cmd/gc: fix capturing by value for range statements

Kindly detected by race builders by failing TestRaceRange.
ORANGE typecheck does not increment decldepth around body.

Change-Id: I0df5f310cb3370a904c94d9647a9cf0f15729075
Reviewed-on: https://go-review.googlesource.com/3507
Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/gc/range.c

@@ -18,14 +18,25 @@ typecheckrange(Node *n)
 	Node *v1, *v2;
 	NodeList *ll;
 
+	// Typechecking order is important here:
+	// 0. first typecheck range expression (slice/map/chan),
+	//	it is evaluated only once and so logically it is not part of the loop.
+	// 1. typcheck produced values,
+	//	this part can declare new vars and so it must be typechecked before body,
+	//	because body can contain a closure that captures the vars.
+	// 2. decldepth++ to denote loop body.
+	// 3. typecheck body.
+	// 4. decldepth--.
+
+	typecheck(&n->right, Erv);
+	if((t = n->right->type) == T)
+		goto out;
+
 	// delicate little dance.  see typecheckas2
 	for(ll=n->list; ll; ll=ll->next)
 		if(ll->n->defn != n)
 			typecheck(&ll->n, Erv | Easgn);
 
-	typecheck(&n->right, Erv);
-	if((t = n->right->type) == T)
-		goto out;
 	if(isptr[t->etype] && isfixedarray(t->type))
 		t = t->type;
 	n->type = t;
@@ -106,7 +117,9 @@ out:
 		if(ll->n->typecheck == 0)
 			typecheck(&ll->n, Erv | Easgn);
 
+	decldepth++;
 	typechecklist(n->nbody, Etop);
+	decldepth--;
 }
 
 void

src/cmd/gc/typecheck.c

@@ -2828,7 +2828,8 @@ checkassign(Node *stmt, Node *n)
 {
 	Node *r, *l;
 
-	if(n->defn != stmt) {
+	// Variables declared in ORANGE are assigned on every iteration.
+	if(n->defn != stmt || stmt->op == ORANGE) {
 		r = outervalue(n);
 		for(l = n; l != r; l = l->left) {
 			l->assigned = 1;

test/closure2.go

@@ -10,6 +10,7 @@
 package main
 
 func main() {
+	{
 		type X struct {
 			v int
 		}
@@ -31,7 +32,9 @@ func main() {
 		if y.v != 1 {
 			panic("y.v != 1")
 		}
+	}
 
+	{
 		type Z struct {
 			a [3]byte
 		}
@@ -44,7 +47,9 @@ func main() {
 		if z.a[1] != 1 {
 			panic("z.a[1] != 1")
 		}
+	}
 
+	{
 		w := 0
 		tmp := 0
 		f := func() {
@@ -63,3 +68,51 @@ func main() {
 		}()
 		f()
 	}
+
+	{
+		var g func() int
+		for i := range [2]int{} {
+			if i == 0 {
+				g = func() int {
+					return i // test that we capture by ref here, i is mutated on every interation
+				}
+			}
+		}
+		if g() != 1 {
+			panic("g() != 1")
+		}
+	}
+
+	{
+		var g func() int
+		q := 0
+		for range [2]int{} {
+			q++
+			g = func() int {
+				return q // test that we capture by ref here
+					 // q++ must on a different decldepth than q declaration
+			}
+		}
+		if g() != 2 {
+			panic("g() != 2")
+		}
+	}
+
+	{
+		var g func() int
+		var a [2]int
+		q := 0
+		for a[func() int {
+			q++
+			return 0
+		}()] = range [2]int{} {
+			g = func() int {
+				return q // test that we capture by ref here
+					 // q++ must on a different decldepth than q declaration
+			}
+		}
+		if g() != 2 {
+			panic("g() != 2")
+		}
+	}
+}