junit-team/junit5 · error · JUnitException

Unsupported parameter kind

Error message

Unsupported parameter kind: ${parameter.getKind()}

What it means

JUnitException thrown by KotlinFunctionUtils.toArgumentMap when a Kotlin KParameter's kind is not INSTANCE, VALUE, or EXTENSION_RECEIVER. JUnit only knows how to bind those three kinds when invoking Kotlin functions reflectively; any other kind (e.g. HEADER from inline-class metadata, or kinds introduced by newer Kotlin compilers) is rejected.

Solutions

1. Restructure the Kotlin test method to use standard VALUE parameters
2. Upgrade JUnit to a version with broader Kotlin kind support
3. Avoid inline/value classes as direct test method parameters
4. Report the case to JUnit with the Kotlin compiler version and a reproducer

static boolean supportsKotlinParam(kotlin.reflect.KParameter p) {
    var k = p.getKind();
    return k == KParameter.Kind.INSTANCE
        || k == KParameter.Kind.VALUE
        || k == KParameter.Kind.EXTENSION_RECEIVER;
}

When it happens

Trigger: Reflective invocation of a Kotlin @Test method or lifecycle method whose Kotlin metadata declares a parameter kind JUnit does not handle - e.g. a value parameter of an inline class exposed with HEADER kind, or a kind introduced by a newer kotlin-compiler version.

Common situations: Using Kotlin inline/value classes as direct test method parameters; new Kotlin compiler version emitting metadata kinds JUnit does not yet recognize; Kotlin suspend functions routed through the non-suspend invocation path.

Related errors

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• Detected cycle in inner class hierarchy between
• Failed to find constructor for
• Failed to inject parameter value into field
• LocaleProvider instance could not be constructed because of…

AI-assisted analysis of junit-team/junit5@f070c699a0 (2026-08-11). Data as JSON: /api/errors/a6fbb3242a271979. Report an issue: GitHub.

Appendix: source

Thrown at junit-platform-commons/src/main/java/org/junit/platform/commons/util/KotlinFunctionUtils.java:123

			}
			catch (Exception e) {
				throw throwAsUncheckedException(getUnderlyingCause(e));
			}
		});
	}

	private static Map<KParameter, @Nullable Object> toArgumentMap(@Nullable Object target, @Nullable Object[] args,
			KFunction<?> function) {
		Map<KParameter, @Nullable Object> arguments = new HashMap<>(args.length + 1);
		int index = 0;
		for (KParameter parameter : function.getParameters()) {
			switch (parameter.getKind()) {
				case INSTANCE -> arguments.put(parameter, target);
				case VALUE, EXTENSION_RECEIVER -> {
					arguments.put(parameter, args[index]);
					index++;
				}
				default -> throw new JUnitException("Unsupported parameter kind: " + parameter.getKind());
			}
		}
		return arguments;
	}

	private static KFunction<?> getKotlinFunction(Method method) {
		return Preconditions.notNull(ReflectJvmMapping.getKotlinFunction(method),
			() -> "Failed to get Kotlin function for method: " + method);
	}

	private KotlinFunctionUtils() {
	}
}

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