cmd/cgo: type error when using pointers to C functions

[bcmills]

If I try to pass a C function pointer back to C from Go, I get a type error. It appears that cgo is mapping the function pointer type to *[0]byte (since there isn't a direct way to express C function-pointer types in the Go type system), but it isn't inserting the correct type conversions when referring to function-pointer values:

cgocallback/main.go:

package main

/*
#include <stdio.h>

static void invoke(void (*f)()) {
	f();
}

void print_hello() {
	printf("Hello, !");
}
*/
import "C"

func main() {
	C.invoke(C.print_hello)
}

bcmills:~$ go build cgocallback
# cgocallback
src/cgocallback/main.go:17: cannot use _Cgo_ptr(_Cfpvar_fp_print_hello) (type unsafe.Pointer) as type *[0]byte in argument to _Cfunc_invoke

The workaround is to define a typedef for the function pointer type an explicitly convert to the typedef, but it would be nice if that workaround were not necessary.

package main

/*
…
typedef void (*closure)();
*/
import "C"

func main() {
	C.invoke(C.closure(C.print_hello))
}

bcmills:~$ go version
go version devel +2bbfa6f746 Thu Mar 9 15:36:43 2017 -0500 linux/amd64

[bcmills]

(CC: @ianlancetaylor @thanm)