Originally reported by Evan Kroske via e-mail to golang-dev (July 27, 2014):

=================
In the process of working on [issue #6764][1], I realized that gc permits declaring a
variable twice in the same short variable declaration if the variable is shadowing
another variable.

For example, gc reports an error for the following code:

    a, a := 1, 2
    _ = a

However, gc compiles this code without complaint.

    var a int8
    {
        a, a := 1, 2
        println(a)
    }
    _ = a
=================

The compiler appears to accept this 2nd short variable declaration as a redeclaration of
the outer a, and new declaration of the inner a:

http://play.golang.org/p/Vy0kYBj82w

(running this program prints 1, 2, 3).

- gccgo also accepts this w/o errors.

- gotype complains with an error for this program:

$ gotype x.go
x.go:7:6: a redeclared in this block
x.go:7:3:   other declaration of a

The spec is not 100% clear, but the compiler behavior is odd at any rate: 

- it is not clear which one (the 1st or the 2nd) variable is the redeclaration, and thus
the value of the redeclared variable is unknown from the program
- code relying on this mechanism is likely to have bugs
- there's no such code in the std library since gotype passes it w/o errors

I think this is a compiler bug.

See also related issue #6764.