oracle/graal · error · IllegalArgumentException

Element {} should be an espresso object constant, got {}

Error message

Element {} should be an espresso object constant, got {}

What it means

In asArrayConstant's object-array branch, every non-null element must be an EspressoExternalObjectConstant so the guest Value can be stored into the guest array with setArrayElement. An element constant from another JVMCI backend (or a primitive-boxed constant where an object constant was expected) has no guest Value and throws IllegalArgumentException naming the offending index.

Source

Thrown at espresso-compiler-stub/src/com.oracle.truffle.espresso.vmaccess/src/com/oracle/truffle/espresso/vmaccess/EspressoExternalVMAccess.java:566

            JavaKind javaKind = elementalType.getJavaKind();
            for (int i = 0; i < elements.length; i++) {
                JavaConstant element = elements[i];
                if (javaKind != element.getJavaKind()) {
                    throw new IllegalArgumentException("Element " + i + " should be a " + javaKind + " but was " + element.getJavaKind());
                }
                if (element.isDefaultForKind()) {
                    continue;
                }
                array.setArrayElement(i, element.asBoxedPrimitive());
            }
        } else {
            for (int i = 0; i < elements.length; i++) {
                JavaConstant element = elements[i];
                if (element.isNull()) {
                    continue;
                }
                if (!(element instanceof EspressoExternalObjectConstant objectElement)) {
                    throw new IllegalArgumentException("Element " + i + " should be an espresso object constant, got " + safeGetClass(element));
                }
                try {
                    array.setArrayElement(i, objectElement.getValue());
                } catch (ClassCastException e) {
                    throw new IllegalArgumentException("Element " + i + " is not an instance of the component type", e);
                }
            }
        }
        return new EspressoExternalObjectConstant(this, array);
    }

    @Override
    public JavaConstant createPrimitiveArray(JavaKind kind, int length) {
        if (kind == null || !kind.isPrimitive() || kind == JavaKind.Void) {
            throw new IllegalArgumentException("Expected a non-void primitive kind, got " + kind);
        }
        if (length < 0) {
            throw new NegativeArraySizeException("Negative array size: " + length);

View on GitHub (pinned to a66e9ccd1d)

Solutions

1. Produce every element through this Espresso provider (its constant reflection / previous asArrayConstant results).
2. For nested arrays, build each inner array with asArrayConstant first so elements are EspressoExternalObjectConstant.
3. Filter or convert foreign constants before the call; keep nulls (they are skipped).

Example fix

// before
elems[i] = JavaConstant.forObject(hostString);

// after
elems[i] = espressoConstantReflection.forObject(guestString); // same context

for (int i = 0; i < elements.length; i++) {
    JavaConstant c = elements[i];
    if (!c.isNull() && !(c instanceof EspressoExternalObjectConstant)) {
        // convert or reject before the call
    }
}

static boolean allEspressoOrNull(JavaConstant[] elems) {
    for (JavaConstant c : elems) {
        if (!c.isNull() && !(c instanceof EspressoExternalObjectConstant)) return false;
    }
    return true;
}

catch (IllegalArgumentException e) naming the index — use the index to locate and re-create the offending element through the Espresso provider, then retry.

When it happens

Trigger: Building an object array (or a multi-dimensional primitive array, dimensions > 1) where some elements are host constants, JavaConstant.forObject host values, or constants from another Espresso context.

Common situations: Mixed-origin element lists in substitutions; element constants cached from a previous provider version; multi-dimensional array construction where callers wrongly pass primitive constants instead of nested Espresso array constants.

Related errors

• Invalid component type
• Expected an EspressoExternalObjectConstant, got {}
• Expected an EspressoExternalObjectConstant for src, got {}
• Expected an EspressoExternalObjectConstant for dest, got
• Expected an EspressoExternalObjectConstant, got

AI-assisted analysis of oracle/graal@a66e9ccd1d (2026-08-14). Data as JSON: /api/errors/0502fdc3f83b39dc. Report an issue: GitHub.