Inference of recursive types

[Vicente-Arturo Romero-Zaldivar]

Hi Liam,

Thanks for the bug report. I also think that this is a bug. I have filed 
this entry: https://bugs.openjdk.java.net/browse/JDK-8035713 to track it.

Vicente

On 21/02/14 21:09, Liam Miller-Cushon wrote:
> I was surprised that it starts working again with javac 8 source 8, 
> but I appreciate that it may not be a regression. Thanks for 
> investigating.
>
> Liam
>
>
> On Fri, Feb 21, 2014 at 1:02 PM, Vicente-Arturo Romero-Zaldivar 
> <vicente.romero at oracle.com <mailto:vicente.romero at oracle.com>> wrote:
>
>     On 21/02/14 19:31, Liam Miller-Cushon wrote:
>
>         +lambda-dev
>
>         Any thoughts on this?
>
>
>     Hi Liam,
>
>     I will look at this and get back to you. The fact that something
>     is working find in 7 and not in 8 source 7 doesn't imply that
>     there is a regression in the compiler. It can be a bug in 7 that
>     we don't have to reproduce in 8 just to keep the compatibility.
>     Every case is different and this could be a regression.
>
>     Thanks,
>     Vicente
>
>
>
>         This affects a significant amount of code, and the fact that
>         it only
>         appears when compiling java 7 with javac8 makes it look like a
>         regression.
>
>         The following is a less contrived example of code with
>         recursive types that
>         doesn't currently compile with javac8 -source 7 -target 7:
>
>         class Test {
>            <T extends Enum<T>> T getEnum() { return null; }
>            <U extends Enum<U>> void m() {
>              U e = getEnum();
>            }
>         }
>
>         $ javac -source 7 -target 7 Test.java
>         Test.java:4: error: incompatible types: inference variable T#1 has
>         incompatible upper bounds Enum<T#2>,U
>              U e = getEnum();
>                           ^
>            where T#1,T#2,U are type-variables:
>              T#1 extends Enum<T#1> declared in method <T#1>getEnum()
>              T#2 extends U
>              U extends Enum<U> declared in method <U>m()
>
>         On Fri, Feb 14, 2014 at 5:07 PM, Liam Miller-Cushon
>         <cushon at google.com <mailto:cushon at google.com>>wrote:
>
>             Hi -
>
>             Sorry if this has already been discussed, but I ran into a
>             difference in
>             behaviour between javac8 -source 7/-target 7 and both
>             javac7 and javac8
>             -source 8/-target 8.
>
>             The following program compiles with everything except
>             javac8 -source
>             7/-target 7.
>
>             Is this an intentional change? Having to make g's type
>             parameter explicit
>             feels like a regression.
>
>             ===
>             class Test {
>                static class One<T> {}
>                static class Two extends One<Two> {}
>
>                <T extends One<T>> T f(T a, String s) {
>                  T t = g(s);
>                  return t;
>                }
>                static <U extends One<U>> U g(String s) {
>                  throw new RuntimeException();
>                }
>             }
>             ===
>
>             Test.java:6: error: incompatible types: inference variable
>             U#1 has
>             incompatible upper bounds One<U#2>,T
>                  T t = g(s);
>                         ^
>                where U#1,U#2,T are type-variables:
>                  U#1 extends One<U#1> declared in method <U#1>g(String)
>                  U#2 extends T
>                  T extends One<T> declared in method <T>f(T,String)
>
>
>