[boost] [build] [config] check_target_builds and feature.subfeature composition

Subject: [boost] [build] [config] check_target_builds and feature.subfeature composition
From: Edward Diener (eldiener_at_[hidden])
Date: 2017-01-22 12:58:32

The Boost library config uses the check_target_builds rule to provide 
build time feature testing. Other Boost libraries use config checks in 
their own build and test jamfiles.

I have found that this works reliably when a particular toolset is used 
in a normal manner. However when using the feature.subfeature and 
feature.compose rules to extend the toolset-version designation with a 
subfeature, such that the toolset designation now becomes 
toolset-version-subfeature, the use of the check_target_builds rule does 
not work as expected. In particular the individual check_target_builds 
rule results are put into a subdirectory based on the toolset-version 
and not based on the toolset-version-subfeature when toolset subfeature 
composition is used. I also suspect that the cached value reflects 
setup. This causes wrong results in tests where the toolset designation 
uses subfeatures, and a library's jamfile is using config and/or predef 
checks.

As an example

My toolset designation looks like:

using gcc : 6.3 : some_path ;
feature.subfeature toolset gcc : cxxstd : c03 c11 c14 c17 : optional 
composite propagated ;
feature.compose <toolset-gcc:cxxstd>c03 : <cxxflags>-std=c++03 ;
feature.compose <toolset-gcc:cxxstd>c11 : <cxxflags>-std=c++11 ;
feature.compose <toolset-gcc:cxxstd>c14 : <cxxflags>-std=c++14 ;
feature.compose <toolset-gcc:cxxstd>c17 : <cxxflags>-std=c++1z ;

A library's test looks like:

import ../../config/checks/config : requires ;
run some_test.cpp : : : [ requires cxx11_hdr_forward_list ] ;

If I run some some_test with toolset=gcc-6.3-c03, the config result for 
checking if forward list is supported is put into a subdirectory 
reflecting the toolset-version while the test result will be put into a 
subdirectory reflecting the toolset-version-subfeature. Subsequently if 
I run some_test with toolset=gcc-6.3-c11 the config result is taken from 
the subdirectory reflect the toolset-version even though whether the 
forward list is supported or not depends very much on the subfeature I 
am designating as part of the toolset. This naturally causes incorrect 
results for the test.

Is this a known problem ? Is it a problem with Boost Build or is it a 
problem with how config uses the check_target_builds rule ? I have run 
into this problem extensively in my testing once I started to use the 
feature.subfeature ability of Boost Build.