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https://issues.apache.org/jira/browse/GROOVY-11683?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17958794#comment-17958794
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Eric Milles commented on GROOVY-11683:
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Replacing "List::stream" with "list -> list.stream()" allows the type of the 
{{flatMap}} stage to be determined to be {{Stream<UserType>}}.  Groovy 4 visits 
method reference call args before visiting the call expr.  Closures and lambdas 
are visited after and benefit from some additional inferencing provided by the 
method's generics.  Groovy 5 visits method pointers and references after -- 
matching closures and lambdas.

Some more detail in this specific scenario: when visiting "List<Stream>" the 
type checker looks for methods of {{Class<List>}} and {{List}} (raw type).  
From this it works out "Stream<Object>" as the return type for the {{flatMap}} 
stage.

Java lets you add type args to method references, like 
"List::<UserType>stream".  GROOVY-9239 requests support for this.  Groovy 
parses "List<UserType>::stream" and this makes its way through STC.  I haven't 
found exactly where this type info is lost.

You can also add a type hint like this: {{.<UserType>flatMap(List::stream)}}

> 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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