We recommend using Visual Studio 2017

Compiler Warning (level 4) C4480

nonstandard extension used: specifying underlying type for enum 'enum'

An extension to the language under /clr was used without /clr. You can disable C4480 with the warning pragma.

For more information, see /clr (Common Language Runtime Compilation), /Za, /Ze (Disable Language Extensions), enum class (C++ Component Extensions), and C++ Enumeration Declarations.

The following sample generates C4480.

// C4480.cpp
// compile with: /W4 /c
enum E : char {};   // C4480
enum F {};