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```perl
type ID => Str;

ID->check('foo'); # OK
```

Internally `ID` is equivalent to the following type:

```perl
sub ID() { Str }
```

#### `$type_args` is an undefined value

Given a undefined value in `$type_args`, it returns the type constraint defined by Type::Tiny's Undef type.

```perl
type Foo => Undef;

Foo->check(undef); # OK