See https://golang.org/cl/118690044/#msg3

$ cat -n issue7757.go
     1  // Copyright 2014 The Go Authors.  All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  package main
     6  
     7  /*
     8  void foo() {}
     9  */
    10  import "C"
    11  
    12  func main() {
    13      C.foo = C.foo // ERROR HERE
    14  }

$ go tool cgo issue7757.go

$ cat _obj/issue7757.cgo1.go
// Created by cgo - DO NOT EDIT

//line /usr/local/google/home/mdempsky/wd/go2/misc/cgo/errors/issue7757.go:5
package main
//line /usr/local/google/home/mdempsky/wd/go2/misc/cgo/errors/issue7757.go:13

//line /usr/local/google/home/mdempsky/wd/go2/misc/cgo/errors/issue7757.go:12
func main() {
//line /usr/local/google/home/mdempsky/wd/go2/misc/cgo/errors/issue7757.go:12
    _Cgo_ptr(_Cfpvar_fp_foo) = _Cgo_ptr(_Cfpvar_fp_foo)
//line /usr/local/google/home/mdempsky/wd/go2/misc/cgo/errors/issue7757.go:14
}


Because of the "//line" output, the go compiler thinks the illegal assignment
error is on line 12, when it actually came from line 13.