On
go version devel +81a8f1a Mon May 30 05:24:16 2016 +0000 linux/amd64

the following program (https://play.golang.org/p/DbyUqeHamB), compiled with go run real.go

package main
const A = real(1.1)

exits with error message

/real.go:3: invalid argument 1.1 (type float64) for real

and behaves analogously with other kinds of untyped constants, passed as an argument
to real or imag.

The Go Language Specification seems to allow any kind of
untyped numeric constant to be passed as an argument to real or imag.

https://golang.org/ref/spec#Complex_numbers

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.

Both gotypes and gccgo accept the program without diagnostics.