interp: fix type switch on arbitrary expressions

If the value on which to type-switch was already set (i.e. a variable),
there was no problem. But if it had to be obtained through a complex
expression (func call, array index, etc...), then the code to retrieve
the value prior type-switch was not scheduled. This is now fixed.

This issue is nasty because the behavior is silently changed,
leading potentially to further unrelated issues or runtime panics.

Fixes #1444.

_test/switch39.go

@@ -0,0 +1,17 @@
+package main
+
+func f(params ...interface{}) {
+	switch p0 := params[0].(type) {
+	case string:
+		println("string:", p0)
+	default:
+		println("not a string")
+	}
+}
+
+func main() {
+	f("Hello")
+}
+
+// Output:
+// string: Hello

_test/switch40.go

@@ -0,0 +1,17 @@
+package main
+
+func f(params ...interface{}) {
+	switch params[0].(type) {
+	case string:
+		println("a string")
+	default:
+		println("not a string")
+	}
+}
+
+func main() {
+	f("Hello")
+}
+
+// Output:
+// a string

interp/cfg.go

@@ -2051,7 +2051,14 @@ func (interp *Interpreter) cfg(root *node, sc *scope, importPath, pkgName string
 			}
 			sbn.start = clauses[0].start
 			n.start = n.child[0].start
-			n.child[0].tnext = sbn.start
+			if n.kind == typeSwitch {
+				// Handle the typeSwitch init (the type assert expression).
+				init := n.child[1].lastChild().child[0]
+				init.tnext = sbn.start
+				n.child[0].tnext = init.start
+			} else {
+				n.child[0].tnext = sbn.start
+			}
 
 		case switchIfStmt: // like an if-else chain
 			sc = sc.pop()