Am I using --with-source improperly?

[Jesse Gibbons]

I asked about this on the IRC but had no luck.

IIUC --with-source=package=source should set the source for "package" to 
"source" when something being built depends on "package". For example, 
`guix build --with-soure=python2=/tmp/python2 lci` should build the lci 
with the python2 source in /tmp/python2, and should work because python2 
is a dependency of lci. However, when I try that, I get the message 
"guix build: warning: transformation 'with-source' had no effect on 
lci@0.11.2". Am I using the --with-source option incorrectly, or is this 
some sort of bug? I replace "--with-source=python2=/tmp/python2" with 
"--with-source=lci=/tmp/python2" when /tmp/python2 doesn't even exist, 
and it gives me the result I would expect: 'guix build: error: lstat: No 
such file or directory: "/tmp/python2"'

Should I file this as a bug report?