Merge pull request #2775 from corob-msft/cr-c2555

Fix 2036 add explanation in C2555

Author: GitHubber17

docs/error-messages/compiler-errors-2/compiler-error-c2555.md

@@ -1,27 +1,36 @@
 ---
 title: "Compiler Error C2555"
-ms.date: "11/04/2016"
+description: "Reference for Visual Studio C++ compiler error C2555."
+ms.date: "03/30/2020"
 f1_keywords: ["C2555"]
 helpviewer_keywords: ["C2555"]
 ms.assetid: 5e49ebb8-7c90-457a-aa12-7ca7ab6574b2
 ---
 # Compiler Error C2555
 
-' class1::function1': overriding virtual function return type differs and is not covariant from 'class2::function2'
+> '*class1*::*function1*': overriding virtual function return type differs and is not covariant from '*class2*::*function2*'
 
-A virtual function and a derived overriding function have identical parameter lists but different return types. An overriding function in a derived class cannot differ from a virtual function in a base class only by its return type.
+A virtual function and a derived overriding function have identical parameter lists but different return types.
 
-To resolve this error, cast the return value after the virtual function has been called.
+## Remarks
 
-You may also see this error if you compile with /clr.   For example, the Visual C++ equivalent to the following C# declaration:
+In C++, an overriding function in a derived class can't differ only by return type from a virtual function in a base class.
 
-```
+There's an exception to this rule for certain return types. When a derived class overrides a public base class, it may return a pointer or reference to the derived class instead of a base-class pointer or reference. These return types are called *covariant*, because they vary together with the type. This rule exception doesn't allow covariant reference-to-pointer or pointer-to-pointer types.
+
+One way to resolve the error is to return the same type as the base class. Then, cast the return value after the virtual function has been called. Another is to also change the parameter list, to make the derived class member function an overload instead of an override.
+
+## Examples
+
+You may see this error if you compile with **`/clr`**. For example, the C++ equivalent to the following C# declaration:
+
+```csharp
 Guid[] CheckSources(Guid sourceID, Guid[] carouselIDs);
 ```
 
 is
 
-```
+```cpp
 Guid CheckSources(Guid sourceID, Guid carouselIDs[]) [];
 ```
 
@@ -38,3 +47,5 @@ struct Y : X {
    void func2();   // OK
 };
 ```
+
+To fix it, change the return type of `Y::func` to `void`.