What version of Go are you using (go version)?

go1.10 darwin/amd64

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/ben/Library/Caches/go-build"
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/ben/Documents/Coding/go"
GORACE=""
GOROOT="/usr/local/go"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/darwin_amd64"
GCCGO="gccgo"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/gb/hp3qsnwd0jz64k6gpnnt2ws40000gn/T/go-build705521711=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

Below code demonstrates the problem.

package cgotest

/*
#cgo LDFLAGS: -framework CoreFoundation
#include <CoreFoundation/CFBase.h>
#include <CoreFoundation/CFString.h>
*/
import "C"

// MyFunc demonstrates compilation problem.
func MyFunc() {

	var typeRef C.CFTypeRef

	cfStringRef := C.CFCopyDescription(typeRef)
	// This cast triggers 'cannot convert cfStringRef (type _Ctype_CFStringRef) to type _Ctype_CFTypeRef'
	defer C.CFRelease(C.CFTypeRef(cfStringRef))

	// Reference C.CFStringRef explicitly in code. Uncomment this line to allow compilation.
	// var _ C.CFStringRef

}

Wordy description...
I want to cast from C.CFStringRef to C.CFTypeRef in order to call C.CFRelease(...). This is a valid cast as both types are represented as uintptrs (as per https://go-review.googlesource.com/c/go/+/66332).

What is weird (and I believe a bug) is that the code does indeed compile if the C.CFStringRef type is mentioned in the Go code. i.e. uncomment the last line in that func and the compilation error goes away.

A possibly related thing that I also find weird is that you need the CFString.h include even though the CFStringRef type is defined in CFBase.h. Without that (and the last line uncommented) I get the same compilation error.

What did you expect to see?

Successful compilation

What did you see instead?

cannot convert cfStringRef (type _Ctype_CFStringRef) to type _Ctype_CFTypeRef