It often happens that a derived class constructor that should invoke a
non-default base class constructor omits it, thereby silently invoking
the default base class constructor. I have repeatedly found this (e.g.
e.g. in copy constructors) when called in to track down other people's bugs.

How about saying that "explicit C()" would imply that a derived
class constructor would need to explicitly invoke the base default ctor
if that constructor is intended.

In other words,
class C
{
   int _i ;
   explicit C(){}
   C(int j): _i(j){}
};
class D: public C
{
    D():C(){}  // rather than just D(){}
    D(){}                // Would at least warn that default ctor C() will
be used
    D(int j):C(j){}    // ok
    D(int j){}          // Would at least warn that default ctor C() will be
used
};

This should be easy to implement, one would think, even if merely as a
compiler warning,
without waiting for a language spec change.

Is there any compelling reason not to do this?

   David
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to '@'.
   cNpOp$aStPl-iAnMtSl.cPoAmM
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