The MethodHandles.foldArguments method that accepts a position argument does not specify any behavior for positions out of bounds.

For positive values, the method currently throws an IllegalArgumentException:

jshell> MethodHandles.foldArguments(MethodHandles.zero(void.class), 1, MethodHandles.zero(void.class))
| Exception java.lang.IllegalArgumentException: target and combiner types must match: ()void != ()void
| at MethodHandleStatics.newIllegalArgumentException (MethodHandleStatics.java:177)
| at MethodHandles.misMatchedTypes (MethodHandles.java:6053)
| at MethodHandles.foldArgumentChecks (MethodHandles.java:5902)
| at MethodHandles.foldArguments (MethodHandles.java:5873)
| at (#3:1)

The exception message is at least confusing.

For negative values, an ArrayIndexOutOfBoundsException is thrown:

jshell> MethodHandles.foldArguments(MethodHandles.zero(void.class), -1, MethodHandles.zero(int.class))
| Exception java.lang.ArrayIndexOutOfBoundsException: Index -1 out of bounds for length 0
| at MethodType.parameterType (MethodType.java:826)
| at MethodHandles.foldArgumentChecks (MethodHandles.java:5899)
| at MethodHandles.foldArguments (MethodHandles.java:5873)
| at (#6:1)

Alternatively, it silently accepts negative values in other scenarios:

jshell> MethodHandles.foldArguments(MethodHandles.zero(void.class), -1, MethodHandles.zero(void.class))
$7 ==> MethodHandle()void

Or throws an ArrayIndexOutOfBoundsException with a different message:

jshell> MethodHandles.foldArguments(MethodHandles.zero(void.class), -2, MethodHandles.zero(void.class))
| Exception java.lang.ArrayIndexOutOfBoundsException: arraycopy: source index -1 out of bounds for object array[1]
| at System.arraycopy (Native Method)
| at LambdaFormEditor.makeArgumentCombinationForm (LambdaFormEditor.java:840)
| at LambdaFormEditor.foldArgumentsForm (LambdaFormEditor.java:1049)
| at MethodHandles.foldArguments (MethodHandles.java:5876)
| at (#8:1)