Compile error with Generic wildcards in 4.37 M3 that was not present in M2

[lukeu]

In Eclipse 4.37 M3, JDT started giving a compile error in some unit test code that was not present in M2 and is not reported by javac.

FYI @stephan-herrmann as the PR in this timeframe that seems the most related is #4251 (tweaking some prior work I see was done on generics recently)

I've extracted the following minimal reproducer "simulating" some Hamcrest matcher code, but without dependencies:

class MyUnitTest {

    interface Matcher<T> {
        boolean matches(Object actual);

        static <E> Matcher<java.util.Collection<? extends E>> empty() {
            return null;
        }
        static <E> Matcher<java.lang.Iterable<? extends E>> contains(E... itemMatcher) {
            return null;
        }
    }

    void testOne() {
        validate(List.of("one"), Matcher.empty());
    }

    void testTwo() {
        validate(List.of("two"), Matcher.contains("two"));
    }

    private static void validate(
            List<String> strings, Matcher<? extends Iterable<? extends String>> matcher) {
        // do some validation
    }
}

The error appears on the call to local helper function validate within testOne, and states:

The method validate(List, PathValidatorTest.Matcher<? extends Iterable<? extends String>>) in the type PathValidatorTest is not applicable for the arguments (List, PathValidatorTest.Matcher<Collection<? extends Object>>)

I would hazard a guess that issues arises because the Generic <E> in method empty is only inferred to Object, whereas before it was determined to be String. My reasoning is simply that if I change the method signature of validate by substituting {Iterable => Collection} then:

• An error is now given on the call to validate within testTwo (as I would expect), and
• The error in testOne goes away and the tooltip when hovering over its call to validate show the inferred type of E as String.

---

Specific JDT versions reported in Help > About > Installation details > Installation History:

• "Good": Eclipse Java Development Tools 3.20.300.v20250724-1800
• "Bad": Eclipse Java Development Tools 3.20.300.v20250814-1944

[stephan-herrmann]

Thanks for reporting.

In HEAD of master, ecj fails to infer E because the following constraint (with inference variable E#2 representing E) reduces to false:

• ⟨capture#1-of ? extends E#2 <= ? extends String⟩

Re-checking JLS I don't see any other option once we get there.

The last state that accepted the program was 1d4f5c9 right before #4251 . Of that change reverting only the part in Scope.parameterCompatibilityLevel(MethodBinding, TypeBinding[], boolean, InvocationSite) would let us still accept the program, in that ParameterizedTypeBinding.findSuperTypeOriginatingFrom() would no longer apply capture. This, however, is not a good option, as capturing is actually required here.

In the example at hand we are computing a supertype of Collection<? extends E#2> that has the same generic type as Iterable<? extends String>. This supertype is Iterable<capture#1-of ? extends E#2>, not Iterable<? extends E#2> as we previously computed.

As long as https://bugs.openjdk.org/browse/JDK-8016207 is unresolved (12 years and counting), this kind of difference between javac and ecj is to be expected, much to my regret. This is definitely nothing that we can fix at the current stage of release candidates.

I don't like shipping a release with a known regression, but with all those uncertainties about JLS in this area, we can't even know if the change regarding this particular program is a regression or an improvement.

[stephan-herrmann]

The fix/workaround for your code would be to say:

			        validate(List.of("one"), Matcher.<String>empty());

[lukeu]

Thanks for the quick preliminary analysis! I too have a trivial workaround (for the non-minimal version) which I can easily push before other teammates encounter it (also via neo-vim using some Java LSP plugin).

we can't even know if the change regarding this particular program is a regression or an improvement.

Yeah I also certainly couldn't, so I figured better to raise the detected change & make you folks aware. (I did briefly toy with the idea of converting it to Scala to see what its type-system had to say about the validity of this, but nah...) If it turns out to be the latter, I'd personally be happy to be told that EJC is just being "more correct" than javac, but I'll trust you to judge the tradeoffs there vs eliminating corner-cases differences with javac to reduce friction.

[stephan-herrmann]

eliminating corner-cases differences with javac to reduce friction.

I'm all for eliminating any such differences, but experience tells me, that fixing one case can easily break five other cases, because much of what works today, does so only because we filled gaps in the spec in a particular way. As long as there are differences between javac and JLS it's all a very brittle situation.

In due time I will consult with experts at oracle about this. Maybe they can shed some light on what should or shouldn't happen.

[stephan-herrmann]

@srikanth-sankaran this is another example where it would be interesting to see how reduction works in javac.

