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cmd/compile: type checking fails in cycle involving recursive struct and unsafe.Sizeof #14620

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rsc opened this Issue Mar 3, 2016 · 6 comments

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rsc commented Mar 3, 2016

package p

import "unsafe"

type hdr struct {
    next *msg
}

type msg struct {
    hdr
    pad [1024-hdrsize]uint8
}

const hdrsize = unsafe.Sizeof(hdr{})

This should type check but does not:

$ go tool compile /tmp/x.go
/tmp/x.go:14: invalid type for composite literal: hdr
/tmp/x.go:14: invalid expression unsafe.Sizeof(hdr literal)

Looks like the compiler gets confused by the not-quite-cycle in the definition.

http://play.golang.org/p/Hbmv1j_UrR

/cc @RLH

@rsc rsc added this to the Go1.7 milestone Mar 3, 2016

@davmaz

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davmaz commented Mar 11, 2016

Could this also be the case (when using cgo):
phySize := unsafe.Sizeof(C.struct_ad9361_rf_phy)
giving a compile time error:
invalid expression unsafe.Sizeof(_Ctype_struct_ad9361_rf_phy)

@ianlancetaylor

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ianlancetaylor commented Mar 11, 2016

@davmaz No, the problem there is simply that unsafe.Sizeof takes a value, not a type. It's not the same as the C sizeof operation.

In general, for the Go project, please ask questions in a forum rather than on the issue tracker. See https://golang.org/wiki/Questions. Thanks.

@mdempsky

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mdempsky commented Apr 18, 2016

The problem is that when typechecking a pointer to a named type like *msg, cmd/compile recurses and tries to completely typecheck msg. Instead, we should just do the minimum to get a *Type value for msg (in this case a TFORW type) so we can construct the *msg pointer type. Later when we visit msg's type declaration, we can finish typechecking it.

A workaround is to move the hdr type declaration after the msg type declaration.

@rsc

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rsc commented May 17, 2016

Quite possibly Robert's front end revamp will take care of this. Maybe not worth worrying about before then.

@rsc rsc modified the milestones: Go1.8, Go1.7 May 17, 2016

@griesemer

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griesemer commented May 18, 2016

FWIW, gotype accepts this code w/o problems. Since there's a work-around per @mdempsky I don't think there's any urgency here.

@griesemer griesemer self-assigned this May 18, 2016

@griesemer griesemer modified the milestones: Go1.8Maybe, Go1.8 May 18, 2016

@quentinmit quentinmit added the NeedsFix label Oct 10, 2016

@rsc rsc modified the milestones: Go1.9, Go1.8Maybe Oct 20, 2016

@josharian

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josharian commented May 11, 2017

Reproduced at tip (00263a8) on May 10, 2017.

Still seems low priority; punting to 1.10.

@josharian josharian modified the milestones: Go1.10, Go1.9 May 11, 2017

@griesemer griesemer modified the milestones: Go1.10, Go1.11 Nov 29, 2017

@griesemer griesemer modified the milestones: Go1.11, Go1.12 Jun 5, 2018

@griesemer griesemer modified the milestones: Go1.12, Go1.13 Oct 24, 2018

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