/Zc:externConstexpr (Enable extern constexpr variables)
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.
Syntax
/Zc:externConstexpr[-]
Remarks
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, 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.
As of Visual Studio 2022 Update 17.6, the /permissive- option enables both /Zc:externConstexpr and /Zc:lambda. In prior versions, /permissive- didn't enable either one.
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.
To set this compiler option in Visual Studio
Open the project's Property Pages dialog box. For details, see Set C++ compiler and build properties in Visual Studio.
Select the Configuration Properties > C/C++ > Command Line property page.
Add
/Zc:externConstexpror/Zc:externConstexpr-to the Additional options: pane.