Here's how ecj concludes FALSE:

Reduced ⟨MyUnitTest.Matcher<Collection<? extends E#2>> → MyUnitTest.Matcher<? extends Iterable<? extends String>>⟩
  to    ⟨MyUnitTest.Matcher<Collection<? extends E#2>> <: MyUnitTest.Matcher<? extends Iterable<? extends String>>⟩
  to    ⟨Collection<? extends E#2> <= ? extends Iterable<? extends String>⟩
  to    ⟨Collection<? extends E#2> <: Iterable<? extends String>⟩
  to    ⟨capture#1-of ? extends E#2 <= ? extends String⟩
  to    ⟨capture#1-of ? extends E#2 <: String⟩
  to    FALSE
Infer invocation type for Matcher.empty() with target Matcher<? extends Iterable<? extends String>>
Result=
null

From javac tracing we get this:

TraceOut: (Speculative) (BASIC)  attempt to instantiate method: <E> empty()MyUnitTest.Matcher<java.util.Collection<? extends E>>
TraceOut: Initial Bound Set (B0):
TraceOut: Inference variable = E 
TraceOut: Bound set after Invocation Applicability Inference (B2): <Ditto>
TraceOut: Bound set after resolution: 
TraceOut: Inference variable = E instantiation = java.lang.Object 
TraceOut: Instantiated Method Type : ()MyUnitTest.Matcher<java.util.Collection<? extends java.lang.Object>>
TraceOut: -----------------------------------------------
TraceOut: (Final) (BASIC)  attempt to instantiate method: <E> empty()MyUnitTest.Matcher<java.util.Collection<? extends E>>
TraceOut: Initial Bound Set (B0):
TraceOut: Inference variable = E 
TraceOut: Bound set after Invocation Applicability Inference (B2): <Ditto>
TraceOut: Bound set after incorporation: <Ditto>
TraceOut: Bound set after generation of return constraints (B3):
TraceOut: Inference variable = E 
TraceOut:     E <: java.lang.String
TraceOut: Bound set after resolution: 
TraceOut: Inference variable = E instantiation = java.lang.String 
TraceOut:     E <: java.lang.String
TraceOut: Instantiated Method Type : ()MyUnitTest.Matcher<java.util.Collection<? extends java.lang.String>>

Speculative inference going to Object is OK. But where does Final inference get this from: E <: java.lang.String ?

[srikanth-sankaran]

Speculative inference going to Object is OK. But where does Final inference get this from: E <: java.lang.String ?

The expanded tracing as of #4838 (comment) produces this:

