Proposal for "Do the Right Thing" Alternative Selection

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

Note: the proposal is intended to be a pure extension. That is, I believe it doesn't change the semantics of any bjam invocation that works today -- it only changes bjam invocations that are currently errors into non-errors.

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. For example, given the following Jamfile:

lib a : a.cpp : <debug-symbols>off ;
lib a : a-dbg.cpp : <debug-symbols>on <profiling>on ;

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