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go/types: incorrect error about missing return #11698

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dvyukov opened this issue Jul 13, 2015 · 7 comments
Closed

go/types: incorrect error about missing return #11698

dvyukov opened this issue Jul 13, 2015 · 7 comments
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@dvyukov
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dvyukov commented Jul 13, 2015

gc successfully compiles the following program:

package a
func f() int {
    for {};
    ;
}

while gotype says:

go.go:5: missing return at end of function

go version devel +9b04852 Sat Jul 11 00:08:50 2015 +0000 linux/amd64

@dvyukov
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dvyukov commented Jul 13, 2015

@mdempsky
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I think the Go spec unambiguously supports the gotype error. Functions with result parameters need to end in a terminating statement, and empty statements are not terminating statements.

@ianlancetaylor ianlancetaylor added this to the Go1.6 milestone Jul 13, 2015
@rsc rsc modified the milestones: Unplanned, Go1.6 Dec 5, 2015
@odeke-em
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odeke-em commented Oct 3, 2016

Interestingly if we change the body of that function from for {} to add in conditions ie for true {} or for i := 0; i < 10; i++ {} the compiler catches the missing return https://play.golang.org/p/tMrrNlXJI6 or inlined here

package main

import "fmt"

func f() int {
    for {
    }
}

func f1() int {
    for true {
    }
}

func main() {
    fmt.Printf("hello world")
}
$ tmp/sandbox109964380/main.go:13: missing return at end of function

@extemporalgenome
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Per https://golang.org/ref/spec#Terminating_statements these programs contain a for loop without a loop condition or break statement, therefore the for loop is a terminating statement. These programs are thus valid and gotype is failing to detect the terminating statement?

@minux
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minux commented Oct 3, 2016

Isn't that this is working as intended for cmd/gc, and a bug
for gotype?

The for loop doesn't terminate, so the function won't ever
return, thus there is no need for a return.

@griesemer
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griesemer commented Oct 3, 2016

This is now a go/types error. The relevant lines from the spec are:

"If the function's signature declares result parameters, the function body's statement list must end in a terminating statement." (https://tip.golang.org/ref/spec#Function_declarations)

"A statement list ends in a terminating statement if the list is not empty and its final non-empty statement is terminating." (https://golang.org/ref/spec#Terminating_statements, end of paragraph).

Thus:

  1. The function must end in a terminating statement since it declares a result; and
  2. The final non-empty statement is the for loop which neither has a loop condition nor a break statement and thus is terminating.

@mdempsky was correct, too, because in 2015 this was still a gc bug :-) The relevant sentence in the spec changed in 2016 (b5ddbb9).

@griesemer griesemer modified the milestones: Go1.8, Unplanned Oct 3, 2016
@griesemer griesemer assigned griesemer and unassigned rsc Oct 3, 2016
@griesemer griesemer changed the title cmd/compile: does not detect a missing return go/types: incorrect error about missing return Oct 3, 2016
@griesemer
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With the spec change, this became a duplicate of #14537 which has been fixed. Not-reproducible anymore. Closing.

@golang golang locked and limited conversation to collaborators Oct 5, 2017
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