[boost] [proto v4 docs]Suggest warning about transform composition failure

From: Larry Evans (cppljevans_at_[hidden])
Date: 2008-05-17 11:52:56

The attached fails to compile because when the output of one when
transform is immediately fed to another transform, the compiler
complains because it expects Expr& instead of Expr as 1st arg
to the composition.  Although the docs on:

    libs/proto/doc/html/boost_proto/appendices/rationale/result_of.html

*hint* that this could happen, it's not obvious.  It would have helped
me to see this on that page.  IOW, that page could specifically say that
transform compositions are not possible because a transform
expects Expr& and when transforms are composed without storing
intermediate values in a variable, then the 1st transform produces
an Expr and not an Expr&; so, the second transform rejects it.