/home/srikanth/jdk/jdk/build/linux-x86_64-server-release/images/jdk/bin/java --enable-preview -javaagent:/home/srikanth/intellij/idea-IU-253.31033.145/lib/idea_rt.jar=35231 -Dfile.encoding=UTF-8 -Dsun.stdout.encoding=UTF-8 -Dsun.stderr.encoding=UTF-8 com.sun.tools.javac.Main -cp /home/srikanth/tmp -XDdebug.dumpInferenceGraphsTo=/home/srikanth/tmp/javac-inference-trace -g /home/srikanth/tmp/X.java
TraceOut: (Speculative) (BASIC)  attempt to instantiate method: <E> of(E[])java.util.List<E> with arguments: java.lang.String
TraceOut: Initial Bound Set (B0):
TraceOut:         Inference variable = E 
TraceOut: ***************************************  FAILURE!!!  ************** com.sun.tools.javac.comp.Infer$InferenceException
TraceOut: (Speculative) (BASIC)  attempt to instantiate method: <E> of(E,E,E,E,E,E,E,E,E,E)java.util.List<E> with arguments: java.lang.String
TraceOut: Initial Bound Set (B0):
TraceOut:         Inference variable = E 
TraceOut: ***************************************  FAILURE!!!  ************** com.sun.tools.javac.comp.Infer$InferenceException
TraceOut: (Speculative) (BASIC)  attempt to instantiate method: <E> of(E,E,E,E,E,E,E,E,E)java.util.List<E> with arguments: java.lang.String
TraceOut: Initial Bound Set (B0):
TraceOut:         Inference variable = E 
TraceOut: ***************************************  FAILURE!!!  ************** com.sun.tools.javac.comp.Infer$InferenceException
TraceOut: (Speculative) (BASIC)  attempt to instantiate method: <E> of(E,E,E,E,E,E,E,E)java.util.List<E> with arguments: java.lang.String
TraceOut: Initial Bound Set (B0):
TraceOut:         Inference variable = E 
TraceOut: ***************************************  FAILURE!!!  ************** com.sun.tools.javac.comp.Infer$InferenceException
TraceOut: (Speculative) (BASIC)  attempt to instantiate method: <E> of(E,E,E,E,E,E,E)java.util.List<E> with arguments: java.lang.String
TraceOut: Initial Bound Set (B0):
TraceOut:         Inference variable = E 
TraceOut: ***************************************  FAILURE!!!  ************** com.sun.tools.javac.comp.Infer$InferenceException
TraceOut: (Speculative) (BASIC)  attempt to instantiate method: <E> of(E,E,E,E,E,E)java.util.List<E> with arguments: java.lang.String
TraceOut: Initial Bound Set (B0):
TraceOut:         Inference variable = E 
TraceOut: ***************************************  FAILURE!!!  ************** com.sun.tools.javac.comp.Infer$InferenceException
TraceOut: (Speculative) (BASIC)  attempt to instantiate method: <E> of(E,E,E,E,E)java.util.List<E> with arguments: java.lang.String
TraceOut: Initial Bound Set (B0):
TraceOut:         Inference variable = E 
TraceOut: ***************************************  FAILURE!!!  ************** com.sun.tools.javac.comp.Infer$InferenceException
TraceOut: (Speculative) (BASIC)  attempt to instantiate method: <E> of(E,E,E,E)java.util.List<E> with arguments: java.lang.String
TraceOut: Initial Bound Set (B0):
TraceOut:         Inference variable = E 
TraceOut: ***************************************  FAILURE!!!  ************** com.sun.tools.javac.comp.Infer$InferenceException
TraceOut: (Speculative) (BASIC)  attempt to instantiate method: <E> of(E,E,E)java.util.List<E> with arguments: java.lang.String
TraceOut: Initial Bound Set (B0):
TraceOut:         Inference variable = E 
TraceOut: ***************************************  FAILURE!!!  ************** com.sun.tools.javac.comp.Infer$InferenceException
TraceOut: (Speculative) (BASIC)  attempt to instantiate method: <E> of(E,E)java.util.List<E> with arguments: java.lang.String
TraceOut: Initial Bound Set (B0):
TraceOut:         Inference variable = E 
TraceOut: ***************************************  FAILURE!!!  ************** com.sun.tools.javac.comp.Infer$InferenceException
TraceOut: (Speculative) (BASIC)  attempt to instantiate method: <E> of(E)java.util.List<E> with arguments: java.lang.String
TraceOut: Initial Bound Set (B0):
TraceOut:         Inference variable = E 
TraceOut: Bound set after Invocation Applicability Inference (B2): 
TraceOut:         Inference variable = E 
TraceOut:             E :> java.lang.String
TraceOut: -------------  Inference variables to be solved: E
TraceOut: -------------   Performing graph inference step EQ_LOWER ----------
TraceOut: Bound set after resolution: 
TraceOut:         Inference variable = E instantiation = java.lang.String 
TraceOut:             E :> java.lang.String
TraceOut: Instantiated Method Type : (java.lang.String)java.util.List<java.lang.String>
TraceOut: -----------------------------------------------
TraceOut: (Speculative) (BASIC)  attempt to instantiate method: <E> of()java.util.List<E> with arguments: java.lang.String
TraceOut: Initial Bound Set (B0):
