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.
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