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

[3v1n0]

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]

see #60725 it's considered a bug as it's not allowed on go type aliases.

[gabyhelp]

Related Issues

• runtime/cgo: bitfields not supported #59982 (closed)
• cmd/cgo: types regression for anonymous structs #38408 (closed)
• cmd/cgo: exported go functions can't use C struct types (regression from 1.2) #8148 (closed)
• cmd/cgo: exported function name shadowing issue #44974
• cmd/cgo: don't accept mangled C names in Cgo programs #28721 (closed)
• go/types: invalid identifiers are no longer type checked #71777
• cmd/cgo: C struct embedding error #2552 (closed)
• cmd/cgo: -godefs doesn't work with structures fields inside another structure. #26644 (closed)
• cmd/cgo: ptr type casts of struct fields cause the whole struct to be checked #32970 (closed)
• cmd/cgo: sometimes when passing a struct to C containing a go pointer to a struct field, the entire go struct is checked for go pointers instead of just the field #63460 (closed)

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[3v1n0]

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

[ianlancetaylor]

@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.