TraceOut:         Inference variable = E 
TraceOut: ***************************************  FAILURE!!!  ************** com.sun.tools.javac.comp.Infer$InferenceException
TraceOut: (Speculative) (BASIC)  attempt to instantiate method: <E> empty()MyUnitTest.Matcher<java.util.Collection<? extends E>>
TraceOut: Initial Bound Set (B0):
TraceOut:         Inference variable = E 
TraceOut: Bound set after Invocation Applicability Inference (B2): <Ditto>
TraceOut: -------------  Inference variables to be solved: E
TraceOut: -------------   Performing graph inference step EQ_LOWER_THROWS_UPPER_CAPTURED ----------
TraceOut: Bound set after resolution: 
TraceOut:         Inference variable = E instantiation = java.lang.Object 
TraceOut: Instantiated Method Type : ()MyUnitTest.Matcher<java.util.Collection<? extends java.lang.Object>>
TraceOut: -----------------------------------------------
TraceOut: (Speculative) (BASIC)  attempt to instantiate method: <E> of(E[])java.util.List<E> with arguments: java.lang.String
TraceOut: Initial Bound Set (B0):
TraceOut:         Inference variable = E 
TraceOut: ***************************************  FAILURE!!!  ************** com.sun.tools.javac.comp.Infer$InferenceException
TraceOut: (Speculative) (BASIC)  attempt to instantiate method: <E> of(E,E,E,E,E,E,E,E,E,E)java.util.List<E> with arguments: java.lang.String
TraceOut: Initial Bound Set (B0):
TraceOut:         Inference variable = E 
TraceOut: ***************************************  FAILURE!!!  ************** com.sun.tools.javac.comp.Infer$InferenceException
TraceOut: (Speculative) (BASIC)  attempt to instantiate method: <E> of(E,E,E,E,E,E,E,E,E)java.util.List<E> with arguments: java.lang.String
TraceOut: Initial Bound Set (B0):
TraceOut:         Inference variable = E 
TraceOut: ***************************************  FAILURE!!!  ************** com.sun.tools.javac.comp.Infer$InferenceException
TraceOut: (Speculative) (BASIC)  attempt to instantiate method: <E> of(E,E,E,E,E,E,E,E)java.util.List<E> with arguments: java.lang.String
TraceOut: Initial Bound Set (B0):
TraceOut:         Inference variable = E 
TraceOut: ***************************************  FAILURE!!!  ************** com.sun.tools.javac.comp.Infer$InferenceException
TraceOut: (Speculative) (BASIC)  attempt to instantiate method: <E> of(E,E,E,E,E,E,E)java.util.List<E> with arguments: java.lang.String
TraceOut: Initial Bound Set (B0):
TraceOut:         Inference variable = E 
TraceOut: ***************************************  FAILURE!!!  ************** com.sun.tools.javac.comp.Infer$InferenceException
TraceOut: (Speculative) (BASIC)  attempt to instantiate method: <E> of(E,E,E,E,E,E)java.util.List<E> with arguments: java.lang.String
TraceOut: Initial Bound Set (B0):
TraceOut:         Inference variable = E 
TraceOut: ***************************************  FAILURE!!!  ************** com.sun.tools.javac.comp.Infer$InferenceException
TraceOut: (Speculative) (BASIC)  attempt to instantiate method: <E> of(E,E,E,E,E)java.util.List<E> with arguments: java.lang.String
TraceOut: Initial Bound Set (B0):
TraceOut:         Inference variable = E 
TraceOut: ***************************************  FAILURE!!!  ************** com.sun.tools.javac.comp.Infer$InferenceException
TraceOut: (Speculative) (BASIC)  attempt to instantiate method: <E> of(E,E,E,E)java.util.List<E> with arguments: java.lang.String
TraceOut: Initial Bound Set (B0):
TraceOut:         Inference variable = E 
TraceOut: ***************************************  FAILURE!!!  ************** com.sun.tools.javac.comp.Infer$InferenceException
TraceOut: (Speculative) (BASIC)  attempt to instantiate method: <E> of(E,E,E)java.util.List<E> with arguments: java.lang.String
TraceOut: Initial Bound Set (B0):
TraceOut:         Inference variable = E 
TraceOut: ***************************************  FAILURE!!!  ************** com.sun.tools.javac.comp.Infer$InferenceException
TraceOut: (Speculative) (BASIC)  attempt to instantiate method: <E> of(E,E)java.util.List<E> with arguments: java.lang.String
TraceOut: Initial Bound Set (B0):
TraceOut:         Inference variable = E 
TraceOut: ***************************************  FAILURE!!!  ************** com.sun.tools.javac.comp.Infer$InferenceException
TraceOut: (Speculative) (BASIC)  attempt to instantiate method: <E> of(E)java.util.List<E> with arguments: java.lang.String
TraceOut: Initial Bound Set (B0):
TraceOut:         Inference variable = E 
TraceOut: Bound set after Invocation Applicability Inference (B2): 
TraceOut:         Inference variable = E 
TraceOut:             E :> java.lang.String
TraceOut: -------------  Inference variables to be solved: E
TraceOut: -------------   Performing graph inference step EQ_LOWER ----------
