diff --git a/src/testdir/test_vim9_class.vim b/src/testdir/test_vim9_class.vim
index 2f2f62a..ef9ef55 100644
--- a/src/testdir/test_vim9_class.vim
+++ b/src/testdir/test_vim9_class.vim
@@ -627,6 +627,19 @@
           echo nr
         enddef
       endclass
+
+      interface Another
+        this.member: string
+      endinterface
+
+      class SomeImpl implements Some, Another
+        this.member = 'abc'
+        static count: number
+        def Method(nr: number)
+          echo nr
+        enddef
+      endclass
+
   END
   v9.CheckScriptSuccess(lines)
 
@@ -635,6 +648,32 @@
 
       interface Some
         static counter: number
+      endinterface
+
+      class SomeImpl implements Some implements Some
+        static count: number
+      endclass
+  END
+  v9.CheckScriptFailure(lines, 'E1350:')
+
+  lines =<< trim END
+      vim9script
+
+      interface Some
+        static counter: number
+      endinterface
+
+      class SomeImpl implements Some, Some
+        static count: number
+      endclass
+  END
+  v9.CheckScriptFailure(lines, 'E1351: Duplicate interface after "implements": Some')
+
+  lines =<< trim END
+      vim9script
+
+      interface Some
+        static counter: number
         def Method(nr: number)
       endinterface