final class Bool {

    immutable bool value;

    alias value this;

    this(bool b) nothrow pure @safe { this.value = b; }

}

struct ConstBool {

    private immutable Bool _b;

    @property Bool _uncast() const nothrow pure @trusted {

        // It is safe to convert an immutable Bool to a mutable Bool 

        // because even a mutable Bool has no mutable fields (and

        // cannot have any because Bool is a final class).

        return cast() this._b;

    }

    alias _uncast this;

    this(const Bool b) nothrow pure @trusted {

        // Can convert mutable bool to immutable by same logic as above.

        this._b = cast(immutable) b;

    }

    // https://forum.dlang.org/post/eicpsjarvxvahknluqwu@forum.dlang.org

    T opCast(T)() const {

        static if (is(bool : T))

            return _b.value;

        else

            return cast(T) this._uncast;

    }

}

immutable TRUE = ConstBool(new Bool(true)), FALSE = ConstBool(new Bool(false));

static assert(!FALSE); // Works in DMD 2.067.1 through 2.087.1 but in DMD 2.088.0-beta.1-master-ede5969 fails with "Error: expression Bool(false) is not constant"

static assert(!!TRUE); // Works in DMD 2.067.1 through 2.087.1 but in DMD 2.088.0-beta.1-master-ede5969 fails with "Error: expression Bool(true) is not constant"

​

void main()

{

}

​