()
+  END
+  v9.CheckScriptFailure(lines, 'E1012: Type mismatch; expected object<One> but got object<Two>')
+enddef
+
 def Test_class_member()
   # check access rules
   var lines =<< trim END
@@ -750,7 +788,7 @@ def Test_class_used_as_type()
       var p: Point
       p = 'text'
   END
-  v9.CheckScriptFailure(lines, 'E1012: Type mismatch; expected object but got string')
+  v9.CheckScriptFailure(lines, 'E1012: Type mismatch; expected object<Point> but got string')
 enddef
 
 def Test_class_extends()
@@ -895,6 +933,27 @@ def Test_class_extends()
       echo o.ToString()
   END
   v9.CheckScriptFailure(lines, 'E1358:')
+
+  lines =<< trim END
+      vim9script
+      class Base
+        this.name: string
+        static def ToString(): string
+          return 'Base class'
+        enddef
+      endclass
+
+      class Child extends Base
+        this.age: number
+        def ToString(): string
+          return Base.ToString() .. ': ' .. this.age
+        enddef
+      endclass
+
+      var o = Child.new('John', 42)
+      assert_equal('Base class: 42', o.ToString())
+  END
+  v9.CheckScriptSuccess(lines)
 enddef