Detect never-nil interface returns

[FiloSottile]

The following program prints false because the foo() return value is always a non-nil interface wrapping a nil bar() return value.

package main

import "errors"

func main() {
	x, _ := foo()
	println(x == nil)
}

func foo() (interface{}, error) {
	return bar()
}

func bar() (*struct{}, error) {
	return nil, errors.New("error")
}

Today I was about to ship something similar to an exposed standard library interface when a test stopped me.

Not sure it can be done with few enough false positives, but I would have loved for staticcheck to point out that foo() always returns non-nil, which given the error return is almost certainly not intended.

[dominikh]

ISTM that we already catch this?

bar.go:7:10: this comparison is never true (SA4023)
	bar.go:6:2: the lhs of the comparison is the 1st return value of this function call
	bar.go:10:6: command-line-arguments.foo never returns a nil interface value