The cgo tool works with C, not Objective C. Sorry.

As far as I understand Objective-C translates to C code and this is why GoLang C package can work with Objective-C. And it looks inconsistent that godefs tools can not do this. Looks like (I'm not sure) that godefs just use wrong/ignore some compile options.

It may well be that it sometimes works. Let me put it this way: we only support using the cgo tool with C, not Objective C. And I will add that the -godefs option is mainly used for internal purposes only, and is not intended to be a general purpose tool.

What are you really trying to do?

I need to pass variable of type T from Go code to Objective-C code. T is structure. So I need to create compatible (in terms of memory layout) type/variable in GoLang and pass it to Objective-C via unsafe.Pointer casting. For generic C code I use godefs in such cases.

I see. I don't think there is a way to do that if the type is only defined in Objective C.

I see that godefs compile source to extract information about type (I would like to say that godefs does not parse source directly). If so then whats the diffences C code or Objective-C code if last converted to C code?
I just want to understand and may be report future request.

I'm sorry, I don't know what the difference is. I see that you tried it, and that it doesn't work. I know for sure that we do not want to take on the burden of supporting Objective C going forward. I understand that it would be useful for you.

Finally I found a solution. It is possible to do what I want. The only thing I missed: godefs ignores #cgo CFLAGS: -x objective-c line. Solution - pass required CFLAGS to go binary as arguments (-- -x objective-c).
Working example:

// +build ignore

package tmp

//go:generate go tool cgo -godefs -- -x objective-c cgodefs.go

//#import <Cocoa/Cocoa.h>
import "C"

type CGPoint C.CGPoint

output:

// Created by cgo -godefs - DO NOT EDIT
// cgo -godefs -- -x objective-c cgodefs.go

package tmp

type CGPoint struct {
	X	float64
	Y	float64
}