Spotted (only) once in the jdk/hs CI build on solaris/sparc. The build before and after it both succeeded, so the failure seems to be intermittent.

open/src/java.base/share/classes/java/util/stream/Collectors.java:1564: error: incompatible types: inference variable K#1 has incompatible bounds
    return toMap(keyMapper, valueMapper, mergeFunction, HashMap::new);
                    ^
    equality constraints: V,U#2,K#3
    lower bounds: CAP#1,CAP#2
  where K#1,T#1,U#1,M,V,U#2,T#2,K#2,K#3 are type-variables:
    K#1 extends Object declared in method <T#1,K#1,U#1,M>toMap(Function<? super T#1,? extends K#1>,Function<? super T#1,? extends U#1>,BinaryOperator<U#1>,Supplier<M>)
    T#1 extends Object declared in method <T#1,K#1,U#1,M>toMap(Function<? super T#1,? extends K#1>,Function<? super T#1,? extends U#1>,BinaryOperator<U#1>,Supplier<M>)
    U#1 extends Object declared in method <T#1,K#1,U#1,M>toMap(Function<? super T#1,? extends K#1>,Function<? super T#1,? extends U#1>,BinaryOperator<U#1>,Supplier<M>)
    M extends Map<K#1,U#1> declared in method <T#1,K#1,U#1,M>toMap(Function<? super T#1,? extends K#1>,Function<? super T#1,? extends U#1>,BinaryOperator<U#1>,Supplier<M>)
    V extends Object declared in class HashMap
    U#2 extends Object declared in method <T#2,K#2,U#2>toMap(Function<? super T#2,? extends K#2>,Function<? super T#2,? extends U#2>,BinaryOperator<U#2>)
    T#2 extends Object declared in method <T#2,K#2,U#2>toMap(Function<? super T#2,? extends K#2>,Function<? super T#2,? extends U#2>,BinaryOperator<U#2>)
    K#2 extends Object declared in method <T#2,K#2,U#2>toMap(Function<? super T#2,? extends K#2>,Function<? super T#2,? extends U#2>,BinaryOperator<U#2>)
    K#3 extends Object declared in class HashMap
  where CAP#1,CAP#2 are fresh type-variables:
    CAP#1 extends U#2 from capture of ? extends U#2
    CAP#2 extends K#2 from capture of ? extends K#2
1 error