codeconsole opened a new pull request, #3133:
URL: https://github.com/apache/groovy/pull/3133
The static type checker rejects a closure or lambda that returns a
`Closure<R>` value where the expected return type is a functional interface:
```groovy
import java.util.function.Supplier
@groovy.transform.CompileStatic
class C {
static String take(Supplier<Supplier<String>> s) { s.get().get() }
static void main(args) {
Closure<String> c = { 'x' }
assert take { c } == 'x' // Cannot return value of type
groovy.lang.Closure<java.lang.String> for closure expecting
java.util.function.Supplier<java.lang.String>
assert take(() -> c) == 'x' // same, "for lambda expecting"
}
}
```
4.0.33, 5.0.8 and 6.0.0 reject it the same way. The closure branch of
`StaticTypeCheckingVisitor.checkReturnType` only allowed covariant returns. It
now also accepts a Closure value when the expected type is a SAM type whose
method returns void or a supertype of the Closure's type argument (a raw
Closure is accepted). In that case it records the SAM type as the closure's
return type. For closures this is enough: the closure method already declares
the inferred return type, and the returned value gets a Groovy cast, which
converts the Closure to the interface (as for GROOVY-9971).
A statically compiled lambda's method declared the SAM method's erased
return type, so the Closure was returned unconverted and the caller failed with
a ClassCastException. `StaticTypesLambdaWriter` now declares the lambda's
inferred return type when the SAM method returns a type variable and that
inferred type is a SAM type, so the same Groovy cast is written at the return.
A `Closure<Integer>` returned for `Supplier<String>` is still an error. This
PR, #3113 (GROOVY-12588) and #3126 (GROOVY-12599) each add an
`isClosureCoercibleToSAM` helper; whichever lands later should fold its copy
into the one already on master.
## Tests
`CoercionSTCTest.testCoerceClosureReturnToFunctionalInterface` fails without
the change, under both `CoercionSTCTest` and `CoercionStaticCompileTests`. Its
lambda cases fail under static compilation without the
`StaticTypesLambdaWriter` change.
`testCoerceClosureReturnToFunctionalInterface2` checks that mismatched return
types are still rejected. The `groovy.transform.stc`,
`org.codehaus.groovy.classgen.asm.sc`, `groovy.bugs` and `gls` tests pass. The
Checkstyle and CodeNarc gates pass.
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