TraceOut: Bound set after resolution: 
TraceOut:         Inference variable = E instantiation = java.lang.String 
TraceOut:             E :> java.lang.String
TraceOut: Instantiated Method Type : (java.lang.String)java.util.List<java.lang.String>
TraceOut: -----------------------------------------------
TraceOut: (Speculative) (BASIC)  attempt to instantiate method: <E> of()java.util.List<E> with arguments: java.lang.String
TraceOut: Initial Bound Set (B0):
TraceOut:         Inference variable = E 
TraceOut: ***************************************  FAILURE!!!  ************** com.sun.tools.javac.comp.Infer$InferenceException
TraceOut: (Final) (BASIC)  attempt to instantiate method: <E> of(E)java.util.List<E> with arguments: java.lang.String
TraceOut: Initial Bound Set (B0):
TraceOut:         Inference variable = E 
TraceOut: Bound set after Invocation Applicability Inference (B2): 
TraceOut:         Inference variable = E 
TraceOut:             E :> java.lang.String
TraceOut: Bound set after incorporation: <Ditto>
TraceOut: Bound set after generation of return constraints (B3):
TraceOut:         Inference variable = E 
TraceOut:             E :> java.lang.String
TraceOut:             E = java.lang.String
TraceOut: -------------  Inference variables to be solved: E
TraceOut: -------------   Performing graph inference step EQ ----------
TraceOut: Bound set after resolution: 
TraceOut:         Inference variable = E instantiation = java.lang.String 
TraceOut:             E :> java.lang.String
TraceOut:             E = java.lang.String
TraceOut: Instantiated Method Type : (java.lang.String)java.util.List<java.lang.String>
TraceOut: -----------------------------------------------
TraceOut: (Speculative) (BASIC)  attempt to instantiate method: <E> empty()MyUnitTest.Matcher<java.util.Collection<? extends E>>
TraceOut: Initial Bound Set (B0):
TraceOut:         Inference variable = E 
TraceOut: Bound set after Invocation Applicability Inference (B2): <Ditto>
TraceOut: -------------  Inference variables to be solved: E
TraceOut: -------------   Performing graph inference step EQ_LOWER_THROWS_UPPER_CAPTURED ----------
TraceOut: Bound set after resolution: 
TraceOut:         Inference variable = E instantiation = java.lang.Object 
TraceOut: Instantiated Method Type : ()MyUnitTest.Matcher<java.util.Collection<? extends java.lang.Object>>
TraceOut: -----------------------------------------------
TraceOut: (Final) (BASIC)  attempt to instantiate method: <E> empty()MyUnitTest.Matcher<java.util.Collection<? extends E>>
TraceOut: Initial Bound Set (B0):
TraceOut:         Inference variable = E 
TraceOut: Bound set after Invocation Applicability Inference (B2): <Ditto>
TraceOut: Bound set after incorporation: <Ditto>
TraceOut: Bound set after generation of return constraints (B3):
TraceOut:         Inference variable = E 
TraceOut:             E <: java.lang.String
TraceOut: -------------  Inference variables to be solved: E
TraceOut: -------------   Performing graph inference step EQ_LOWER_THROWS_UPPER_CAPTURED ----------
TraceOut: Bound set after resolution: 
TraceOut:         Inference variable = E instantiation = java.lang.String 
TraceOut:             E <: java.lang.String
TraceOut: Instantiated Method Type : ()MyUnitTest.Matcher<java.util.Collection<? extends java.lang.String>>
TraceOut: -----------------------------------------------
TraceOut: (Speculative) (BASIC)  attempt to instantiate method: <E> of(E[])java.util.List<E> with arguments: java.lang.String
TraceOut: Initial Bound Set (B0):
TraceOut:         Inference variable = E 
TraceOut: ***************************************  FAILURE!!!  ************** com.sun.tools.javac.comp.Infer$InferenceException
TraceOut: (Speculative) (BASIC)  attempt to instantiate method: <E> of(E,E,E,E,E,E,E,E,E,E)java.util.List<E> with arguments: java.lang.String
TraceOut: Initial Bound Set (B0):
TraceOut:         Inference variable = E 
TraceOut: ***************************************  FAILURE!!!  ************** com.sun.tools.javac.comp.Infer$InferenceException
TraceOut: (Speculative) (BASIC)  attempt to instantiate method: <E> of(E,E,E,E,E,E,E,E,E)java.util.List<E> with arguments: java.lang.String
TraceOut: Initial Bound Set (B0):
TraceOut:         Inference variable = E 
TraceOut: ***************************************  FAILURE!!!  ************** com.sun.tools.javac.comp.Infer$InferenceException
TraceOut: (Speculative) (BASIC)  attempt to instantiate method: <E> of(E,E,E,E,E,E,E,E)java.util.List<E> with arguments: java.lang.String
TraceOut: Initial Bound Set (B0):
TraceOut:         Inference variable = E 
TraceOut: ***************************************  FAILURE!!!  ************** com.sun.tools.javac.comp.Infer$InferenceException
