cmd/compile: add write barrier to type switch

Type switches need write barriers if the written-to
variable is heap allocated.

For the added needwritebarrier call, the right arg doesn't
really matter, I just pass something that will never disqualify
the write barrier.  The left arg is the one that matters.

Fixes #14306

Change-Id: Ic2754167cce062064ea2eeac2944ea4f77cc9c3b
Reviewed-on: https://go-review.googlesource.com/19481
Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/compile/internal/gc/gen.go

@@ -836,7 +836,7 @@ func gen(n *Node) {
 		Cgen_as_wb(n.Left, n.Right, true)
 
 	case OAS2DOTTYPE:
-		cgen_dottype(n.Rlist.N, n.List.N, n.List.Next.N, false)
+		cgen_dottype(n.Rlist.N, n.List.N, n.List.Next.N, needwritebarrier(n.List.N, n.Rlist.N))
 
 	case OCALLMETH:
 		cgen_callmeth(n, 0)

test/writebarrier.go

@@ -144,3 +144,17 @@ type T8 struct {
 func f16(x []T8, y T8) []T8 {
 	return append(x, y) // ERROR "write barrier"
 }
+
+func t1(i interface{}) **int {
+	// From issue 14306, make sure we have write barriers in a type switch
+	// where the assigned variable escapes.
+	switch x := i.(type) { // ERROR "write barrier"
+	case *int:
+		return &x
+	}
+	switch y := i.(type) { // no write barrier here
+	case **int:
+		return y
+	}
+	return nil
+}