tried to make type restriction a compile time constraint. that did not seem to work. so made it a run-time constraint.

optional_marshaljson.go

@@ -5,14 +5,20 @@ import (
 	"encoding/json"
 )
 
-type jsonMarshaler interface {
-	bool | string
-}
+var _ json.Marshaler = Nothing[bool]()
+var _ json.Marshaler = Nothing[string]()
 
 // MarshalJSON makes it so json.Marshaler is implemented.
-func (receiver Optional[jsonMarshal]) MarshalJSON() ([]byte, error) {
-	if !receiver.something {
-		return nil, fmt.Errorf("cannot marshal opt.Nothing[%T]()", receiver.value)
+func (receiver Optional[T]) MarshalJSON() ([]byte, error) {
+	switch interface{}(receiver.value).(type) {
+	case bool, string:
+		// these are OK.
+	default:
+		return nil, fmt.Errorf("cannot marshal something of type %T into JSON because parameterized type is ‘%T’ rather than ‘bool’ or ‘string’", receiver, receiver.value)
+	}
+
+	if receiver.isnothing() {
+		return nil, fmt.Errorf("cannot marshal opt.Nothing[%T]() into JSON", receiver.value)
 	}
 
 	return json.Marshal(receiver.value)