[boost] Did Boost 1.59.0 change the meaning of "using python : 3.4 : ..." in user-config.jam?

Subject: [boost] Did Boost 1.59.0 change the meaning of "using python : 3.4 : ..." in user-config.jam?
From: Jonathan Wakely (jwakely.boost_at_[hidden])
Date: 2015-08-21 13:11:50

Hi,

I'm trying to update the Fedora rawhide package to Boost 1.59.0 and
the build fails with the output below (ignore the MPI warnings).

We build with the following in user-config.jam:

# This _adds_ extra python version.  It doesn't replace whatever
# python 2.X is default on the system.
using python : 3.4 : /usr/bin/python3 : /usr/include/python3.4m : : : : m ;

Is that comment no longer true?

Did something change in Boost 1.59.0 so that that line now sets the
only allowed python version?

I can make the build work with the following but I'd like to
understand the change:

using python : 2.7 : /usr/bin/python2 : /usr/include/python2.7 : : : :
;
using python : 3.4 : /usr/bin/python3 : /usr/include/python3.4m : : : : m ;

Build failure:

+ echo ============================= build serial ==================
+ ./b2 -d+2 -q -j4 --without-mpi --without-graph_parallel
--build-dir=serial variant=release threading=multi debug-symbols=on
pch=off python=2.7 stage
MPI auto-detection failed: unknown wrapper compiler mpic++
Please report this error to the Boost mailing list: http://www.boost.org
You will need to manually configure MPI support.
/builddir/build/BUILD/boost_1_59_0/tools/build/src/build/feature.jam:494:
in feature.validate-value-string from module feature
error: "2.7" is not a known value of feature <python>
error: legal values: "3.4"
/builddir/build/BUILD/boost_1_59_0/tools/build/src/build/property.jam:276:
in validate1 from module property
/builddir/build/BUILD/boost_1_59_0/tools/build/src/build/property.jam:302:
in property.validate from module property
/builddir/build/BUILD/boost_1_59_0/tools/build/src/build/build-request.jam:216:
in convert-command-line-element from module build-request
/builddir/build/BUILD/boost_1_59_0/tools/build/src/build/build-request.jam:154:
in build-request.from-command-line from module build-request
/builddir/build/BUILD/boost_1_59_0/tools/build/src/build-system.jam:583:
in load from module build-system
/builddir/build/BUILD/boost_1_59_0/tools/build/src/kernel/modules.jam:295:
in import from module modules
/builddir/build/BUILD/boost_1_59_0/tools/build/src/kernel/bootstrap.jam:139:
in boost-build from module
/builddir/build/BUILD/boost_1_59_0/boost-build.jam:17: in module scope
from module