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Eric Milles commented on GROOVY-11683: -------------------------------------- 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. -- This message was sent by Atlassian Jira (v8.20.10#820010)