Use of 'feature' in user-config.jam file

In my user-config.jam I have:

import toolset ;
import feature ;
using msvc : 14.1 ;
feature.subfeature toolset msvc-14.1 : standard : c14 c17 : optional 
composite propagated ;
feature.compose <toolset-msvc-14.1:standard>c14 : <cxxflags>"/std:c++14" ;
feature.compose <toolset-msvc-14.1:standard>c17 : 
<cxxflags>"/std:c++latest" ;

I invoke b2 with toolset=msvc-14.1-c14 only to get, among a slew of 
Boost Build output:

error: "c14" is not a known subfeature value of <toolset>msvc

What am I doing wrong, and how do I correct it ?



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