Type argument inference crash: lambda passed as Supplier<? extends List<? extends T>> to a ? extends target #2159

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1. aosen-xiong

   added

   bug

   on Sep 26, 2026
2. aosen-xiong commented on Sep 26, 2026

   

   aosen-xiong

   on Sep 26, 2026

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   Two smaller programs fail on the same kind of constraint (capture of ? extends T <: …) with no lambda. They crash at different places in InvocationTypeInference.createB3, so they may not share a root cause with the original.

   Double wildcard in a return type:

   class NoLambda {
       static class Box<T> {}
   
       static <T> Box<? extends Box<? extends T>> make() {
           throw new Error();
       }
   
       void m() {
           Box<? extends Box<? extends String>> b = make();
       }
   }

   NoLambda.java:9: error: [type.argument.inference.crashed] Type argument inference crashed for NoLambda.make
           Box<? extends Box<? extends String>> b = make();
                                                        ^
     error: An exception occurred:  False bound for: Constraint: inference type: capture#485 of ? extends T <: String Result: FALSE
   

   FalseBoundException: False bound for: Constraint: inference type: capture#154 of ? extends T <: String Result: FALSE
     at ...typeinference8.constraint.ConstraintSet.reduceOneStep(ConstraintSet.java:458)
     at ...typeinference8.constraint.ConstraintSet.reduce(ConstraintSet.java:391)
     at ...typeinference8.bound.CaptureBound.incorporate(CaptureBound.java:156)
     at ...typeinference8.bound.CaptureBound.createAndIncorporateCaptureConstraint(CaptureBound.java:111)
     at ...typeinference8.InvocationTypeInference.createB3(InvocationTypeInference.java:381)
   

   Single wildcard inside java.util.function.Function:

   import java.util.function.Function;
   
   class FunctionReturn {
       static class Box<T> {}
   
       static <T> Function<Throwable, Box<? extends T>> make() {
           throw new Error();
       }
   
       void m() {
           Function<Throwable, Box<? extends String>> f = make();
       }
   }

   FunctionReturn.java:11: error: [type.argument.inference.crashed] Type argument inference crashed for FunctionReturn.make
     error: An exception occurred:  False bound for: Constraint: inference type: capture#4 of ? extends T <: String Result: FALSE
   

   FalseBoundException: False bound for: Constraint: inference type: capture#360 of ? extends T <: String Result: FALSE
     at ...typeinference8.constraint.ConstraintSet.reduceOneStep(ConstraintSet.java:458)
     at ...typeinference8.constraint.ConstraintSet.reduce(ConstraintSet.java:391)
     at ...typeinference8.InvocationTypeInference.createB3(InvocationTypeInference.java:443)
   

   The second one also crashes with java.util.function.Supplier. It does not crash with a user-defined functional interface interface Fn<A, B> { B apply(A a); }, or with a single Box<Box<? extends T>>.

   Spring-framework sites with this failure:

   • Mono.defer(() -> …) passed to switchIfEmpty(Mono<? extends T>) or concatWith(Publisher<? extends T>): TransactionAspectSupport.java:981, LiteWebJarsResourceResolver.java:74/90, VersionResourceResolver.java:165 and AbstractCharSequenceDecoder.java:107.
   • onErrorResume(predicate, exceptionWrappingFunction(response)), where the method returns Function<Throwable, Mono<? extends T>>: DefaultWebClient.java:672. This is the FunctionReturn shape.
   • Nested then(Mono.defer(() -> x.then(Mono.defer(() -> Mono.empty())))): AbstractReactiveTransactionManager.java:506.
3. aosen-xiong

   added this to the next-release milestone on Sep 26, 2026
4. aosen-xiong commented on Sep 28, 2026

   

   aosen-xiong

   on Sep 28, 2026

   Last edited by aosen-xiong

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   On master aea76ae, this still causes type-argument inference crashes in micrometer (2 sites) and spring-framework (7 sites).
5. wmdietl

   closed this as completedin #2254on Oct 13, 2026