[complex.literals]

This subclause describes literal suffixes for constructing complex number literals.

The suffixes i, il, and if create complex numbers of the types complex<double>, complex<long double>, and complex<float> respectively, with their imaginary part denoted by the given literal number and the real part being zero.

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constexpr complex<long double> operator""il(long double d);
constexpr complex<long double> operator""il(unsigned long d);

Returns: complex<long double>{0.0L, static_cast<long double>(d)}.

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constexpr complex<double> operator""i(long double d);
constexpr complex<double> operator""i(unsigned long d);

Returns: complex<double>{0.0, static_cast<double>(d)}.

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constexpr complex<float> operator""if(long double d);
constexpr complex<float> operator""if(unsigned long d);

Returns: complex<float>{0.0f, static_cast<float>(d)}.