[GROOVY-10859] Regression in invoking method references to methods from List class when called on empty lists

• Type: Bug
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 4.0.0
• Fix Version/s: 4.0.7
• Assignee: Eric Milles
• Reporter: Marcin Erdmann
• Created: 02/Dec/22 16:14
• Updated: 10/Feb/23 21:40
• Resolved: 04/Dec/22 15:54

Description

While migrating from Groovy 3 to Groovy 4 I have uncovered a regression in how method references to methods on List are invoked when called on empty lists. The following test case passes on Groovy 3:

    def "method references from List class are invoked correctly on empty lists"() {
        given:
        def streamOfEmptyCollections = [[]].stream()

        when:
        def flattened = streamOfEmptyCollections.flatMap(List::stream).toList()

        then:
        flattened == []
    }

But on Groovy 4 it fails with the following error:

Condition not satisfied:

flattened == []
|         |
|         false
[interface java.util.List]

I believe that this regression was introduced by this change to groovy.lang.MetaClassImpl.invokeMethodClosure() which was part of the fix for GROOVY-10057.