These warnings all emerged as the result of the introduction of the checks framework. Thanks to Anssi Kääriäinen for the report.
This is the result of Christopher Medrela's 2013 Summer of Code project. Thanks also to Preston Holmes, Tim Graham, Anssi Kääriäinen, Florian Apolloner, and Alex Gaynor for review notes along the way. Also: Fixes #8579, fixes #3055, fixes #19844.