Currently the constraint type needs to be specified twice when using GenericConstraintDef:

new GenericConstraintDef<MyConstraint>( MyConstraint.class )
    .param( "myParam", 2 );

This could be avoided e.g. by providing a static factory method which leverages type inference:

GenericConstraintDef.getInstance( MyConstraint.class )
    .param( "myParam", 2 );

Or we provide a parameterless constructor used by anonymous subclasses and retrieve the value of the type parameter internally:

new GenericConstraintDef<LuggageCountMatchesPassengerCount>(){}
    .param( "myParam", 2 );

The latter would break compatability though if we want to enforce subclassing by making GenericConstraintDef an anonymous class.