Alternatives and underfined feature values

From: Vladimir Prus (ghost_at_[hidden])
Date: 2005-02-03 10:57:23

I'm looking at http://zigzag.cs.msu.su:7814/scarab/issues/id/BB83
which says that if you write:

alias a : a_gcc.cpp : <toolset>gcc ;
alias a : a_msvc.cpp : <toolset>msvc ;

and don't initialise both gcc and msvc toolset, you'll get errors. This fact 
makes target alternatives not so usefull.

I see three approaches:

1. Do not validate features here at all.
2. Introduce special syntax for properties used for alternative selection:

alias a : a_gcc.cpp : when <toolset>gcc ;
alias a : a_msvc.cpp : when <toolset>msvc ;

and don't validate features specified in "when" clause. This is related to
http://zigzag.cs.msu.su:7814/scarab/issues/id/BB78, which says we need 
explicit control over what features are used for alternative selection.
3. Allow the user to say that <toolset>msvc is valid, without initializing the 
toolset.

Any opinions on the best approach.

Regarding #1, it's implementable by a tweak to property.translate-paths: we 
need to comment out the following lines:

# need to do this here to get reasonable error messages for
# unrecognized implicit features.
validate $(p) ; 

I don't really understand the comment, and the log message (by Dave), only 
says: "restore from disk crash".

- Volodya