TraceOut: (Speculative) (BASIC)  attempt to instantiate method: <E> of(E,E,E,E,E,E,E)java.util.List<E> with arguments: java.lang.String
TraceOut: Initial Bound Set (B0):
TraceOut:         Inference variable = E 
TraceOut: ***************************************  FAILURE!!!  ************** com.sun.tools.javac.comp.Infer$InferenceException
TraceOut: (Speculative) (BASIC)  attempt to instantiate method: <E> of(E,E,E,E,E,E)java.util.List<E> with arguments: java.lang.String
TraceOut: Initial Bound Set (B0):
TraceOut:         Inference variable = E 
TraceOut: ***************************************  FAILURE!!!  ************** com.sun.tools.javac.comp.Infer$InferenceException
TraceOut: (Speculative) (BASIC)  attempt to instantiate method: <E> of(E,E,E,E,E)java.util.List<E> with arguments: java.lang.String
TraceOut: Initial Bound Set (B0):
TraceOut:         Inference variable = E 
TraceOut: ***************************************  FAILURE!!!  ************** com.sun.tools.javac.comp.Infer$InferenceException
TraceOut: (Speculative) (BASIC)  attempt to instantiate method: <E> of(E,E,E,E)java.util.List<E> with arguments: java.lang.String
TraceOut: Initial Bound Set (B0):
TraceOut:         Inference variable = E 
TraceOut: ***************************************  FAILURE!!!  ************** com.sun.tools.javac.comp.Infer$InferenceException
TraceOut: (Speculative) (BASIC)  attempt to instantiate method: <E> of(E,E,E)java.util.List<E> with arguments: java.lang.String
TraceOut: Initial Bound Set (B0):
TraceOut:         Inference variable = E 
TraceOut: ***************************************  FAILURE!!!  ************** com.sun.tools.javac.comp.Infer$InferenceException
TraceOut: (Speculative) (BASIC)  attempt to instantiate method: <E> of(E,E)java.util.List<E> with arguments: java.lang.String
TraceOut: Initial Bound Set (B0):
TraceOut:         Inference variable = E 
TraceOut: ***************************************  FAILURE!!!  ************** com.sun.tools.javac.comp.Infer$InferenceException
TraceOut: (Speculative) (BASIC)  attempt to instantiate method: <E> of(E)java.util.List<E> with arguments: java.lang.String
TraceOut: Initial Bound Set (B0):
TraceOut:         Inference variable = E 
TraceOut: Bound set after Invocation Applicability Inference (B2): 
TraceOut:         Inference variable = E 
TraceOut:             E :> java.lang.String
TraceOut: -------------  Inference variables to be solved: E
TraceOut: -------------   Performing graph inference step EQ_LOWER ----------
TraceOut: Bound set after resolution: 
TraceOut:         Inference variable = E instantiation = java.lang.String 
TraceOut:             E :> java.lang.String
TraceOut: Instantiated Method Type : (java.lang.String)java.util.List<java.lang.String>
TraceOut: -----------------------------------------------
TraceOut: (Speculative) (BASIC)  attempt to instantiate method: <E> of()java.util.List<E> with arguments: java.lang.String
TraceOut: Initial Bound Set (B0):
TraceOut:         Inference variable = E 
TraceOut: ***************************************  FAILURE!!!  ************** com.sun.tools.javac.comp.Infer$InferenceException
TraceOut: (Speculative) (BASIC)  attempt to instantiate method: <E> contains(E[])MyUnitTest.Matcher<java.lang.Iterable<? extends E>> with arguments: java.lang.String
TraceOut: Initial Bound Set (B0):
TraceOut:         Inference variable = E 
TraceOut: ***************************************  FAILURE!!!  ************** com.sun.tools.javac.comp.Infer$InferenceException
TraceOut: (Speculative) (BOX)  attempt to instantiate method: <E> contains(E[])MyUnitTest.Matcher<java.lang.Iterable<? extends E>> with arguments: java.lang.String
TraceOut: Initial Bound Set (B0):
TraceOut:         Inference variable = E 
TraceOut: ***************************************  FAILURE!!!  ************** com.sun.tools.javac.comp.Infer$InferenceException
TraceOut: (Speculative) (VARARITY)  attempt to instantiate method: <E> contains(E[])MyUnitTest.Matcher<java.lang.Iterable<? extends E>> with arguments: java.lang.String
TraceOut: Initial Bound Set (B0):
TraceOut:         Inference variable = E 
TraceOut: Bound set after Invocation Applicability Inference (B2): 
TraceOut:         Inference variable = E 
TraceOut:             E :> java.lang.String
TraceOut: -------------  Inference variables to be solved: E
TraceOut: -------------   Performing graph inference step EQ_LOWER ----------
TraceOut: Bound set after resolution: 
TraceOut:         Inference variable = E instantiation = java.lang.String 
TraceOut:             E :> java.lang.String
TraceOut: Instantiated Method Type : (java.lang.String[])MyUnitTest.Matcher<java.lang.Iterable<? extends java.lang.String>>
TraceOut: -----------------------------------------------
