Update C2993

Author: Rageking8

docs/error-messages/compiler-errors-2/compiler-error-c2993.md

@@ -1,7 +1,7 @@
 ---
 description: "Learn more about: Compiler Error C2993"
 title: "Compiler Error C2993"
-ms.date: "11/04/2016"
+ms.date: "10/03/2023"
 f1_keywords: ["C2993"]
 helpviewer_keywords: ["C2993"]
 ms.assetid: 4ffd2b78-654b-46aa-95a6-b62101cf91c8
@@ -10,34 +10,50 @@ ms.assetid: 4ffd2b78-654b-46aa-95a6-b62101cf91c8
 
 'identifier' : illegal type for non-type template parameter 'parameter'
 
-You cannot declare a template with a structure or union argument. Use pointers to pass structures and unions as template parameters.
+You cannot declare a template with a structure, class or union argument. However, pointers can be used in place of those types as template parameters. Since C++20, structure, class or unions can be used as non-type template parameters.
 
 The following sample generates C2993:
 
 ```cpp
 // C2993.cpp
-// compile with: /c
-// C2993 expected
-struct MyStruct {
-   int a;char b;
-};
+// compile with: /c /std:c++17
+struct MyStruct {};
 
-template <class T, struct MyStruct S>   // C2993
+template <class T, MyStruct S>   // C2993
+class MyClass1 {};
 
-// try the following line instead
-// template <class T, struct MyStruct * S>
-class CMyClass {};
+template <class T, MyStruct* S>   // OK
+class MyClass2 {};
 ```
 
-This error will also be generated as a result of compiler conformance work that was done in Visual Studio .NET 2003: floating point non-type template parameters no longer allowed. The C++ standard does not allow floating point non-type template parameters.
+> [!NOTE]
+> The sample above no longer emits C2993 since MSVC v19.26 and instead shows C7592.
+
+With C++17 and earlier, you cannot have floating point non-type template parameters. Since C++20, floating point non-type template parameters are allowed as with literal class types.
 
-If it is a function template, use a function argument to pass in the floating point non-type template parameter (this code will be valid in the Visual Studio .NET 2003 and Visual Studio .NET versions of Visual C++). If it is a class template, there is no easy workaround.
+If it is a function template, use a function argument to pass in the floating point non-type template parameter.
 
 ```cpp
 // C2993b.cpp
+// compile with: /c /std:c++17
+template<class T, float F>   // C2993
+void func1(T t) {}
+
+template<class T>   // OK
+void func2(T t, float F) {}
+```
+
+> [!NOTE]
+> The sample above no longer emits C2993 since MSVC v19.26 and instead shows C7592.
+
+Non-type template parameters cannot be of rvalue reference type. Similarly, a parameter pack of such types are not allowed.
+
+```cpp
+// C2993c.cpp
 // compile with: /c
-template<class T, float f> void func(T) {}   // C2993
+template <int&& I>   // C2993
+struct S1 {};
 
-// OK
-template<class T>   void func2(T, float) {}
+template <int&&... Is>   // C2993
+struct S2 {};
 ```