Re: [boost] [type_traits] is_convertible<noncopyable,To> crashes

From: John Maddock (john_at_[hidden])
Date: 2006-04-14 06:19:11

> In a post from yesterday (see http://tinyurl.com/hzk5w),
> Alexei Alexandrov found a rather serious problem with
> is_convertible in at least MSVC 7.1 and Intel 9.0, namely
> that is_convertible<From,To> fails to compile when
>> From is non-copyable, emitting an error message like this:

Wait a second, I've just looked at the original error messages, and the test 
case is very specific, it fails for:

is_convertible<const noncopyable, const noncopyable>

That is known not to work, *and will never work*, there is no way we can 
define a trait that detects whether a type is copy-constructable or not. In 
other words the From and To parameters must be different types.

Likewise is_convertible<From, To> will fail if To has a private constructor 
taking a From as argument.

Sorry but it's an inherent limitation of the trait implemented without 
core-language support.

John.