interp: fix a wrong control flow in switch

In switch case expressions, the condition on case clause was
not always properly evaluated. Reverse the order of case clause
evaluations (as already done for if-else-if fashion), and fix the
wiring to false-next and true-next nodes.

Fixes #1126.

_test/addr2.go

@@ -57,7 +57,7 @@ func main() {
 	fmt.Println(err, vvv)
 }
 
-// Ouput:
+// Output:
 // <nil> {work bob@work.com}
 // <nil> {work bob@work.com}
 // <nil> {work bob@work.com}

_test/addr4.go

@@ -103,7 +103,7 @@ func main() {
 	intoMap()
 }
 
-// Ouput:
+// Output:
 // 0 : foo
 // 1 : bar
 // 0 : foo

_test/issue-1094.go

@@ -8,5 +8,5 @@ func main() {
 	fmt.Printf("%v %T\n", x, x)
 }
 
-// Ouput:
+// Output:
 // ab string

_test/issue-1126.go

@@ -0,0 +1,23 @@
+package main
+
+import (
+	"errors"
+	"fmt"
+	"strings"
+)
+
+func main() {
+	err := errors.New("hello there")
+
+	switch true {
+	case err == nil:
+		break
+	case strings.Contains(err.Error(), "hello"):
+		fmt.Println("True!")
+	default:
+		fmt.Println("False!")
+	}
+}
+
+// Output:
+// True!

interp/cfg.go

@@ -1737,15 +1737,17 @@ func (interp *Interpreter) cfg(root *node, importPath string) ([]*node, error) {
 				break
 			}
 			// Chain case clauses.
-			for i, c := range clauses[:l-1] {
-				// Chain to next clause.
-				setFNext(c, clauses[i+1])
+			for i := l - 1; i >= 0; i-- {
+				c := clauses[i]
 				if len(c.child) == 0 {
 					c.tnext = n // Clause body is empty, exit.
 				} else {
 					body := c.lastChild()
 					c.tnext = body.start
-					if len(body.child) > 0 && body.lastChild().kind == fallthroughtStmt {
+					c.child[0].tnext = c
+					c.start = c.child[0].start
+
+					if i < l-1 && len(body.child) > 0 && body.lastChild().kind == fallthroughtStmt {
 						if n.kind == typeSwitch {
 							err = body.lastChild().cfgErrorf("cannot fallthrough in type switch")
 						}
@@ -1758,15 +1760,16 @@ func (interp *Interpreter) cfg(root *node, importPath string) ([]*node, error) {
 						body.tnext = n // Exit switch at end of clause body.
 					}
 				}
-			}
-			c := clauses[l-1] // Last clause.
-			c.fnext = n
-			if len(c.child) == 0 {
-				c.tnext = n // Clause body is empty, exit.
-			} else {
-				body := c.lastChild()
-				c.tnext = body.start
-				body.tnext = n
+
+				if i == l-1 {
+					setFNext(clauses[i], n)
+					continue
+				}
+				if len(clauses[i+1].child) > 1 {
+					setFNext(c, clauses[i+1].start)
+				} else {
+					setFNext(c, clauses[i+1])
+				}
 			}
 			n.start = n.child[0].start
 			n.child[0].tnext = sbn.start