If this is acceptable - I'll add a bunch of missing tests (related to non-string scalars):

• int $opt = 0 (valid)
• ?int $opt = null (valid)
• ?int $opt = 0 (invalid)
• int|bool = false (valid)
• int|bool = 0 (invalid)
• float $opt = 0.0 (valid)
• ?float $opt = null (valid)
• ?float $opt = 0.0 (invalid)
• float|bool = false (valid)
• float|bool = 0.0 (invalid)

Makes sense to me, thank you.

@yceruto, the full list:

Any string can be swapped with int or float.

@kbond do you think/confirm we can document these rules like this? + sample table above

Signature rules (<type> $opt = <default-value>)

• <type> must be one of: bool, string, int, float, or array
  • Nullable types (?<type>) require <default-value> to be null
  • Union types are only allowed for bool|string, bool|int, or bool|float
    • In these cases, <default-value> must be false
• All options must define a <default-value>

Definition rules

• VALUE_NONE: set for bool or ?bool
  • VALUE_NEGATABLE: same as VALUE_NONE, but only if <default-value> is true or null
• VALUE_REQUIRED: set for non-boolean single types (<type>) or nullable types (?<type>)
  • VALUE_ARRAY: same as VALUE_REQUIRED, but specifically for array type
• VALUE_OPTIONAL: set only for union types combining bool|<allowed-type>

Usage/Value rules

• If the option is omitted, <default-value> is given
• If passed without a value (--opt), true is given
• If negate passed (--no-opt), false is given
• If passed with a value (--opt=val), val is given

@yceruto those rules look great to me!

Thank you @kbond.