gc: check for assignment to private fields during initialization

R=ken2
https://golang.org/cl/165055

src/cmd/gc/typecheck.c

@@ -1961,8 +1961,12 @@ typecheckas(Node *n)
 
 	checkassign(n->left);
 	typecheck(&n->right, Erv);
-	if(n->left->type != T && n->right && n->right->type != T)
+	if(n->right && n->right->type != T) {
-		n->right = typecheckconv(nil, n->right, n->left->type, 0, nil);
+		if(n->left->type != T)
+			n->right = typecheckconv(nil, n->right, n->left->type, 0, "assignment");
+		else
+			exportassignok(n->right->type, "assignment");
+	}
 	if(n->left->defn == n && n->left->ntype == N) {
 		defaultlit(&n->right, T);
 		n->left->type = n->right->type;

test/assign.go

@@ -42,4 +42,12 @@ func main() {
 		x := sync.Mutex{key: 0};	// ERROR "(unknown|assignment).*Mutex"
 		_ = x;
 	}
+	{
+		x := &sync.Mutex{};	// ok
+		var y sync.Mutex;	// ok
+		y = *x;	// ERROR "assignment.*Mutex"
+		*x = y;	// ERROR "assignment.*Mutex"
+		_ = x;
+		_ = y;
+	}		
 }

test/fixedbugs/bug226.dir/x.go

@@ -0,0 +1,9 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package x
+
+type T struct { x, Y int }
+
+func (t T) M()

test/fixedbugs/bug226.dir/y.go

@@ -0,0 +1,31 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package y
+
+import "./x"
+
+func f() {
+	ok := new(x.T);
+	var ok1 x.T;
+	ok2 := &ok1;
+	ok3 := &x.T{};
+	ok4 := &x.T{Y:2};
+	_ = x.T{};
+	_ = x.T{Y:2};
+	
+	ok1.M();	// ERROR "assignment.*T"
+	bad1 := *ok;	// ERROR "assignment.*T"
+	bad2 := ok1;	// ERROR "assignment.*T"
+	*ok4 = ok1;	// ERROR "assignment.*T"
+	*ok4 = *ok2;	// ERROR "assignment.*T"
+	ok1 = *ok4;	// ERROR "assignment.*T"
+	_ = bad1;
+	_ = bad2;
+	_ = ok4;
+	_ = ok3;
+	_ = ok2;
+	_ = ok1;
+	_ = ok;
+}

test/fixedbugs/bug226.go

@@ -0,0 +1,7 @@
+// $G $D/$F.dir/x.go && errchk $G $D/$F.dir/y.go
+
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+ignored