The issue here is that declval<T>().d is considered instantiation-dependent
within a template, as the access to 'd' might depend on the particular
specialization.  But when we're deducing template arguments for a call, we
know that the call and the arguments are non-dependent, so we can do the
substitution as though we aren't in a template.  Which strictly speaking we
aren't, since the default argument is considered a separate definition.

	* pt.c (type_unification_real): Accept a dependent result in
	template context.

From-SVN: r269826