Possible compiler bug? Code compiles on 8 but not 11

[B. Blaser]

My reproducer seems to be missing one line, see below.
Bernard

On Fri, 7 Feb 2020 at 21:22, B. Blaser <bsrbnd at gmail.com> wrote:
>
> Alex, David,
>
> Here is a simpler reproducer:
>
> public class Factory {
>     public Factory() {
>         String ok = gen(StringGenerator.class);
> //        @SuppressWarnings("unchecked")
>         String ko = gen(Generator.class);
>     }
>
>     public <T, F extends SimpleGenerator<T>> T gen(Class<F> c) { return null; }
> }
>
> class StringGenerator extends SimpleGenerator<String> {}
> class Generator<T> extends SimpleGenerator<T> {}

abstract class SimpleGenerator<T> {}

> $ javac -Xlint:unchecked Factory.java
> Factory.java:5: warning: [unchecked] unchecked method invocation:
> method gen in class Factory is applied to given types
>         String ko = gen(Generator.class);
>                        ^
>   required: Class<F>
>   found: Class<Generator>
>   where F,T are type-variables:
>     F extends SimpleGenerator<T> declared in method <T,F>gen(Class<F>)
>     T extends Object declared in method <T,F>gen(Class<F>)
> Factory.java:5: error: incompatible types: Object cannot be converted to String
>         String ko = gen(Generator.class);
>                        ^
> 1 error
> 1 warning
>
> JLS §15.8.2 states:
>
> "The type of C.class, where C is the name of a class, interface, or
> array type (§4.3), is Class<C>"
>
> which means that the found type 'Class<Generator>' triggers the
> following rule from §18.2.2:
>
> "A constraint formula of the form ‹ S → T › is reduced as follows:
> [...]
> • Otherwise, if T is a parameterized type of the form G<T 1 , ..., T n
> > , and there exists no type of the form G< ... > that is a supertype
> of S , but the raw type G is a supertype of S , then the constraint
> reduces to true.
> [...]
> The fourth and fifth cases are implicit uses of unchecked conversion (§5.1.9)."
>
> implying then §18.5.2.1:
>
> "If unchecked conversion was necessary for the method to be applicable
> during constraint set reduction in §18.5.1, the constraint formula ‹|
> R | → T › is reduced and incorporated with B 2 ."
>
> meaning that erasing the return type of 'gen()' to 'Object' looks, at
> first sight, correct. I'm not exactly sure what could have changed
> between 8 and 11, but we've still made a small fix in this area some
> years ago [1], as far as I can remember.
>
> Any feedback is welcome,
> Bernard
>
> [1] http://hg.openjdk.java.net/jdk/jdk/rev/39446351e625
>
> On Fri, 31 Jan 2020 at 20:00, Alex Buckley <alex.buckley at oracle.com> wrote:
> >
> > Hi David,
> >
> > This is a question of whether javac is conforming to the JLS, and if so,
> > whether the JLS is correct. It's best in such cases to quote the
> > smallest possible code sample in inline text. Oracle's javac team is not
> > looking to reproduce issues in the manner of a support organization, but
> > rather to understand as precisely as possible what's going on in the
> > type system.
> >
> > Alex
> >
> > On 1/31/2020 10:36 AM, David Grieve wrote:
> > > I have some code that compiles on 8 but not on 11. With 11 (11.0.5, to be exact), the compiler gives an error:
> > >
> > >> Factory.java
> > >> Error:(9, 25) java: incompatible types: java.lang.Object cannot be converted to Manager<java.lang.String>
> > >
> > > This can be fixed with an explicit cast. But it seems to me that the cast should not be needed.
> > >
> > > I'm hoping to get an answer for why this compiles cleanly on 8, but not 11.
> > >
> > > I have uploaded a small sample that reproduces to http://cr.openjdk.java.net/~dgrieve/javac-bug.zip
> > >