interpreterbase: Add validator keyword argument to typed_kwargs

This attribute is a callable that returns a string if the value is
invalid, otherwise None. This intended for cases like the `install_*`
function's `install_mode` paramater, which is either an int or the
string "preserve", which allows us to do nice things like:

```python

class Kwargs(TypedDict):

    install_mode: T.Union[int, T.Literal['preserve']]

@typed_kwargs(
  'foo', KwargInfo('install_mode', ...,
  validator=lambda x: None if isinstance(x, int) or x == 'preserve' else 'must be the literal "preserve"),
)
def install_data(self, node, args, kwargs: 'Kwargs'):
  ...
```

In this case mypy *knows* that the string is preserve, as do we, and we
can simply do tests like:
```python
if kwargs['install_mode'] == 'preserve':
   ...
else:
   # this is an int
```

1 files changed, 15 insertions, 0 deletions

diff --git a/run_unittests.py b/run_unittests.py
index 39b3496..9f66d9c 100755
--- a/run_unittests.py
+++ b/run_unittests.py
@@ -1629,6 +1629,21 @@ class InternalTests(unittest.TestCase):
             self.assertRegex(out.getvalue(), r'WARNING:.*deprecated.*input arg in testfunc')
             self.assertNotRegex(out.getvalue(), r'WARNING:.*introduced.*input arg in testfunc')
 
+    def test_typed_kwarg_validator(self) -> None:
+        @typed_kwargs(
+            'testfunc',
+            KwargInfo('input', str, validator=lambda x: 'invalid!' if x != 'foo' else None)
+        )
+        def _(obj, node, args: T.Tuple, kwargs: T.Dict[str, str]) -> None:
+            pass
+
+        # Should be valid
+        _(None, mock.Mock(), tuple(), dict(input='foo'))
+
+        with self.assertRaises(MesonException) as cm:
+            _(None, mock.Mock(), tuple(), dict(input='bar'))
+        self.assertEqual(str(cm.exception), "testfunc keyword argument \"input\" invalid!")
+
 
 @unittest.skipIf(is_tarball(), 'Skipping because this is a tarball release')
 class DataTests(unittest.TestCase):