The /Zc:externConstexpr compiler option tells the compiler to conform to the C++ standard and allow external linkage for constexpr variables. By default, Visual Studio always gives a constexpr variable internal linkage, even if you specify the extern keyword.

/Zc:externConstexpr[-]

The /Zc:externConstexpr compiler option causes the compiler to apply external linkage to variables declared by using extern constexpr. In earlier versions of Visual Studio, and by default or if /Zc:externConstexpr- is specified, Visual Studio applies internal linkage to constexpr variables even if the extern keyword is used. The /Zc:externConstexpr option is available starting in Visual Studio 2017 Update 15.6. and is off by default. The /permissive- option does not enable /Zc:externConstexpr.

If a header file contains a variable declared extern constexpr, it must be marked __declspec(selectany) in order to merge the duplicate declarations into a single instance in the linked binary. Otherwise you may see linker errors, for example, LNK2005, for violations of the one-definition rule.

1. Open the project's Property Pages dialog box. For details, see Set C++ compiler and build properties in Visual Studio.

2. Select the Configuration Properties > C/C++ > Command Line property page.

3. Add /Zc:externConstexpr or /Zc:externConstexpr- to the Additional options: pane.

/Zc (Conformance)
auto Keyword