Problem

Frozen Unit instances raise FrozenError on the first comparison call. normalize lazily memoizes via @normalized ||= dup.normalize!; when the object is frozen that write fails. Since <=> calls normalize, every comparison operator (<, >, >=, <=, ==) is affected.

Fix

Override freeze to pre-populate @normalized while the object is still mutable:

def freeze
  normalize  # populate @normalized while still mutable
  super
end

Once @normalized is set the ||= short-circuits on every subsequent call, so the frozen instance is safe to compare indefinitely.

Tests

• Unit(1, "m").freeze >= Unit("0.9 m") → true (no error)
• Unit(60, "Hz").freeze < Unit("61 Hz") → true (no error)
• Unit(1, "kg").freeze == Unit("1 kg") → true (no error)