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https://issues.apache.org/jira/browse/GROOVY-12617?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=18135646#comment-18135646
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ASF GitHub Bot commented on GROOVY-12617:
-----------------------------------------
codeconsole opened a new pull request, #3143:
URL: https://github.com/apache/groovy/pull/3143
Under `@CompileStatic`, a method reference whose method's return value must
be converted by Groovy to the functional interface's primitive return type
compiled, but failed at bootstrap:
```groovy
import java.util.function.Predicate
@groovy.transform.CompileStatic
class C {
static String str(String s) { s }
static void main(args) {
Predicate<String> p = C::str // LambdaConversionException: class
java.lang.String is not convertible to boolean
assert !p.test('') && p.test('x')
}
}
```
Dynamic Groovy, `@TypeChecked` and the method pointer `C.&str` apply Groovy
truth and give `[false, true]`. 4.0.33, 5.0.8 and 6.0.0 fail the same way. The
same failure affected the other primitive returns the type checker accepts but
`LambdaMetafactory` cannot convert, for example `ToDoubleFunction` from a
`BigDecimal` method and `ToIntFunction` from a `long` or `Long` method.
`LambdaMetafactory` converts a return value to a primitive type only by
unboxing and widening primitive conversion.
`StaticTypesMethodReferenceExpressionWriter` already routes some method
references through a synthetic static adapter to convert `GString` parameters.
That adapter is now also used when the return value needs a conversion the
metafactory does not perform. The adapter then returns the SAM's primitive
type, and its statically compiled `return` converts the value like a cast:
Groovy truth for `boolean`, Number conversion for the numeric types. Method
references whose return value the metafactory can convert still bind the method
directly.
## Tests
`MethodReferenceTest` adds three tests that fail without the change:
`testPrimitiveBooleanSAMFromNonBooleanReturnType` (dynamic, `@TypeChecked` and
`@CompileStatic`, with static, instance, bound, extension, constructor and
variadic references),
`testPrimitiveBooleanSAMFromNonBooleanReturnTypeSerializable` and
`testPrimitiveNumberSAMFromNonWideningReturnType`. The `groovy.transform.stc`,
`org.codehaus.groovy.classgen.asm.sc`, `bugs` and `groovy.bugs` tests and
`LambdasTest` pass. The Checkstyle and CodeNarc gates pass.
> SC: a method reference to a functional interface with a primitive return type
> fails at bootstrap when the return value needs a Groovy conversion (e.g.
> String to boolean)
> -------------------------------------------------------------------------------------------------------------------------------------------------------------------------
>
> Key: GROOVY-12617
> URL: https://issues.apache.org/jira/browse/GROOVY-12617
> 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 String str(String s) { s }
> static void main(args) {
> Predicate<String> p = C::str
> assert !p.test('') && p.test('x')
> }
> }
> {code}
> Actual: the code compiles, but evaluating the method reference throws
> {{java.lang.BootstrapMethodError}}, caused by
> {{java.lang.invoke.LambdaConversionException: Type mismatch for lambda
> expected return: class java.lang.String is not convertible to boolean}}. The
> same happens for every primitive return that the type checker accepts but the
> JVM cannot convert: boolean from any reference type or from int,
> ToDoubleFunction from a BigDecimal method, and ToIntFunction from a long or
> Long method.
> Expected: the value is converted as in dynamic Groovy, @TypeChecked code and
> a method pointer ({{C.&str}}), which all give [false, true] here (Groovy
> truth).
> Cause: the static compiler binds the method directly through
> LambdaMetafactory, which converts a return value to a primitive type only by
> unboxing and widening primitive conversion.
> Seen in 4.0.33, 5.0.8 and 6.0.0.
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