class C {
  static {
    this.foo = arguments;
  }
}

In Class Fields Proposal, it is a syntax error if the initializer contains arguments.

class C {
  static foo = arguments; // throws
}

The spec currently does not throw for arguments. However, to align with fields initializer, I think it makes sense to disallow arguments directly inside static block.

Note that it is still valid if arguments is inside a function declared in the static block:

class C {
  static {
    this.foo = function() { arguments };
  }
}