fix: do type check on assignment (#738)

* fix: do type check on assignment

* fix: check for recursive type rather than field

_test/assign15.go

@@ -0,0 +1,11 @@
+package main
+
+func main() {
+	var c chan<- struct{} = make(chan struct{})
+	var d <-chan struct{} = c
+
+	_ = d
+}
+
+// Error:
+// _test/assign15.go:5:26: cannot use type chan<- struct{} as type <-chan struct{} in assignment

interp/cfg.go

@@ -518,6 +518,22 @@ func (interp *Interpreter) cfg(root *node, pkgID string) ([]*node, error) {
 						err = src.cfgErrorf("invalid float truncate")
 						return
 					}
+
+					// TODO: Rudimentary type check at this point,
+					// improvements need to be made to make it better.
+					switch {
+					case dest.typ.untyped || src.typ.untyped:
+						// Both side of the assignment must be typed.
+					case isRecursiveType(dest.typ, dest.typ.rtype) || isRecursiveType(src.typ, src.typ.rtype):
+						// Recursive types cannot be type checked.
+					case t0.Kind() == reflect.Interface || t0.Kind() == reflect.Func:
+						// We have no way of checking interfaces and functions.
+					case t1.AssignableTo(t0):
+						// All is well when they are assignable.
+					default:
+						err = src.cfgErrorf("cannot use type %s as type %s in assignment", src.typ.id(), dest.typ.id())
+						return
+					}
 				}
 				n.findex = dest.findex
 				n.level = dest.level
@@ -1157,6 +1173,7 @@ func (interp *Interpreter) cfg(root *node, pkgID string) ([]*node, error) {
 				case n.ident == "iota":
 					n.rval = reflect.ValueOf(sc.iota)
 					n.kind = basicLit
+					n.typ.untyped = true
 				case n.ident == "nil":
 					n.kind = basicLit
 				case sym.kind == binSym:

interp/interp_consistent_test.go

@@ -35,6 +35,7 @@ func TestInterpConsistencyBuild(t *testing.T) {
 		if filepath.Ext(file.Name()) != ".go" ||
 			file.Name() == "assign11.go" || // expect error
 			file.Name() == "assign12.go" || // expect error
+			file.Name() == "assign15.go" || // expect error
 			file.Name() == "bad0.go" || // expect error
 			file.Name() == "const9.go" || // expect error
 			file.Name() == "export1.go" || // non-main package

interp/type.go

@@ -1013,7 +1013,11 @@ func (t *itype) id() (res string) {
 	case arrayT:
 		res = "[" + strconv.Itoa(t.size) + "]" + t.val.id()
 	case chanT:
-		res = "<-" + t.val.id()
+		res = "chan " + t.val.id()
+	case chanSendT:
+		res = "chan<- " + t.val.id()
+	case chanRecvT:
+		res = "<-chan " + t.val.id()
 	case funcT:
 		res = "func("
 		for _, t := range t.arg {