Go spec says:

For real and imag, the argument must be of complex type, and the return type is the corresponding floating-point type: float32 for a complex64 argument, and float64 for a complex128 argument. If the argument evaluates to an untyped constant, it must be a number, and the return value of the function is an untyped floating-point constant.

$ cat /tmp/cplx.go
package cplx

const (
        a int = real(0i)
        b = imag(0)
)
$ go tool compile /tmp/cplx.go
/tmp/cplx.go:4: cannot use real(0i) (type float64) as type int in const initializer
/tmp/cplx.go:5: invalid argument 0 (type int) for imag
$ gccgo-4.8 -c /tmp/cplx.go
$ gotype /tmp/cplx.go