From Vim's help (":he ft"):

When a dot appears in the value then this separates two filetype
names. Example:
/* vim: set filetype=c.doxygen : */ ~
This will use the "c" filetype first, then the "doxygen" filetype.
This works both for filetype plugins and for syntax files. More than
one dot may appear.

Unfortunately, taglist doesn't support dotted filetypes. For instance, I open a
C++ file and set the filetype to "c.cpp". This shouldn't affect the behavior of
taglist, but when I :TlistOpen, I don't see anything. With a filetype of cpp or
c alone, it works just fine.

Is this know/expected behavior?

Thanks for a wonderful plugin!