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cmd/cgo: cannot define new methods on non-local type in 1.24 #71917

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3v1n0 opened this issue Feb 23, 2025 · 4 comments
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cmd/cgo: cannot define new methods on non-local type in 1.24 #71917

3v1n0 opened this issue Feb 23, 2025 · 4 comments
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compiler/runtime Issues related to the Go compiler and/or runtime.

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@3v1n0
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3v1n0 commented Feb 23, 2025 •
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Go version

go version go1.24.0 linux/amd64

Output of go env in your module/workspace:

AR='ar'
CC='gcc'
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_ENABLED='1'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
CXX='g++'
GCCGO='gccgo'
GO111MODULE=''
GOAMD64='v1'
GOARCH='amd64'
GOAUTH='netrc'
GOBIN=''
GOCACHE='/home/marco/.cache/go-build'
GOCACHEPROG=''
GODEBUG=''
GOENV='/home/marco/.config/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFIPS140='off'
GOFLAGS=''
GOGCCFLAGS='-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=/tmp/go-build3394646905=/tmp/go-build -gno-record-gcc-switches'
GOHOSTARCH='amd64'
GOHOSTOS='linux'
GOINSECURE=''
GOMOD='/home/marco/Dev/authd/go.mod'
GOMODCACHE='/tmp/go-path/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='linux'
GOPATH='/tmp/go-path'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/snap/go/10853'
GOSUMDB='sum.golang.org'
GOTELEMETRY='local'
GOTELEMETRYDIR='/home/marco/.config/go/telemetry'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/snap/go/10853/pkg/tool/linux_amd64'
GOVCS=''
GOVERSION='go1.24.0'
GOWORK=''
PKG_CONFIG='pkg-config'

What did you do?

We had some code doing:

package main

/*
#include <stdlib.h>

typedef struct {
        int value;
} Struct;
*/
import "C"
import "fmt"

type CStruct = C.Struct

func (s *CStruct) init(value int) {
	s.value = C.int(value)
}

func main() {
	s := &CStruct{}
	s.init(1234)
	fmt.Println(s.value)
}

What did you see happen?

./main.go:16:10: cannot define new methods on non-local type CStruct

What did you expect to see?

This used to work until go 1.23, now it's not allowed anymore.
@seankhliao seankhliao changed the title go 1.24 breaks some CGO usage: cannot define new methods on non-local type CStruct cmd/cgo: cannot define new methods on non-local type in 1.24 Feb 23, 2025
@gopherbot gopherbot added the compiler/runtime Issues related to the Go compiler and/or runtime. label Feb 23, 2025
@seankhliao
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see #60725 it's considered a bug as it's not allowed on go type aliases.

@gabyhelp
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Related Issues

(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)

@3v1n0
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3v1n0 commented Feb 24, 2025

So using instead type CStruct C.Struct fixes it, is that considered fine instead?

3v1n0 added a commit to 3v1n0/authd that referenced this issue Feb 24, 2025
@3v1n0
pam/gdm/extension: Redefine jsonProtoMessage type
8896399 
By using type alias instead we were getting compile errors in Go 1.24

See: golang/go#71917
@3v1n0 3v1n0 mentioned this issue Feb 24, 2025
Merged
3v1n0 added a commit to ubuntu/authd that referenced this issue Feb 24, 2025
@3v1n0
pam/gdm: Redefine jsonProtoMessage type for go 1.24 (#808)
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71483d8 
Fix the definition as it wouldn't work in newer go:

```
# github.com/ubuntu/authd/pam/internal/gdm [github.com/ubuntu/authd/pam/internal/gdm.test]
./extension.go:91:12: cannot define new methods on non-local type jsonProtoMessage
./extension.go:106:12: cannot define new methods on non-local type jsonProtoMessage
./extension.go:115:12: cannot define new methods on non-local type jsonProtoMessage
./extension.go:119:12: cannot define new methods on non-local type jsonProtoMessage
./extension.go:123:12: cannot define new methods on non-local type jsonProtoMessage
./extension.go:137:12: cannot define new methods on non-local type jsonProtoMessage
FAIL	github.com/ubuntu/authd/pam/internal/gdm [build failed]
```

See: golang/go#71917

UDENG-6160
@ianlancetaylor
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@3v1n0 Sure, that is fine. What is forbidden is defining methods on a C type. With type CStruct C.Struct you will be defining methods on the Go type CStruct.

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@3v1n0 @ianlancetaylor @gopherbot @seankhliao @gabyhelp