D-Programming-Language/phobos — pull request #1079 by AndrejMitrovic (2013-01-19 02:02:55+00:00)
Title: Issue 5065 - Format specifier must be %s for some types

http://d.puremagic.com/issues/show_bug.cgi?id=5065

Essentially the issue is that a format specifier such as `%f` doesn't make sense for aggregate types unless they define the `toString` version of the function which takes a format specifier (they can throw internally if they want to on invalid specs).