Document Kotlin declaration site variance subtleties #31370
Description
Affects: 5.3.x
For background, this problem was identified while looking at https://stackoverflow.com/questions/77233335/how-to-register-new-collection-factories-in-spring-boot.
With a Converter implemented in Kotlin for a List to Guava's ImmutableList conversion:
class ListToImmutableListConverter : Converter<List<Any>, ImmutableList<Any>> { override fun convert(source: List<Any>): ImmutableList<Any> = ImmutableList.copyOf(source) }ResolvableType resolves the first Any to ? and the second to Object:
val type: ResolvableType = ResolvableType.forClass(ListToImmutableListConverter::class.java).`as`(Converter::class.java) println(type) org.springframework.core.convert.converter.Converter<java.util.List<?>, com.google.common.collect.ImmutableList<java.lang.Object>> Contrastingly, using Java and ? results in both being resolved to ?:
static class ListToImmutableList implements Converter<List<?>, ImmutableList<?>> { @Override public ImmutableList<?> convert(List<?> source) { return ImmutableList.copyOf(source); } }ResolvableType type = ResolvableType.forClass(ListToImmutableList.class).as(Converter.class); System.out.println(type); org.springframework.core.convert.converter.Converter<java.util.List<?>, com.google.common.collect.ImmutableList<?>> Resolving the target type to ImmutableList<Object> rather than ImmutableList<?> is problematic as it prevents the converter from matching a List<String> to ImmutableList<String> conversion.