[cfe-dev] Feature Idea: Error Message Replacing

[Sebastian Redl]

Hi,

This is a feature idea I just came up with. I have no concrete plans to
implement this, but I thought I'd post it to the list to see if others
like it as well, and to preserve it for future reference.

I'm currently working on a Boost library, and in the compile errors this
shows up:

error: invalid use of incomplete type ‘struct
boost::property_tree::path_of<boost::any>’

Because I've written this library, I know that this error really means,
"You haven't specialized the path_of struct for that template argument
you're trying to use, but this is a prerequisite." If I were a user of
the library, I'd have no clue.
I think there should be a way for the library author to communicate this
knowledge to the user. I'm imagining something like this:

--- File: boost/property_tree/ptree.hpp (the library's main include) ---
#pragma clang error_catalog "boost/property_tree/property_tree.en.ec"

// Users have to specialize this
template <typename Key> struct path_of;


--- File: boost/property_tree/property_tree.en.ec (pseudo-syntax) ---
match "invalid use of incomplete type 'struct
boost::property_tree::path_of<{T}>'"
replacement "you need to specialize 'boost::property_tree::path_of' for
'$T' in order to use it as a property tree key"

Then, when I do

boost::basic_ptree<boost::any, int> pt;

I get the error

error: you need to specialize 'boost::property_tree::path_of' for
'boost::any' in order to use it as a property tree key


Error catalogs would be looked up either in the include path or some
separate lookup path; I would prefer the include path because it makes
using error catalogs "just work" for the library user. Error catalogs
are cumulative, i.e. you can have any number of error_catalog pragmas in
a translation unit.

Comments? With C++0x concepts, the need for this is obviously smaller
(my example is a result of a pseudo-concept emulation), but I think it
could still be useful.

Sebastian