TraceOut: (Speculative) (BASIC)  attempt to instantiate method: <E> of(E[])java.util.List<E> with arguments: java.lang.String
TraceOut: Initial Bound Set (B0):
TraceOut:         Inference variable = E 
TraceOut: ***************************************  FAILURE!!!  ************** com.sun.tools.javac.comp.Infer$InferenceException
TraceOut: (Speculative) (BASIC)  attempt to instantiate method: <E> of(E,E,E,E,E,E,E,E,E,E)java.util.List<E> with arguments: java.lang.String
TraceOut: Initial Bound Set (B0):
TraceOut:         Inference variable = E 
TraceOut: ***************************************  FAILURE!!!  ************** com.sun.tools.javac.comp.Infer$InferenceException
TraceOut: (Speculative) (BASIC)  attempt to instantiate method: <E> of(E,E,E,E,E,E,E,E,E)java.util.List<E> with arguments: java.lang.String
TraceOut: Initial Bound Set (B0):
TraceOut:         Inference variable = E 
TraceOut: ***************************************  FAILURE!!!  ************** com.sun.tools.javac.comp.Infer$InferenceException
TraceOut: (Speculative) (BASIC)  attempt to instantiate method: <E> of(E,E,E,E,E,E,E,E)java.util.List<E> with arguments: java.lang.String
TraceOut: Initial Bound Set (B0):
TraceOut:         Inference variable = E 
TraceOut: ***************************************  FAILURE!!!  ************** com.sun.tools.javac.comp.Infer$InferenceException
TraceOut: (Speculative) (BASIC)  attempt to instantiate method: <E> of(E,E,E,E,E,E,E)java.util.List<E> with arguments: java.lang.String
TraceOut: Initial Bound Set (B0):
TraceOut:         Inference variable = E 
TraceOut: ***************************************  FAILURE!!!  ************** com.sun.tools.javac.comp.Infer$InferenceException
TraceOut: (Speculative) (BASIC)  attempt to instantiate method: <E> of(E,E,E,E,E,E)java.util.List<E> with arguments: java.lang.String
TraceOut: Initial Bound Set (B0):
TraceOut:         Inference variable = E 
TraceOut: ***************************************  FAILURE!!!  ************** com.sun.tools.javac.comp.Infer$InferenceException
TraceOut: (Speculative) (BASIC)  attempt to instantiate method: <E> of(E,E,E,E,E)java.util.List<E> with arguments: java.lang.String
TraceOut: Initial Bound Set (B0):
TraceOut:         Inference variable = E 
TraceOut: ***************************************  FAILURE!!!  ************** com.sun.tools.javac.comp.Infer$InferenceException
TraceOut: (Speculative) (BASIC)  attempt to instantiate method: <E> of(E,E,E,E)java.util.List<E> with arguments: java.lang.String
TraceOut: Initial Bound Set (B0):
TraceOut:         Inference variable = E 
TraceOut: ***************************************  FAILURE!!!  ************** com.sun.tools.javac.comp.Infer$InferenceException
TraceOut: (Speculative) (BASIC)  attempt to instantiate method: <E> of(E,E,E)java.util.List<E> with arguments: java.lang.String
TraceOut: Initial Bound Set (B0):
TraceOut:         Inference variable = E 
TraceOut: ***************************************  FAILURE!!!  ************** com.sun.tools.javac.comp.Infer$InferenceException
TraceOut: (Speculative) (BASIC)  attempt to instantiate method: <E> of(E,E)java.util.List<E> with arguments: java.lang.String
TraceOut: Initial Bound Set (B0):
TraceOut:         Inference variable = E 
TraceOut: ***************************************  FAILURE!!!  ************** com.sun.tools.javac.comp.Infer$InferenceException
TraceOut: (Speculative) (BASIC)  attempt to instantiate method: <E> of(E)java.util.List<E> with arguments: java.lang.String
TraceOut: Initial Bound Set (B0):
TraceOut:         Inference variable = E 
TraceOut: Bound set after Invocation Applicability Inference (B2): 
TraceOut:         Inference variable = E 
TraceOut:             E :> java.lang.String
TraceOut: -------------  Inference variables to be solved: E
TraceOut: -------------   Performing graph inference step EQ_LOWER ----------
TraceOut: Bound set after resolution: 
TraceOut:         Inference variable = E instantiation = java.lang.String 
TraceOut:             E :> java.lang.String
TraceOut: Instantiated Method Type : (java.lang.String)java.util.List<java.lang.String>
TraceOut: -----------------------------------------------
TraceOut: (Speculative) (BASIC)  attempt to instantiate method: <E> of()java.util.List<E> with arguments: java.lang.String
TraceOut: Initial Bound Set (B0):
TraceOut:         Inference variable = E 
TraceOut: ***************************************  FAILURE!!!  ************** com.sun.tools.javac.comp.Infer$InferenceException
TraceOut: (Final) (BASIC)  attempt to instantiate method: <E> of(E)java.util.List<E> with arguments: java.lang.String
TraceOut: Initial Bound Set (B0):
TraceOut:         Inference variable = E 
