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https://issues.apache.org/jira/browse/GROOVY-11683?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17959351#comment-17959351
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Eric Milles edited comment on GROOVY-11683 at 6/10/25 4:04 PM:
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For the case of "stream" specifically, there are a few options:
* {{default Stream<E> stream()}} of {{Collection<E>}}
* {{public static <T> Stream<T> stream(final Iterable<T> self)}} of
{{StreamGroovyMethods}}
* {{public static <T> Stream<T> stream(final T self)}} of
{{StreamGroovyMethods}}
".flatMap(List<UserType>::stream)" does propagate to the Collection method.
There is a bug in the static check that prevents it from filling in the
Iterable method. The Object/T method is not a good match, but is not filtered
out (in Groovy 4) due to not knowing the expected arguments. This is
[StaticTypeCheckingVisitor#filterMethodCandidates|https://github.com/apache/groovy/blob/31ad6a762af79e58a2a44d26105825337799badd/src/main/java/org/codehaus/groovy/transform/stc/StaticTypeCheckingVisitor.java#L2631].
was (Author: emilles):
For the case of "stream" specifically, there are a few options:
* {{default Stream<E> stream()}} of {{Collection<E>}}
* {{public static <T> Stream<T> stream(final Iterable<T> self)}} of
{{StreamGroovyMethods}}
* {{public static <T> Stream<T> stream(final T self)}} of
{{StreamGroovyMethods}}
".flatMap(List<UserType>::stream)" does propagate to the Collection method.
There is a bug in the static check that prevents it from filling in the
Iterable method. The Object/T method is not a good match, but is not filtered
out due to not knowing the expected arguments. This is
[StaticTypeCheckingVisitor#filterMethodCandidates|https://github.com/apache/groovy/blob/31ad6a762af79e58a2a44d26105825337799badd/src/main/java/org/codehaus/groovy/transform/stc/StaticTypeCheckingVisitor.java#L2631].
> STC loses generic information on method reference
> -------------------------------------------------
>
> Key: GROOVY-11683
> URL: https://issues.apache.org/jira/browse/GROOVY-11683
> Project: Groovy
> Issue Type: Bug
> Components: Static Type Checker
> Affects Versions: 4.0.27
> Reporter: Christopher Smith
> Assignee: Eric Milles
> Priority: Minor
>
> I am using a typical stream pipeline to get a list of pages of results and
> flatMap them to the individual records:
> {code}
> //
> software.amazon.awssdk.services.cognitoidentityprovider.CognitoIdentityProviderClient
> cognito
> cognito.listUsersPaginator { it.userPoolId(cognitoUserPool) }
> .stream()
> .map(ListUsersResponse::users)
> .flatMap(List::stream) // produces UserType
> .map(UserType::username)
> .toList()
> {code}
> Both groovyc and GRECLIPSE report the compile-time error
> {code}
> Failed to find class method 'username(java.lang.Object)' or instance method
> 'username()' for the type:
> software.amazon.awssdk.services.cognitoidentityprovider.model.UserType
> {code}
> The cause appears to be a failure to propagate the type information out of
> the {{flatMap}} call: On hover, Eclipse correctly identifies the
> {{List::stream}} as returning {{Stream<UserType>}}, but the {{flatMap}} is
> reported as {{Stream<Object>}}. Inserting a type witness before the
> {{flatMap}} call resolves the error. Both the loss of type information and
> the incorrect error message appear to be bugs.
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