Compiler Error C2429

'language feature' requires compiler flag 'compiler option'

The language feature requires a specific compiler option for support.

The error C2429: language feature 'nested-namespace-definition' requires compiler flag '/std:c++17' is generated if you try to define a compound namespace, a namespace that contains one or more scope-nested namespace names, starting in Visual Studio 2015 Update 5. (In Visual Studio 2017 version 15.3, the /std:c++latest switch is required.) Compound namespace definitions are not allowed in C++ prior to C++17. The compiler supports compound namespace definitions when the /std:c++17 compiler option is specified:

// C2429a.cpp
namespace a::b { int i; } // C2429 starting in Visual Studio 2015 Update 3.
                          // Use /std:c++17 to fix, or do this:
// namespace a { namespace b { int i; }}

int main() {
   a::b::i = 2;
}