guix build python2 --with-source fails?

[zimoun]

Dear,

As an exercise about rebuilding old versions (e.g. used by old
scientific paper), I have tried to rebuild say the very old Python 2.5.

My naive attempt is to start with:

--8<---------------cut here---------------start------------->8---
wget https://www.python.org/ftp/python/2.5.2/Python-2.5.2.tgz
tar -xzvf Python-2.5.2.tgz

guix build python2 --with-source=/tmp/Python-2.5.2
guix build: warning: transformation 'with-source' had no effect on 
python2@2.7.17
The following graft will be made:
   /gnu/store/ck7swzbk6h7mc7splsih93xsixc6z4ni-python2-2.7.17.drv
[...]
--8<---------------cut here---------------end--------------->8---

Why is it not possible to use '--with-source' with the package python2?
I was expecting fail to build, not fail to apply the transformation.

I mean, I thought that "guix build foo --with-source" was somehow
equivalent to:

--8<---------------cut here---------------start------------->8---
cd /tmp/Python-2.5.2
guix environment -C python2
./configure
make
--8<---------------cut here---------------end--------------->8---

which works.  Is it not?


Thank you in advance.

All the best,
simon