cmd/compile: better error message when attempting to use a shadowed type name

[pwaller]

The compiler will emit a confusing error when a type is shadowed within a function. It should somehow try and make clear that the inner type shadows the global type.

prog.go:13: cannot use Range literal (type Range) as type Range in assignment

Link to play.golang.org example

Link to golang-nuts thread

package main

type Range struct{ Start, End int }

type Foo struct{ rng Range }

func main() {

    type Range struct{ Start, End int }
    foos := []Foo{}

    for i, foo := range foos {
        foos[i].rng = Range{}
    }
}

See also #8853.

Edit: I incorrectly referred to "shadowing" as "aliasing".

[griesemer]

Just for the record: There's no "aliasing" going on here - you are declaring a new Range that happens to shadow an outer Range type; not a recommended practice for types.

It's probably a bit of work to make these cases print more information when it makes sense (such as in this one), and not when it's not needed.

Certainly not urgent.