Go: cmd/compile: != operator incorrect for literal nil pointer compared to literal nil interface

[bcmills]

See https://play.golang.org/p/fAb8AX92nf:

package main

import (
    "fmt"
)

type ET struct{}

func (*ET) Error() string { return "err" }

func main() {
    fmt.Printf("(*ET)(nil) == error(nil): %v\n", (*ET)(nil) == error(nil))
    fmt.Printf("(*ET)(nil) != error(nil): %v\n", (*ET)(nil) != error(nil))

    nilET := (*ET)(nil)
    nilError := error(nil)

    fmt.Printf("nilET == nilError: %v\n", nilET == nilError)
    fmt.Printf("nilET != nilError: %v\n", nilET != nilError)
}

Expected output:

(*ET)(nil) == error(nil): false
(*ET)(nil) != error(nil): true
nilET == nilError: false
nilET != nilError: true

Actual output:

(*ET)(nil) == error(nil): false
(*ET)(nil) != error(nil): false
nilET == nilError: false
nilET != nilError: true

Note that the (*ET)(nil) != error(nil) case returns false. Unless I'm mistaken, it should return true.

[josharian]

Fun! Bisecting.

[josharian]

Go 1.8 agrees with tip. Go 1.7 rejects the code with:

# command-line-arguments
./x.go:12: illegal constant expression: *ET == error
./x.go:13: illegal constant expression: *ET != error

[griesemer]

Go 1.7 would be incorrect as well, as these are not constant expressions (nil is not a constant).

[ianlancetaylor]

gccgo says

(*ET)(nil) == error(nil): false
(*ET)(nil) != error(nil): true
nilET == nilError: false
nilET != nilError: true

[ianlancetaylor]

Go 1.5 and 1.6 also say illegal constant expression. Go 1.0 through Go 1.4 generate the same output as gccgo.