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https://issues.apache.org/jira/browse/GROOVY-12623?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=18135662#comment-18135662
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ASF GitHub Bot commented on GROOVY-12623:
-----------------------------------------
codeconsole opened a new pull request, #3149:
URL: https://github.com/apache/groovy/pull/3149
Under `@CompileStatic`, a method reference to a method that returns a type
parameter fails at run time when the functional interface returns a primitive
and the type argument is not that primitive's wrapper:
```groovy
@CompileStatic
class C {
static <T> T id(T t) { t }
static void main(args) {
Predicate<String> p = C::id
assert [p.test(''), p.test('x')] == [false, true] //
ClassCastException: String cannot be cast to Boolean
}
}
```
Dynamic Groovy and `@TypeChecked` give `[false, true]`. 4.0.33, 5.0.8 and
6.0.0 fail the same way. The static compiler sets the instantiated return type
to the SAM's wrapper type (GROOVY-10975), so `LambdaMetafactory` casts the
erased return value to `Boolean`, or to `Number` for numeric return types, and
unboxes it.
`StaticTypesMethodReferenceExpressionWriter` now resolves the type argument:
a method type parameter from the argument types the type checker inferred, or a
class type parameter from the receiver type. When `LambdaMetafactory` cannot
convert that type, the reference goes through the synthetic adapter that the
GString parameter bridge already uses. The adapter returns the primitive type
and converts the value as a statically compiled cast does: Groovy truth for
`boolean`, a Groovy cast for numbers and `char`. A type argument that
`LambdaMetafactory` already converts binds directly as before. Examples are
`Map<String,Boolean>::get` as a `Predicate<String>`, where a missing key still
throws `NullPointerException` as in Java, and `Long` for an `int` return type.
Method references whose return type is not a type parameter are not changed.
This and #3143 (GROOVY-12617, non-generic return types) both extend the
adapter in `StaticTypesMethodReferenceExpressionWriter`; whichever lands second
keeps both predicates (one for a concrete return type, one for a type-parameter
return) and a single early return.
## Tests
`MethodReferenceTest.testPrimitiveSAMFromTypeParameterReturnType` (run with
`@CompileDynamic`, `@TypeChecked` and `@CompileStatic`) and
`testPrimitiveSAMFromTypeParameterReturnTypeSerializable` fail without the
change. `testPrimitiveSAMFromTypeParameterReturnTypeNoConversion` checks that
type arguments `LambdaMetafactory` already converts keep their behaviour. The
`groovy.transform.stc`, `org.codehaus.groovy.classgen.asm.sc` and `groovy.bugs`
tests pass. The Checkstyle and CodeNarc gates pass.
> SC: a method reference to a generic method used for a primitive SAM return
> type (e.g. Predicate) throws ClassCastException instead of applying Groovy
> truth
> -----------------------------------------------------------------------------------------------------------------------------------------------------------
>
> Key: GROOVY-12623
> URL: https://issues.apache.org/jira/browse/GROOVY-12623
> Project: Groovy
> Issue Type: Bug
> Affects Versions: 4.0.33, 5.0.8, 6.0.0
> Reporter: Scott Murphy Heiberg
> Priority: Major
>
> {code:groovy}
> import java.util.function.Predicate
> @groovy.transform.CompileStatic
> class C {
> static <T> T id(T t) { t }
> static void main(args) {
> Predicate<String> p = C::id
> assert [p.test(''), p.test('x')] == [false, true]
> }
> }
> {code}
> Actual: {{java.lang.ClassCastException: class java.lang.String cannot be cast
> to class java.lang.Boolean}} at {{p.test('x')}}. The same happens for the get
> method of a Map of String to String used as a Predicate (bound or unbound),
> List::first, an Optional's get as a BooleanSupplier, and generic instance
> methods. For numeric SAMs, a ToIntFunction of String or Character with
> {{C::id}} gives "cannot be cast to class java.lang.Number".
> Expected: [false, true], as in dynamic Groovy and under @TypeChecked; 120 and
> 65 for the numeric cases.
> When the referenced method returns a type parameter, the static compiler sets
> the instantiated return type to the SAM's wrapper type (GROOVY-10975) and
> leaves the conversion to LambdaMetafactory, which only casts and unboxes.
> Seen in 4.0.33, 5.0.8 and 6.0.0.
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