This program

http://play.golang.org/p/RXFJJh5KRw

reports an error as expected, albeit a confusing one. This is related to, or the same as
issue #6764.

A short variable declaration in a for range loop can never redeclare an existing
variable because a for statement opens a new scope and thus there are no variables
declared earlier in that scope. So there should be a double declaration error.

Interestingly, adding an external declaration appears to "fix" the problem:

http://play.golang.org/p/4spddNy6Ki

The code compiles and runs without errors. Output:
0
1
2
3

Here it looks like the short variable declaration actually was handled like a
redeclaration of the outer a. Interestingly, value assigned to a is the index and not
the indexed element. Contrast this (and its output) to the following correct program:

http://play.golang.org/p/BWZD1fCWKj

Output:
1
2
3
3