cmd/gc: fix error line in switch expr eval

Fixes #4562.

R=ken2
CC=golang-dev
https://golang.org/cl/7008044

src/cmd/gc/swt.c

@@ -259,10 +259,11 @@ casebody(Node *sw, Node *typeswvar)
 	Node *go, *br;
 	int32 lno, needvar;
 
-	lno = setlineno(sw);
 	if(sw->list == nil)
 		return;
 
+	lno = setlineno(sw);
+
 	cas = nil;	// cases
 	stat = nil;	// statements
 	def = N;	// defaults
@@ -270,7 +271,7 @@ casebody(Node *sw, Node *typeswvar)
 
 	for(l=sw->list; l; l=l->next) {
 		n = l->n;
-		lno = setlineno(n);
+		setlineno(n);
 		if(n->op != OXCASE)
 			fatal("casebody %O", n->op);
 		n->op = OCASE;
@@ -793,7 +794,6 @@ typeswitch(Node *sw)
 void
 walkswitch(Node *sw)
 {
-
 	/*
 	 * reorder the body into (OLIST, cases, statements)
 	 * cases have OGOTO into statements.

test/fixedbugs/issue4562.go

+// run
+
+// Copyright 2012 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 main
+
+import (
+	"fmt"
+	"runtime"
+	"strings"
+)
+
+type T struct {
+	val int
+}
+
+func main() {
+	defer expectError(22)
+	var pT *T
+	switch pT.val { // error should be here - line 22
+	case 0:
+		fmt.Println("0")
+	case 1: // used to show up here instead
+		fmt.Println("1")
+	case 2:
+		fmt.Println("2")
+	}
+	fmt.Println("finished")
+}
+
+func expectError(expectLine int) {
+	if recover() == nil {
+		panic("did not crash")
+	}
+	for i := 1;; i++ {
+		_, file, line, ok := runtime.Caller(i)
+		if !ok {
+			panic("cannot find issue4562.go on stack")
+		}
+		if strings.HasSuffix(file, "issue4562.go") {
+			if line != expectLine {
+				panic(fmt.Sprintf("crashed at line %d, wanted line %d", line, expectLine))
+			}
+			break
+		}
+	}
+}