TraceOut: Bound set after Invocation Applicability Inference (B2): 
TraceOut:         Inference variable = E 
TraceOut:             E :> java.lang.String
TraceOut: Bound set after incorporation: <Ditto>
TraceOut: Bound set after generation of return constraints (B3):
TraceOut:         Inference variable = E 
TraceOut:             E :> java.lang.String
TraceOut:             E = java.lang.String
TraceOut: -------------  Inference variables to be solved: E
TraceOut: -------------   Performing graph inference step EQ ----------
TraceOut: Bound set after resolution: 
TraceOut:         Inference variable = E instantiation = java.lang.String 
TraceOut:             E :> java.lang.String
TraceOut:             E = java.lang.String
TraceOut: Instantiated Method Type : (java.lang.String)java.util.List<java.lang.String>
TraceOut: -----------------------------------------------
TraceOut: (Speculative) (BASIC)  attempt to instantiate method: <E> contains(E[])MyUnitTest.Matcher<java.lang.Iterable<? extends E>> with arguments: java.lang.String
TraceOut: Initial Bound Set (B0):
TraceOut:         Inference variable = E 
TraceOut: ***************************************  FAILURE!!!  ************** com.sun.tools.javac.comp.Infer$InferenceException
TraceOut: (Speculative) (BOX)  attempt to instantiate method: <E> contains(E[])MyUnitTest.Matcher<java.lang.Iterable<? extends E>> with arguments: java.lang.String
TraceOut: Initial Bound Set (B0):
TraceOut:         Inference variable = E 
TraceOut: ***************************************  FAILURE!!!  ************** com.sun.tools.javac.comp.Infer$InferenceException
TraceOut: (Speculative) (VARARITY)  attempt to instantiate method: <E> contains(E[])MyUnitTest.Matcher<java.lang.Iterable<? extends E>> with arguments: java.lang.String
TraceOut: Initial Bound Set (B0):
TraceOut:         Inference variable = E 
TraceOut: Bound set after Invocation Applicability Inference (B2): 
TraceOut:         Inference variable = E 
TraceOut:             E :> java.lang.String
TraceOut: -------------  Inference variables to be solved: E
TraceOut: -------------   Performing graph inference step EQ_LOWER ----------
TraceOut: Bound set after resolution: 
TraceOut:         Inference variable = E instantiation = java.lang.String 
TraceOut:             E :> java.lang.String
TraceOut: Instantiated Method Type : (java.lang.String[])MyUnitTest.Matcher<java.lang.Iterable<? extends java.lang.String>>
TraceOut: -----------------------------------------------
TraceOut: (Final) (VARARITY)  attempt to instantiate method: <E> contains(E[])MyUnitTest.Matcher<java.lang.Iterable<? extends E>> with arguments: java.lang.String
TraceOut: Initial Bound Set (B0):
TraceOut:         Inference variable = E 
TraceOut: Bound set after Invocation Applicability Inference (B2): 
TraceOut:         Inference variable = E 
TraceOut:             E :> java.lang.String
TraceOut: Bound set after incorporation: <Ditto>
TraceOut: Bound set after generation of return constraints (B3):
TraceOut:         Inference variable = E 
TraceOut:             E <: java.lang.String
TraceOut:             E :> java.lang.String
TraceOut: -------------  Inference variables to be solved: E
TraceOut: -------------   Performing graph inference step EQ_LOWER ----------
TraceOut: Bound set after resolution: 
TraceOut:         Inference variable = E instantiation = java.lang.String 
TraceOut:             E <: java.lang.String
TraceOut:             E :> java.lang.String
TraceOut: Instantiated Method Type : (java.lang.String[])MyUnitTest.Matcher<java.lang.Iterable<? extends java.lang.String>>
TraceOut: -----------------------------------------------
Note: /home/srikanth/tmp/X.java uses unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.

So it would appear Bound set after generation of return constraints (B3): has this additional bound E <: java.lang.String and the two bounds E <: java.lang.String and E :> java.lang.String result in instantiation with java.lang.String during graph inference step EQ_LOWER.

See if this sheds additional/needed light.

[stephan-herrmann]

See if this sheds additional/needed light.

Resolution failure of all those wrong of(..) methods kind of obscures the trace ;), but I still can't see the origin of E <: java.lang.String.

We'd need to see javac's equivalent of these:

     ⟨Collection<? extends E#2> <: Iterable<? extends String>⟩
     ⟨capture#1-of ? extends E#2 <= ? extends String⟩
     ⟨capture#1-of ? extends E#2 <: String⟩

Saying E <: java.lang.String could be caused by javac widening capture capture#1-of ? extends E#2 to its bound E#2, leading us once again to https://bugs.openjdk.org/browse/JDK-8016207 - but so far I don't even know if javac has the equivalent of any of these constraints.