Disabling /Qip or /Qipo for that file is a quick workaround, but you will lose the benefits of inlining. The correct solutionis to find the file that is emitting an external unresolved call to this function, then fix the includes for the file so that you include the header file with the definition of this function.
I recently ran into this problem:
It turns out that:
1) test1.cpp has the definition of the function xxxyyy
2) mytest.h has the declaration for this function
3) test2.cpp reference the function xxxyyy.
4) both test1.cpp and test2.cppincludes myheader.h which has the function declaration.
With /Qip enable, function xxxyyy was inlined, and no function body was emitted. To resolve this problem, I move the definition of xxxyyy from test1.cpp to myheader.h.
Hope this helps.