cmd/compile: fix mis-compilation with labeled fallthrough

A fallthrough statement can be a labeled fallthrough per Go spec.
However, the hasFallthrough function is not considering this case,
causing mis-compilation.

Fixing this by un-wrapping (possible nested) labeled fallthrough
statements if any.

Fixes #70173

Change-Id: Ic93d4fb75ff02703a32dfc63c3e84a8b7f78c261
Reviewed-on: https://go-review.googlesource.com/c/go/+/624717
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Youlin Feng <fengyoulin@live.com>
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/cmd/compile/internal/noder/writer.go

@@ -3070,7 +3070,17 @@ func isPtrTo(from, to types2.Type) bool {
 // hasFallthrough reports whether stmts ends in a fallthrough
 // statement.
 func hasFallthrough(stmts []syntax.Stmt) bool {
-	last, ok := lastNonEmptyStmt(stmts).(*syntax.BranchStmt)
+	// From spec: the last non-empty statement may be a (possibly labeled) "fallthrough" statement
+	// Stripping (possible nested) labeled statement if any.
+	stmt := lastNonEmptyStmt(stmts)
+	for {
+		ls, ok := stmt.(*syntax.LabeledStmt)
+		if !ok {
+			break
+		}
+		stmt = ls.Stmt
+	}
+	last, ok := stmt.(*syntax.BranchStmt)
 	return ok && last.Tok == syntax.Fallthrough
 }
 

src/cmd/compile/testdata/script/issue70173.txt

@@ -0,0 +1,23 @@
+go run main.go
+! stdout .
+! stderr .
+
+-- main.go --
+
+package main
+
+func main() {
+	switch {
+	case true:
+	_:
+		fallthrough
+	default:
+	}
+	switch {
+	case true:
+	_:
+	_:
+		fallthrough
+	default:
+	}
+}