std.traits.isBoolean

[Tony]

At
https://dlang.org/library/std/traits/is_boolean.html

it has:


enum isBoolean(T) = is(BooleanTypeOf!T) && !isAggregateType!T;

per:
https://dlang.org/library/std/traits/is_aggregate_type.html

isAggregateType is true for [struct, union, class, interface].

So BooleanTypeOf!T is true for structs, unions, classes and 
interfaces? And if yes, why is that so?