Error Reporting
===============

   The C++ front-end uses a call-back mechanism to allow functions to
print out reasonable strings for types and functions without putting
extra logic in the functions where errors are found.  The interface is
through the `cp_error' function (or `cp_warning', etc.).  The syntax is
exactly like that of `error', except that a few more conversions are
supported:

   * %C indicates a value of `enum tree_code'.

   * %D indicates a *_DECL node.

   * %E indicates a *_EXPR node.

   * %L indicates a value of `enum languages'.

   * %P indicates the name of a parameter (i.e. "this", "1", "2", ...)

   * %T indicates a *_TYPE node.

   * %O indicates the name of an operator (MODIFY_EXPR -> "operator =").

   There is some overlap between these; for instance, any of the node
options can be used for printing an identifier (though only `%D' tries
to decipher function names).

   For a more verbose message (`class foo' as opposed to just `foo',
including the return type for functions), use `%#c'.  To have the line
number on the error message indicate the line of the DECL, use
`cp_error_at' and its ilk; to indicate which argument you want, use
`%+D', or it will default to the first.