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const 2

What does the following program print?
package main

const X = 3

func main() {
	const (
		X = X + X
		Y
	)
	
	println(X, Y)
}

Choices:

Answer: 6 12

Run it on Go play.

Key points:

The consntant declaration in the quiz code is equivalent to the following one:

	const (
		X = X + X // here the two "X" are both the global one
		Y = X + X // here the two "X" are both the local one
	)

The local X is evaluated as 6 at compile time, so the constant Y is evaluaed as 12 (also at compile time).

Please note that, the output result was 6 6 when using Go toolchain v1.17-. The bug has been fixed since Go toochain v1.18.

Similarly, since Go toolchain 1.18, the following program prints 0 0. Before Go toolchain 1.18, it printed 1 2 (a bug).

package main

func main() {
	const (
		iota = iota
		X
		Y
	)
	println(X, Y)
}

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