"explicit" is a great tool for avoiding mistakes in the use of a class. Unfortunately, programmers must remember to use it, and there is not presently a mechanism to diagnose failure to do so.
I would like to request that g++ add (1) a warning if a constructor or conversion operator is not declared "explicit", and (2) some mechanism (e.g. an attribute) to silence this warning for a particular declaration, i.e. to annotate that a constructor or conversion operator is intended to be implicit. This would help to prevent forgetting to make such "explicit" when implicit behavior is not desired.
Confirming as an enhancement. Call it "-Wsuggest-explicit" perhaps?