We were allocating the implicit attribute in the declarator's attribute pool, but putting into the declaration specifier's ParsedAttributesView. If there are multiple declarators, then we'll use the attribute from the declaration specifier after clearing out the declarators attribute pool. Fix this by allocating the attribute in the declaration specifier's pool. This problem was creating some nonsensical diagnostics and crashes on the testcase (only in NDEBUG, though).
Thanks for taking a look!