Spec (https://cr.openjdk.org/~abimpoudis/unnamed/jep443-20230322/specs/unnamed-jls.html#jls-14.20.3) says:
Alternatively, the meaning of a basic try-with-resources statement of the form:

try ({VariableModifier} R _ = Expression ...)
    Block

is given by the following translation to a local variable declaration and a try-catch-finally statement:

{
    final {VariableModifierNoFinal} R #i = Expression;
    Throwable #primaryExc = null;

    try ResourceSpecification_tail
        Block
    catch (Throwable #t) {
        #primaryExc = #t;
        throw #t;
    } finally {
        if (#i != null) {
            if (#primaryExc != null) {
                try {
                    #i.close();
                } catch (Throwable #suppressedExc) {
                    #primaryExc.addSuppressed(#suppressedExc);
                }
            } else {
                #i.close();
            }
        }
    }
}

{VariableModifierNoFinal} is defined as {VariableModifier} without final, if present. [jck-14.20.3.1-220-upv]
#t, #primaryExc, #suppressedExc, and #i are automatically generated identifiers that are distinct from any other identifiers (automatically generated or otherwise) that are in scope at the point where the try-with-resources statement occurs. [jck-14.20.3.1-230-upv]



class Door implements AutoCloseable {
    public void close() { }
}

public class Example {

    public static void main(String argv[]) {
        try ( final Door _ = new Door();) { //compile-time error: java: as of release 21, the underscore keyword '_' is only allowed to declare
  unnamed patterns, local variables, exception parameters or lambda parameters
        }
    }


java --version
java 21-ea 2023-09-19 LTS
Java(TM) SE Runtime Environment (build 21-ea+24-LTS-2081)
Java HotSpot(TM) 64-Bit Server VM (build 21-ea+24-LTS-2081, mixed mode, sharing)