Why an unused for index variable causes an error (but not always)?

[programandala-net]

First I have noticed a for unused index variable causes an error when there's a homonymous variable defined in the outer scope, instead of printing a warning (marked with XXX-4 in the example code below). That is my first doubt.

Then I have noticed the error does not happen with a previous identical loop (marked with XXX-2 in the example code). It seems it happens when the loop is the last one in the function: if loop 4 is moved to the top of the function, it is compiled; if loop 3 is copied below loop 4, i.e. at the bottom of the function, loop 4 is compiled as well. What am I missing? Or is it a bug?

Thank you in advance.

package for_unused_index

import "core:fmt"

x := 0

main :: proc() {

	for x := 0; x < 1; x += 1 do fmt.print(1) // loop 1

	// XXX-2 Why not the same error/warning here than in loop 4 below?:
	for x in 0 ..< 2 do fmt.print(2) // loop 2

	for _ in 0 ..< 3 do fmt.print(3) // loop 3

	// XXX-4 Why error "'x' declared but not used, possibly shadows declaration
	// at line…" instead of a warning?:
	for x in 0 ..< 4 do fmt.print(4) // loop 4
}