[GROOVY-10765] STC: Closure implementation of Java @FunctionalInterface loses type information

• Type: Bug
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 4.0.5
• Fix Version/s: 4.0.6
• Component/s: Static Type Checker
• Assignee: Eric Milles
• Reporter: Kyle Moore
• Created: 21/Sep/22 17:29
• Updated: 10/Feb/23 21:40
• Resolved: 22/Sep/22 15:55

Description

The following fails to compile with Groovy 4.0.5:

import java.util.function.BiFunction

@groovy.transform.CompileStatic
class MapTransformEntries {
    private static <K, V1, V2> Map<K, V2> transformEntries(Map<K, V1> map, BiFunction<? super K, ? super V1, V2> transformer) {
        throw new UnsupportedOperationException("implementation not relevant");
    }

    {
        Map<String, ? extends File> outputFiles
        Map<String, Integer> outputFileSpecs = transformEntries(outputFiles, { key, value -> value.hashCode() }) 
    }
}

Producing result:

1 compilation error:

[Static type checking] - Incompatible generic argument types. Cannot assign java.util.Map<? extends java.lang.Object, java.lang.Integer> to: java.util.Map<java.lang.String, java.lang.Integer>
 at line: 11, column: 48

At first glance, this seems similar to GROOVY-10613 and some of its related issues.

We can workaround by casting the above to Map<String, Integer>, but this was not necessary in Groovy 3.x.

Thanks to otogami for creating the reproducer

Issue Links

• is caused by: GROOVY-9998 Wildcards with a lower bound fail static type checking