On 21.11.2011 09:24, Tobias Pankrath wrote:
Actually, that was my starting point. However, this approach does not
scale up:
A concept typically is a collection of requirements that have to be met
individually. A huge boolean statement inside an assertion does not give
any clue as to which part of it failed.
The isXxx approach works fine for the template constraint itself, but to
assert that a given type meets all requirements of a concept, it is not
very usable at all.
Wouldn't it be preferable to improve the compiler to a degree, that it can
tell, which subexpression of a constraint failed?
Two problems:
a) failing constraints do not lead to a compiler error at all but simply
lead to the template being ignored. It might be possible to improve the
error message if no matching template is found by listing all the
templates that were ignored due to constraints. Still, this would have
to be done carefully, to avoid making the message even more confusing.
b) constraints are simply general expressions that evaluate to a
boolean. It is only the very special situation of chained &&-expressions
that one could possibly "blame" a subexpression for the failure. I am
not sure whether this situation justifies a special handling.
