Why does C++ mandate that complex only be instantiated for float, double, or long double?

Question

According to the C++ ISO spec, §26.2/2:

The effect of instantiating the template complex for any type other than float,

Answer 1

Probably for compatibility with the helper functions. For example:

template T abs (const complex& x);

If T == int, abs would return int, which would mean a massive loss in precision.