staticcheck: detect shadowing of named returns

[florianl]

staticcheck should detect the shadowing of named returns.

In the following example, the function foo() uses the named return err error. In this case, the named return is shadowed by err := bar().

func bar() error {
 return nil
}

func foo() (err error) {
  err := bar()
  return
}

[dominikh]

Note that the provided example doesn't compile, because that's not shadowing but trying to redeclare. And if it had worked, it would've been an unused variable and also hadn't compiled.

Do you have a closer-to-reality example you'd like to see flagged by Staticcheck?

[florianl]

This is the example from the Go specification (last example in the Return statements section):

func f(n int) (res int, err error) {
	if _, err := f(n-1); err != nil {
		return  // invalid return statement: err is shadowed
	}
	return
}