cmd/compile: add test for non interface type switch

Ensure that we have a test for when the compiler
encounters a type switch on a non-interface value.

Change-Id: Icb222f986894d0190e1241ca65396b4950e7d14f
Reviewed-on: https://go-review.googlesource.com/38661Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

test/typeswitch3.go

@@ -18,26 +18,39 @@ type I interface {
 	M()
 }
 
-func main(){
+func main() {
 	var x I
 	switch x.(type) {
-	case string:	// ERROR "impossible"
+	case string: // ERROR "impossible"
 		println("FAIL")
 	}
-	
+
 	// Issue 2700: if the case type is an interface, nothing is impossible
-	
+
 	var r io.Reader
-	
+
 	_, _ = r.(io.Writer)
-	
+
 	switch r.(type) {
 	case io.Writer:
 	}
-	
+
 	// Issue 2827.
-	switch _ := r.(type) {  // ERROR "invalid variable name _|no new variables"
+	switch _ := r.(type) { // ERROR "invalid variable name _|no new variables"
 	}
 }
 
+func noninterface() {
+	var i int
+	switch i.(type) { // ERROR "cannot type switch on non-interface value"
+	case string:
+	case int:
+	}
 
+	type S struct {
+		name string
+	}
+	var s S
+	switch s.(type) { // ERROR "cannot type switch on non-interface value"
+	}
+}