Add test for #34921 (#39365)

src/Compilers/CSharp/Test/Semantic/Semantics/NullableReferenceTypesTests.cs

@@ -117673,5 +117673,47 @@ await foreach (var o in Create(y))
                 //         object y = null; // 1
                 Diagnostic(ErrorCode.WRN_ConvertingNullableToNonNullable, "null").WithLocation(24, 20));
         }
+
+        [Fact]
+        [WorkItem(34921, "https://github.com/dotnet/roslyn/issues/34921")]
+        public void NullableStructMembersOfClassesAndInterfaces()
+        {
+            var source =
+@"#nullable enable
+
+interface I<T>
+{
+    T P { get; }
+}
+
+class C<T>
+{
+    internal T F = default!;
+}
+
+class Program
+{
+    static void F1<T>(I<(T, T)> i) where T : class?
+    {
+        var t = i.P;
+        t.Item1.ToString(); // 1
+    }
+
+    static void F2<T>(C<(T, T)> c) where T : class?
+    {
+        var t = c.F;
+        t.Item1.ToString();// 2
+    }
+}";
+            var comp = CreateCompilation(source);
+            comp.VerifyDiagnostics(
+                // (18,9): warning CS8602: Dereference of a possibly null reference.
+                //         t.Item1.ToString(); // 1
+                Diagnostic(ErrorCode.WRN_NullReferenceReceiver, "t.Item1").WithLocation(18, 9),
+                // (24,9): warning CS8602: Dereference of a possibly null reference.
+                //         t.Item1.ToString();// 2
+                Diagnostic(ErrorCode.WRN_NullReferenceReceiver, "t.Item1").WithLocation(24, 9)
+                );
+        }
     }
 }