Inference of recursive types

[Vicente-Arturo Romero-Zaldivar]

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>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)
>>