Symbols for libraries (.lib)

I have the following structure:

src\a.cpp
bin\a.lib
bin\a.pdb

I have created two nupkg files.
a.nupkg containing binaries bin\a.lib, bin\a.pdb
a-symbols.nupkg containing sources: src\a.cpp

I create a new project and add a package a.nupkg .
Now I want to debug and step into sources.
This doesn't work.
Visual Studio doesn't download the symbols package at all.

However this works in case of dll file for instance a.dll.
That way Visual Studio downloads the symbols and can step into source.

What is the proper way of making symbols package for library ?

Product: ProGet
Version: 4.0.6

This is the first inquiry about .lib files... we haven't tested it before, and are not sure if NuGet with Symbol/Source Serving supports .lib files.

Serving lib works well, the only problem is the source server.
Visual Studio cannot find the relevant source.

I would like to know if anything is possible to do.
Thanks.

Has .lib files been tested as yet? We are experiencing the same problem with the source server. Can we please get confirmation if this is supported?