------- Comment #2 from redi at gcc dot gnu dot org 2010-04-01 10:53 ------- the subject line is misleading, I think the placeholders are still substituted in exactly the same way, but as part of that an invalid instantiation is generated while determining the template_arity of the metafunction.
The invalid instantiation occurs in a sizeof expression and was previously ignored (presumably due to SFINAE) but now causes an error. I don't think SFINAE applies in that context, which would make 4.5 correct -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43608
