Lambda and wildcard

[Glavo]

Hi Tagir,

This is expected behavior. See Chapter 15 of the Java Language
Specification (Java SE 25 Edition) [1]:

Unlike a lambda expression, a method reference can be congruent with a
generic function type (that is, a function type that has type parameters).
This is because the lambda expression would need to be able to declare type
parameters, and no syntax supports this; while for a method reference,
no such declaration is necessary. For example, the following program is
legal:

interface ListFactory {
    <T> List<T> make();
}

ListFactory lf  = ArrayList::new;
List<String> ls = lf.make();
List<Number> ln = lf.make();


Glavo

[1]:
https://docs.oracle.com/javase/specs/jls/se25/html/jls-15.html#jls-15.27

On Wed, Oct 8, 2025 at 12:12 AM Tagir Valeev <amaembo at gmail.com> wrote:

> Hello!
>
> I'm investigating a seemingly weird compilation case. Consider the
> following Java interface:
>
> import java.util.function.Supplier;
>
> interface Main {
>     interface X<T> {
>         X<T> self();
>     }
>
>     static X<?> makeX() {return null;}
>
>     static <R> X<R> create(Supplier<? extends R> supplier) {return null;}
>
>     static X<X<?>> methodRef() {
>         return create(Main::makeX).self();
>     }
>
>     static X<X<?>> lambda() {
>         return create(() -> makeX()).self();
>     }
> }
>
> I expect that either both methods 'methodRef' and 'lambda' should be
> compilable or both should be non-compilable. However, while 'methodRef'
> compiles, 'lambda' is rejected by compiler (using javac build 25+36-3489):
>
> Main.java:17: error: incompatible types: X<X<CAP#1>> cannot be converted
> to X<X<?>>
>         return create(() -> makeX()).self();
>                                          ^
>   where CAP#1 is a fresh type-variable:
>     CAP#1 extends Object from capture of ?
> 1 error
> error: compilation failed
>
> Could you please help me and clarify whether this is an expected behavior
> or not?
>
> With best regards,
> Tagir Valeev
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://mail.openjdk.org/pipermail/compiler-dev/attachments/20251008/943e1513/attachment.htm>