Thanks tobyink. I was only referring to instances with DateTime loaded.

The documentation states that this fails silently: 

use DateTime;
subtype DateTime => as Object => where { $_->isa('DateTime') };

[download]

I am unable to reproduce this failure.

One idea was that if the behaviour of the fat comma quoting mechanism had changed since Perl 5.8, then maybe the failure only happens with earlier versions of Perl. If this wasn't the case, my only other thought was that this was a documentation error - I provided an example of what would fail silently: 

use DateTime;
subtype DateTime, as Object => where { $_->isa(q{DateTime}) };

[download]

-- Ken

In reply to Re^2: Moose::Util::TypeConstraints - Querying subtype() Caveat by kcott
in thread Moose::Util::TypeConstraints - Querying subtype() Caveat by kcott

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