When using the latest MSVC version if I include reflect in 2 different source files (.cpp) I get

LNK2005 ""class reflect::test::expect:: reflect::test::expect" (?expect@test@reflect@@3v@expect@test@reflect@@A) already defined in <file_name>.obj" and
LNK1169 "one or more multiply defined symbols found".

The errors are cause by auto expect = [](bool cond) { if (not cond) { failed(); } }; at line 1086 in file "reflect".

Adding inline to the variable declaration removes the linker errors: inline auto expect = [](bool cond) { if (not cond) { failed(); } };.
I don't think (?) adding inline alters functionality but I have just started using this library, someone with a deeper understanding of it would have to determine that...

The tests were done within a newly created empty project. The command line arguments as displayed in Visual Studio "Project -> Properties -> C/C++ -> Command Line" were "/permissive- /ifcOutput "x64\Release" /GS /GL /W3 /Gy /Zc:wchar_t /Zi /Gm- /O2 /sdl /Fd"x64\Release\vc143.pdb" /Zc:inline /fp:precise /D "NDEBUG" /D "_CONSOLE" /D "_UNICODE" /D "UNICODE" /errorReport:prompt /WX- /Zc:forScope /Gd /Oi /MD /std:c++20 /FC /Fa"x64\Release" /EHsc /nologo /Fo"x64\Release" /Fp"x64\Release\Project1.pch" /diagnostics:column"

I also tried compiling with clang-cl version 18.1.6 which worked fine both with and without inline.