Scott Murphy Heiberg created GROOVY-12606:
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Summary: STC rejects a closure or lambda returning a Closure<R>
value where a functional interface return type is expected
Key: GROOVY-12606
URL: https://issues.apache.org/jira/browse/GROOVY-12606
Project: Groovy
Issue Type: Bug
Affects Versions: 5.0.8, 4.0.33, 6.0.0
Reporter: Scott Murphy Heiberg
{code:groovy}
import java.util.function.Supplier
@groovy.transform.CompileStatic // or @TypeChecked
class C {
static String take(Supplier<Supplier<String>> s) { s.get().get() }
static void main(args) {
Closure<String> c = { 'x' }
assert take { c } == 'x'
assert take(() -> c) == 'x'
}
}
{code}
Actual:
{noformat}
[Static type checking] - Cannot return value of type
groovy.lang.Closure<java.lang.String> for closure expecting
java.util.function.Supplier<java.lang.String>
[Static type checking] - Cannot return value of type
groovy.lang.Closure<java.lang.String> for lambda expecting
java.util.function.Supplier<java.lang.String>
{noformat}
Expected: both calls compile and the returned closure is converted to the
functional interface, as it is when a Closure value is passed as a Supplier
argument. A Closure<Integer> should still be rejected.
The return check for closures and lambdas only accepts subtypes of the expected
return type and does not consider closure-to-SAM coercion. A returned closure
literal and a raw Closure are rejected the same way. Under static compilation,
a lambda also needs its generated method to declare the SAM type, so that the
returned closure is converted instead of being returned as a Closure.
Seen in 4.0.33, 5.0.8 and 6.0.0.
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