On Sun, 19 Feb 2023 23:52:52 GMT, Archie L. Cobbs <[email protected]> wrote:
> This bug relates to the "potentially ambiguous overload" warning which is
> enabled by `-Xlint:overloads`.
>
> The warning detects certain ambiguities that can cause problems for lambdas.
> For example, consider the interface `Spliterator.OfInt`, which declares these
> two methods:
>
> void forEachRemaining(Consumer<? super Integer> action);
> void forEachRemaining(IntConsumer action);
>
> Both methods have the same name, same number of parameters, and take a lambda
> with the same "shape" in the same argument position. This causes an ambiguity
> in any code that wants to do this:
>
> spliterator.forEachRemaining(x -> { ... });
>
> That code won't compile; instead, you'll get this error:
>
> Ambiguity.java:4: error: reference to forEachRemaining is ambiguous
> spliterator.forEachRemaining(x -> { });
> ^
> both method forEachRemaining(IntConsumer) in OfInt and method
> forEachRemaining(Consumer<? super Integer>) in OfInt match
>
>
> The problem reported by the bug is that the warning fails to detect
> ambiguities which are created purely by inheritance, for example:
>
> interface ConsumerOfInteger {
> void foo(Consumer<Integer> c);
> }
>
> interface IntegerConsumer {
> void foo(IntConsumer c);
> }
>
> // We should get a warning here...
> interface Test extends ConsumerOfInteger, IntegerConsumer {
> }
>
>
> The cause of the bug is that ambiguities are detected on a per-method basis,
> by checking whether a method is part of an ambiguity pair when we visit that
> method. So if the methods in an ambiguity pair are inherited from two
> distinct supertypes, we'll miss the ambiguity.
>
> To fix the problem, we need to look for ambiguities on a per-class level,
> checking all pairs of methods. However, it's not that simple - we only want
> to "blame" a class when that class itself, and not some supertype, is
> responsible for creating the ambiguity. For example, any interface extending
> `Spliterator.OfInt` will automatically inherit the two ambiguities mentioned
> above, but these are not the interface's fault so to speak so no warning
> should be generated. Making things more complicated is the fact that methods
> can be overridden and declared in generic classes so they only conflict in
> some subtypes, etc.
>
> So we generate the warning when there are two methods m1 and m2 in a class C
> such that:
>
> * m1 and m2 consitiute a "potentially ambiguous overload" (using the same
> definition as before)
> * There is no direct supertype T of C such that m1 and m2, or some methods
> they override, both exist in T and constitute a "potentially ambiguous
> overload" as members of T
> * We haven't already generated a warning for either m1 or m2 in class C
>
> If either method is declared in C, we locate the warning there, but when both
> methods are inherited, there's no method declaration to point at so the
> warning is instead located at the class declaration.
>
> I noticed a couple of other minor bugs; these are also being fixed here:
>
> (1) For inherited methods, the method signatures were being reported as they
> are declared, rather than in the context of the class being visited. As a
> result, when a methods is inherited from a generic supertype, the ambiguity
> is less clear. Here's an example:
>
> interface Upper<T> {
> void foo(T c);
> }
>
> interface Lower extends Upper<IntConsumer> {
> void foo(Consumer<Integer> c);
> }
>
> Currently, the error is reported as:
>
> warning: [overloads] foo(Consumer<Integer>) in Lower is potentially ambiguous
> with foo(T) in Upper
>
> Reporting the method signatures in the context of the class being visited
> makes the ambiguity clearer:
>
> warning: [overloads] foo(Consumer<Integer>) in Lower is potentially ambiguous
> with foo(IntConsumer) in Upper
>
>
> (2) When a method is identified as part of an ambiguous pair, we were setting
> a `POTENTIALLY_AMBIGUOUS` flag on it. This caused it to be forever excluded
> from future warnings. For methods that are declared in the class we're
> visiting, this makes sense, but it doesn't make sense for inherited methods,
> because it disqualifies them from participating in the analysis of any other
> class that also inherits them.
>
> As a result, for a class like the one below, the compiler was only generating
> one warning instead of three:
>
> public interface SuperIface {
> void foo(Consumer<Integer> c);
> }
>
> public interface I1 extends SuperIface {
> void foo(IntConsumer c); // warning was generated here
> }
>
> public interface I2 extends SuperIface {
> void foo(IntConsumer c); // no warning was generated here
> }
>
> public interface I3 extends SuperIface {
> void foo(IntConsumer c); // no warning was generated here
> }
>
>
> With this patch the `POTENTIALLY_AMBIGUOUS` flag is no longer needed. I
> wasn't sure whether to renumber all the subsequent flags, or just leave an
> empty placeholder, so I chose the latter.
>
> Finally, this fix uncovers new warnings in `java.base` and `java.desktop`, so
> these are now suppressed in the patch.
In the `AWTEventMulticaster` class(es), which interfaces are causing the
ambiguity?
-------------
PR: https://git.openjdk.org/jdk/pull/12645
