cmd/compile and gccgo accept this Go source, but go/types rejects it with "cannot compare x == y (mismatched types b and bool)":

package p

func f() bool {
    var i interface{}
    type b bool
    var x, y b = i.(b)
    return x == y
}

My reading of the Go spec is that cmd/compile and gccgo are correct here:

If a type is present, each variable is given that type.

So x and y should have type b.

A type assertion used in an assignment or initialization of the special form [...] yields an additional untyped boolean value.

So y is initialized with an untyped boolean value, which is assignable to the named boolean type b.

Arguably the syntax var x, y b = i.(b) isn't actually one of the three "special forms" described in the Go spec, but none of cmd/compile, go/types, or gccgo complain about that. It seems reasonable to expect that var x, y b = i.(b) should behave identically to var x, y b; x, y = i.(b) (which all three tools allow).

/cc @griesemer @ianlancetaylor