Add test for reported issue (#41703)

Fixes #41481

src/Compilers/CSharp/Test/Symbol/Symbols/DefaultInterfaceImplementationTests.cs

@@ -59855,5 +59855,32 @@ interface I1<T>
                 Diagnostic(ErrorCode.ERR_ClassDoesntImplementInterface, "I1<int>").WithArguments("I1<T>.P1", "I1<int>").WithLocation(4, 9)
                 );
         }
+
+        [Fact, WorkItem(41481, "https://github.com/dotnet/roslyn/issues/41481")]
+        public void IncompletePropertyImplementationSyntax_01()
+        {
+            var source1 =
+@"interface A
+{
+    object A.B{";
+            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
+                                                 parseOptions: TestOptions.Regular,
+                                                 targetFramework: TargetFramework.NetStandardLatest);
+            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
+            compilation1.VerifyDiagnostics(
+                // (3,12): error CS0540: 'A.B': containing type does not implement interface 'A'
+                //     object A.B{
+                Diagnostic(ErrorCode.ERR_ClassDoesntImplementInterface, "A").WithArguments("A.B", "A").WithLocation(3, 12),
+                // (3,14): error CS0548: 'A.B': property or indexer must have at least one accessor
+                //     object A.B{
+                Diagnostic(ErrorCode.ERR_PropertyWithNoAccessors, "B").WithArguments("A.B").WithLocation(3, 14),
+                // (3,16): error CS1513: } expected
+                //     object A.B{
+                Diagnostic(ErrorCode.ERR_RbraceExpected, "").WithLocation(3, 16),
+                // (3,16): error CS1513: } expected
+                //     object A.B{
+                Diagnostic(ErrorCode.ERR_RbraceExpected, "").WithLocation(3, 16)
+                );
+        }
     }
 }