Given the annotation type:

@interface Foo { int[] value(); }

javac fails to, but should, parse an annotation @Foo({,}) - that is, a single comma as the value of the array which is assigned to Foo's "value" element.

Accepting not merely a trailing comma but a single comma was the clear intent of JLS 9.7.1 "Normal annotations". It aligns with array initializers for variable declarations, where "int[] x = {,}" is accepted.