false negative in defer assignment

This code intends to overwrite a function's return value, but ineffectually assigns to a local variable instead:
```go
package main

import (
	"errors"
	"fmt"
)

func main() {
	fmt.Println("Hello", ineffassignTest()) // Hello <nil>
}

func ineffassignTest() error {
	var err error
	defer func() {
		err = errors.New("ineffectual assignment")
	}()
	return err // always returns nil
}
```