Generics compilation failure casting List<? extends Set...> to List<Set...> under Java 7

[Rémi Forax]

On 12/31/2011 04:17 PM, Andrew Phillips wrote:
> Hi all
>
> During some recent work to get jclouds to compile under Java 7 I came 
> across a generics compilation failure that can be reproduced using the 
> following sample code:
>
> classGenericCompilationFailureDemo{
>     List<?extendsGenericCompilationFailureDemo>newList(){
>         returnnewArrayList<GenericCompilationFailureDemo>();
>     };
>
>     voiduseList(){
>         List<GenericCompilationFailureDemo>list =
>             (List<GenericCompilationFailureDemo>)newList();
>     }
>
>     
> List<?extendsSet<GenericCompilationFailureDemo>>newListOfSpecificSets(){
>         returnnewArrayList<Set<GenericCompilationFailureDemo>>();
>     };
>
>     voiduseListOfSpecificSets(){
>         List<Set<GenericCompilationFailureDemo>>listOfSpecificSets =
>             
> (List<Set<GenericCompilationFailureDemo>>)newListOfSpecificSets();
>     }
>
>     
> List<?extendsSet<?extendsGenericCompilationFailureDemo>>newListOfSets(){
>         returnnewArrayList<Set<?extendsGenericCompilationFailureDemo>>();
>     };
>
>     voiduseListOfSet(){
>         List<Set<?extendsGenericCompilationFailureDemo>>listOfSets =
>             
> (List<Set<?extendsGenericCompilationFailureDemo>>)newListOfSets();
>     }
> }
>
>
> This passes under Sun JDK 1.6.0_20 and _24 but fails under Oracle JDK 
> 1.7.0 and _01 with:
>
> [ERROR]src\main\java\GenericCompilationFailureDemo.java:[56,78]error:inconvertible 
> types
>
> The comments and tentative answer to my resulting StackOverflow question
>
> http://stackoverflow.com/questions/8637937/why-does-a-generic-cast-of-a-list-extends-set-to-listset-succeed-on-sun 
>
>
> seem to suggest that this is a compiler bug. We found a simple 
> workaround, by the way: do the cast in a separate method (see 
> https://github.com/jclouds/jclouds/commit/1ba75f623f03908d87c2fddce46e276098f3db40).
>
> Any comments or suggestion much appreciated! And a belated Merry 
> Christmas and Happy New Year, of course ;-)
>
> Regards
>
>
> Andrew

I think it's a bug too, and javac is (was) known to be to strict when 
checking assignment conversions.
Your program can be reduced to

           List<Set<? extends String>> a = null;
           List<? extends Set<? extends String>> b = a;   // 1

           List<? extends Set<? extends String>> c = null;
           List<Set<? extends String>> d = (List<Set<? extends 
String>>)c;  // 2

(1) compiles but the reverse (2) doesn't compile.

You should check in the bug is already in the bug DB (bugs.sun.com) and 
files a bug report if it's not already reported.

cheers,
Rémi