Please answer these questions before submitting your issue. Thanks!

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

go version devel +3868a371a8 Tue May 15 20:47:43 2018 +0000 linux/amd64

Does this issue reproduce with the latest release?

Yes.

Example

Two files t.go and t.h are needed. t.go just calls a static function defined in t.h.

// t.go
package main

// #include "t.h"
import "C"

func main() {
	C.hello()
}

// t.h
#include <stdio.h>
static void hello() {
	printf("hello\n");
}

Reproduction steps

go build ignores changes to the cgo header file.

$ go build t.go
$ ./t
hello
$ sed -i -e 's/"hello/&, world/' t.h    # modify t.h without changing t.go
$ go build t.go  # nothing is rebuild
$ ./t
hello   # output unchanged.
$ GODEBUG=gocacheverify=1 go build a.go  # didn't catch the problem
$ go clean -cache && go build t.go && ./t
hello, world   # clearing the build cache finally fixes the bug

Discussion

Of course, changes like this is arguably hard for the Go tool to detect, and in fact, sometimes, we
don't want the Go tools to read all transitively required header files just to determine whether
to rebuild a package that uses cgo. However, the current behavior is very confusing, as it quietly
disregards changes to the header files, even the one in the package's directory.

I think we either should handle this completely (i.e. tap into gcc's -MM and handle changes to any transitively included files), or at least handle the directly included header file in Go source files (and make this problem clear in release notes.)