bcmills:~/src$ go version
go version devel +12f3099c7d Sat May 13 00:09:49 2017 -0400 linux/amd64
bcmills:~/src$ $(go env CC) --version
gcc (Ubuntu 4.8.4-2ubuntu1~14.04.3) 4.8.4

My use-case is for checking conversions between Go types (such as int) and corresponding C types (such as C.size_t). I'm attempting to define a portable constant for SIZE_MAX:

const SizeMax = uint64(C.SIZE_MAX)

But that gives me a compile error. A simple whole program demonstrating the problem:

package main

/*
#include <limits.h>
#include <stdint.h>
*/
import "C"

import "fmt"

const SizeMax = uint64(C.SIZE_MAX)

func main() {
	fmt.Println(SizeMax)
}

with output:

bcmills:~/src$ go run sizemax/sizemax.go
# command-line-arguments
sizemax/sizemax.go:11[sizemax.cgo1.go:12:23]: constant -1 overflows uint64

A larger example program shows that the C constant is defined in C as a large unsigned number, but cgo is compiling it to the two's-complement signed constant:

package main

/*
#include <limits.h>
#include <stdint.h>

#define xstr(s) str(s)
#define str(s) #s

// size_max_str returns a C string containing the definition of SIZE_MAX.
static const char* size_max_str() {
	return xstr(SIZE_MAX);
}
*/
import "C"

import "fmt"

func main() {
	fmt.Println(C.GoString(C.size_max_str()))
	fmt.Println(C.SIZE_MAX)
}

Actual output:

bcmills:~/src$ go run sizemax/sizemax.go
(18446744073709551615UL)
-1

Expected:
Compile error, because the constant 18446744073709551615 overflows the default int type for integer constants.

(CC @ianlancetaylor @mdempsky; see also #4120 and #2470.)