tracker issue : CF-4202355

Issue: Aether Beta's null support allows non-void UDFs to return null. This breaks backward-compat where users expect the declared return type to be enforced.

Suggestion: Require a leading question mark symbol (?) to explicitly indicate a nullable type; otherwise the type is not nullable. Precedent: PHP 7.1.

Examples (with null support enabled):

<cfscript>
  any function myAny1(){}//OK
  any function myAny2(){return}//OK
  any function myAny3(){return null}//OK
  array function myArray1(){}//error
  array function myArray2(){return}//error
  array function myArray3(){return null}//error
  ?array function myArray1(){}//OK
  ?array function myArray2(){return}//OK
  ?array function myArray3(){return null}//OK
</cfscript>

When subtyping a return type the nullability can be removed by a subclass, but it cannot be added:

MyInterface.cfc:
interface {?array function myArray4()}

MyCFC.cfc:
component implements="MyInterface" {array function myArray4(){}}//valid

Related thread: https://forums.adobeprerelease.com/coldfusionpr/discussion/380/cf-4202295-null-support-removes-return-type-check

Related ticket: CF-4202295

Related URL: https://wiki.php.net/rfc/nullable_types

Credit: Adam Cameron, Sean Corfield, Denard Springle and Charlie Arehart.

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