reflect: implement Type.AssignableTo

src/reflect/type.go

@@ -216,6 +216,18 @@ func (t Type) Size() uintptr {
 	}
 }
 
+// AssignableTo returns whether a value of type u can be assigned to a variable
+// of type t.
+func (t Type) AssignableTo(u Type) bool {
+	if t == u {
+		return true
+	}
+	if t.Kind() == Interface {
+		panic("reflect: unimplemented: assigning to interface of different type")
+	}
+	return false
+}
+
 type StructField struct {
 	Name string
 	Type Type

src/reflect/value.go

@@ -388,12 +388,8 @@ func (v Value) Set(x Value) {
 	if !v.indirect {
 		panic("reflect: value is not addressable")
 	}
-	if v.Type() != x.Type() {
-		if v.Kind() == Interface {
-			panic("reflect: unimplemented: assigning to interface of different type")
-		} else {
-			panic("reflect: cannot assign")
-		}
+	if !v.Type().AssignableTo(x.Type()) {
+		panic("reflect: cannot set")
 	}
 	size := v.Type().Size()
 	xptr := x.value