Specification states:

withElementCountCopy SequenceLayout withElementCount(long elementCount)
Throws: IllegalArgumentException - if elementCount < 0.

Found:
withElementCount doesn't throw IAE if "long" overflow happens.
The method should be fully documented i.e. AE should be either mentioned or wrapped.

jshell> SequenceLayout sl = MemoryLayout.sequenceLayout(4, MemoryLayout.sequenceLayout(4,ValueLayout.JAVA_LONG));
sl ==> [4:[4:j64]]

jshell> sl.withElementCount(Long.MIN_VALUE/(7*32));
| Exception java.lang.ArithmeticException: long overflow
| at Math.multiplyExact (Math.java:1032)
| at SequenceLayoutImpl.<init> (SequenceLayoutImpl.java:43)
| at SequenceLayoutImpl.withElementCount (SequenceLayoutImpl.java:71)
| at (#31:1)

also it would be good to wrap ArithmeticException: long overflow in IAE for positive counts:

jshell> sl.withElementCount(Long.MAX_VALUE/(7*32));
| Exception java.lang.ArithmeticException: long overflow
| at Math.multiplyExact (Math.java:1032)
| at SequenceLayoutImpl.<init> (SequenceLayoutImpl.java:43)
| at SequenceLayoutImpl.withElementCount (SequenceLayoutImpl.java:71)
| at (#34:1)