fix interface methods that return a value

vlib/compiler/cgen.v

@@ -389,6 +389,7 @@ fn sort_structs(types []Type) []Type {
 	return types_sorted
 }
 
+// Generates interface table and interface indexes
 fn (v &V) interface_table() string {
        mut sb := strings.new_builder(100)
        for _, t in v.table.typesmap {

vlib/compiler/fn.v

@@ -735,7 +735,7 @@ fn (p mut Parser) fn_call(f mut Fn, method_ph int, receiver_var, receiver_type s
 				p.cgen.resetln('')
 				var := p.expr_var.name
 				iname := f.args[0].typ // Speaker
-				p.gen('((void (*)())(${iname}_name_table[${var}._interface_idx][$idx]))(${var}._object)')
+				p.gen('(($f.typ (*)())(${iname}_name_table[${var}._interface_idx][$idx]))(${var}._object)')
 			}
 		}
 		//println('r=$receiver.typ RT=$receiver_type')

vlib/compiler/tests/interface_test.v

@@ -27,16 +27,18 @@ struct Foo {
 	speaker Speaker
 }	
 
-fn perform_speak(s Speaker) bool {
+fn perform_speak(s Speaker) {
 	s.speak()
-	return true
+	assert true
+	name := s.name()
+	assert name == 'Dog' || name == 'Cat'
 }
 
 fn test_perform_speak() {
 	d := Dog{}
-	assert perform_speak(d)
+	perform_speak(d)
 	cat := Cat{}
-	assert perform_speak(cat)
+	perform_speak(cat)
 	f := Foo {
 		//speaker: d
 	}