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cmd/compile: != operator incorrect for literal nil pointer compared to literal nil interface #19911

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bcmills opened this issue Apr 10, 2017 · 5 comments
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cmd/compile: != operator incorrect for literal nil pointer compared to literal nil interface #19911

bcmills opened this issue Apr 10, 2017 · 5 comments
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@bcmills
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bcmills commented Apr 10, 2017

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
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Fun! Bisecting.

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

@ianlancetaylor
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gccgo says

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

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

lparth pushed a commit to lparth/go that referenced this issue Apr 13, 2017
@josharian @lparth
cmd/compile: make iface == iface const evaluation respect !=
fe9f69f 
Fixes golang#19911

Change-Id: Ib2b2505fe31ce00c6ffc021a0fe5df510633b44b
Reviewed-on: https://go-review.googlesource.com/40251
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
@golang golang locked and limited conversation to collaborators Apr 10, 2018
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@josharian @ianlancetaylor @bcmills @griesemer @gopherbot