fix: add method checks for interface types

_test/interface14.go

@@ -0,0 +1,17 @@
+package main
+
+type T struct{}
+
+func (t *T) Error() string { return "T: error" }
+
+var invalidT = &T{}
+
+func main() {
+	var err error
+	if err != invalidT {
+		println("ok")
+	}
+}
+
+// Output:
+// ok

_test/interface15.go

@@ -0,0 +1,28 @@
+package main
+
+type Fooer interface {
+	Foo() string
+}
+
+type Barer interface {
+	//fmt.Stringer
+	Fooer
+	Bar()
+}
+
+type T struct{}
+
+func (t *T) Foo() string { return "T: foo" }
+func (*T) Bar()          { println("in bar") }
+
+var t = &T{}
+
+func main() {
+	var f Barer
+	if f != t {
+		println("ok")
+	}
+}
+
+// Output:
+// ok

_test/interface16.go

@@ -0,0 +1,25 @@
+package main
+
+import "fmt"
+
+type Barer interface {
+	fmt.Stringer
+	Bar()
+}
+
+type T struct{}
+
+func (*T) String() string { return "T: nothing" }
+func (*T) Bar()           { println("in bar") }
+
+var t = &T{}
+
+func main() {
+	var f Barer
+	if f != t {
+		println("ok")
+	}
+}
+
+// Output:
+// ok

_test/interface17.go

@@ -0,0 +1,17 @@
+package main
+
+type T struct{}
+
+func (t T) Error() string { return "T: error" }
+
+var invalidT = T{}
+
+func main() {
+	var err error
+	if err != invalidT {
+		println("ok")
+	}
+}
+
+// Output:
+// ok

interp/cfg.go

@@ -547,7 +547,7 @@ func (interp *Interpreter) cfg(root *node) ([]*node, error) {
 			t0, t1 := c0.typ.TypeOf(), c1.typ.TypeOf()
 			// Shift operator type is inherited from first parameter only
 			// All other binary operators require both parameter types to be the same
-			if !isShiftNode(n) && !c0.typ.untyped && !c1.typ.untyped && c0.typ.id() != c1.typ.id() {
+			if !isShiftNode(n) && !c0.typ.untyped && !c1.typ.untyped && !c0.typ.equal(c1.typ) {
 				err = n.cfgErrorf("mismatched types %s and %s", c0.typ.id(), c1.typ.id())
 				break
 			}

interp/run.go

@@ -260,7 +260,7 @@ func assign(n *node) {
 		switch {
 		case dest.typ.cat == interfaceT:
 			svalue[i] = genValueInterface(src)
-		case dest.typ.cat == valueT && dest.typ.rtype.Kind() == reflect.Interface:
+		case (dest.typ.cat == valueT || dest.typ.cat == errorT) && dest.typ.rtype.Kind() == reflect.Interface:
 			svalue[i] = genInterfaceWrapper(src, dest.typ.rtype)
 		case dest.typ.cat == valueT && src.typ.cat == funcT:
 			svalue[i] = genFunctionWrapper(src)

interp/type.go

@@ -636,6 +636,51 @@ func (t *itype) finalize() (*itype, error) {
 	return t, err
 }
 
+// equal returns true if the given type is identical to the receiver one
+func (t *itype) equal(o *itype) bool {
+	if isInterface(t) || isInterface(o) {
+		// Check for identical methods sets
+		return reflect.DeepEqual(t.methods(), o.methods())
+	}
+	return t.id() == o.id()
+}
+
+// methods returns a map of method type strings, indexed by method names
+func (t *itype) methods() map[string]string {
+	res := make(map[string]string)
+	switch t.cat {
+	case interfaceT:
+		// Get methods from recursive analysis of interface fields
+		for _, f := range t.field {
+			if f.typ.cat == funcT {
+				res[f.name] = f.typ.TypeOf().String()
+			} else {
+				for k, v := range f.typ.methods() {
+					res[k] = v
+				}
+			}
+		}
+	case valueT, errorT:
+		// Get method from corresponding reflect.Type
+		for i := t.rtype.NumMethod() - 1; i >= 0; i-- {
+			m := t.rtype.Method(i)
+			res[m.Name] = m.Type.String()
+		}
+	case ptrT:
+		// Consider only methods where receiver is a pointer to type t
+		for _, m := range t.val.method {
+			if m.child[0].child[0].lastChild().typ.cat == ptrT {
+				res[m.ident] = m.typ.TypeOf().String()
+			}
+		}
+	default:
+		for _, m := range t.method {
+			res[m.ident] = m.typ.TypeOf().String()
+		}
+	}
+	return res
+}
+
 // id returns a unique type identificator string
 func (t *itype) id() string {
 	// TODO: if res is nil, build identity from String()