double y = 0.0; // initialized to 0.0
    double x; // initialized to NaN

I don't know of any other programming language with this safety feature.

Also, the D `char` type is initialized to 0xFF, not 0, because Unicode says that 0xFF is an invalid character.

For me, the major advantage of nan boxing is that you don't have to allocate a whole class of types (like floats). That saves so much at garbage collection time.

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[1] https://en.wikipedia.org/wiki/Hilbert%27s_paradox_of_the_Gra...