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cmd/compile: accepts invalid comparisons against converted nil #13480

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mdempsky opened this issue Dec 4, 2015 · 8 comments
Closed

cmd/compile: accepts invalid comparisons against converted nil #13480

mdempsky opened this issue Dec 4, 2015 · 8 comments
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@mdempsky
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mdempsky commented Dec 4, 2015

cmd/compile accepts this source:

package zero

func isZeroSlice(s []byte) bool    { return s == []byte(nil) }
func isZeroMap(m map[int]int) bool { return m == map[int]int(nil) }
func isZeroFunc(f func()) bool     { return f == (func())(nil) }

but gotype rejects it with:

zero.go:3:45: cannot compare s == []byte(nil) (operator == not defined for []byte)
zero.go:4:45: cannot compare m == map[int]int(nil) (operator == not defined for map[int]int)
zero.go:5:45: cannot compare f == (func())(nil) (operator == not defined for func())

The Go spec says "However, as a special case, a slice, map, or function value may be compared to the predeclared identifier nil." which seems to support gotype's behavior.
@bradfitz
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bradfitz commented Dec 4, 2015

/cc @griesemer @alandonovan

@alandonovan
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I'm not certain the behavior of go/types is wrong. Constants have basic types (string, number, or boolean), so nil is not a constant. Thus the compiler is not required to know the value of an expression such as []byte(nil) at compile time.

EDIT: I'm agreeing with mdemsky. I just didn't realize it till now.

@mdempsky
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mdempsky commented Dec 4, 2015

@alandonovan Sorry, I'm not sure I follow your response. To clarify, the issue I'm trying to point out is that cmd/compile does compile the snippet of source I pasted without errors, whereas my understanding of the Go spec (and supported by gotype's behavior) says it should not.

@bradfitz
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bradfitz commented Dec 4, 2015

Also, surely it's a bug that cmd/compile and gotype differ in their opinion. What does gccgo say?
/cc @ianlancetaylor

@alandonovan
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Hi Matthew, sorry for the confusion. I clarified my response but of course that didn't send mail. I agree with you that go/types is correct and cmd/compile has a bug.

@mdempsky
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mdempsky commented Dec 4, 2015

Ah, okay. :)

@ianlancetaylor
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gccgo says:

foo.go:3:47: error: slice can only be compared to nil
 func isZeroSlice(s []byte) bool    { return s == []byte(nil) }
                                               ^
foo.go:4:47: error: map can only be compared to nil
 func isZeroMap(m map[int]int) bool { return m == map[int]int(nil) }
                                               ^
foo.go:5:47: error: func can only be compared to nil
 func isZeroFunc(f func()) bool     { return f == (func())(nil) }
                                               ^

@mdempsky mdempsky assigned mdempsky and unassigned griesemer Dec 4, 2015
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CL https://golang.org/cl/17461 mentions this issue.

@golang golang locked and limited conversation to collaborators Dec 14, 2016
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@bradfitz @mdempsky @ianlancetaylor @alandonovan @griesemer @gopherbot