cmd/cgo: match note following error in compiler errors

With current GCC a macro that refers to another macro can report an
error on the macro definition line, with a note on the use.
When cgo is trying to decide which line an error refers to,
it is looking at the uses. So if we see an error on a line that we
don't recognize followed by a note on a line that we do recognize,
treat the note as an error.

Fixes #20125.

Change-Id: I389cd0eb7d56ad2d54bef70e278d9f76c4d36448
Reviewed-on: https://go-review.googlesource.com/44290
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Hiroshi Ioka <hirochachacha@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

misc/cgo/test/issue18720.go

@@ -8,6 +8,10 @@ package cgotest
 #define HELLO "hello"
 #define WORLD "world"
 #define HELLO_WORLD HELLO "\000" WORLD
+
+struct foo { char c; };
+#define SIZE_OF(x) sizeof(x)
+#define SIZE_OF_FOO SIZE_OF(struct foo)
 */
 import "C"
 import "testing"
@@ -16,4 +20,9 @@ func test18720(t *testing.T) {
 	if C.HELLO_WORLD != "hello\000world" {
 		t.Fatalf(`expected "hello\000world", but got %q`, C.HELLO_WORLD)
 	}
+
+	// Issue 20125.
+	if got, want := C.SIZE_OF_FOO, 1; got != want {
+		t.Errorf("C.SIZE_OF_FOO == %v, expected %v", got, want)
+	}
 }

src/cmd/cgo/gcc.go

@@ -367,10 +367,17 @@ func (p *Package) guessKinds(f *File) []*Name {
 		notDeclared
 		notSignedIntConst
 	)
+	sawUnmatchedErrors := false
 	for _, line := range strings.Split(stderr, "\n") {
-		if !strings.Contains(line, ": error:") {
-			// we only care about errors.
-			// we tried to turn off warnings on the command line, but one never knows.
+		// Ignore warnings and random comments, with one
+		// exception: newer GCC versions will sometimes emit
+		// an error on a macro #define with a note referring
+		// to where the expansion occurs. We care about where
+		// the expansion occurs, so in that case treat the note
+		// as an error.
+		isError := strings.Contains(line, ": error:")
+		isErrorNote := strings.Contains(line, ": note:") && sawUnmatchedErrors
+		if !isError && !isErrorNote {
 			continue
 		}
 
@@ -388,6 +395,9 @@ func (p *Package) guessKinds(f *File) []*Name {
 		i, _ := strconv.Atoi(line[c1+1 : c2])
 		i--
 		if i < 0 || i >= len(names) {
+			if isError {
+				sawUnmatchedErrors = true
+			}
 			continue
 		}
 
@@ -411,7 +421,14 @@ func (p *Package) guessKinds(f *File) []*Name {
 			sniff[i] |= notStrLiteral
 		case "not-signed-int-const":
 			sniff[i] |= notSignedIntConst
+		default:
+			if isError {
+				sawUnmatchedErrors = true
+			}
+			continue
 		}
+
+		sawUnmatchedErrors = false
 	}
 
 	if !completed {