Proposal for "Do the Right Thing" Alternative Selection

This page describes a proposal for a change to the way target alternatives are selected.

Motivation

It seems there are a significant number of bjam invocations for which there is a sensible interpretation, but for which Boost.Build currently generates an error instead and refuses to build.

Example 1

Given the following Jamfile:

lib a : a.cpp : <debug-symbols>off ;                   # alternative 1
lib a : a-dbg.cpp : <debug-symbols>on <profiling>on ;  # alternative 2

In response to

bjam debug-symbols=on

the result is

error: No best alternative for ./a
    next alternative: required properties: <debug-symbols>off
        not matched
    next alternative: required properties: <debug-symbols>on <profiling>on
        not matched

My claim is that it would be more appropriate to build the 2nd alternative because it is a better match for the explicit build request.

Boost.Build generates an error because the default value of the <profiling> feature is off. It combines that default value with the explicit build request to get <debug-symbols>on <profiling>off, which does not match the 2nd alternative.