[Bug] Config composition error with latest version of OmegaConf

[odelalleau]

🐛 Bug

Description

When using OmegaConf at commit 2dd15f9 (first commit where this problem occurs), there are multiple Hydra tests failures, for instance:

pytest "tests/test_basic_launcher.py::TestBasicLauncher::test_sweep_1_job[basic-overrides0]"
(...)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

self = __INVALID__, value = None

    def validate_and_convert(self, value: Any) -> Any:
        """
        Validates input and converts to canonical form
        :param value: input value
        :return: converted value ("100" may be converted to 100 for example)
        """
        if value is None:
            if self._is_optional():
                return None
>           raise ValidationError("Non optional field cannot be assigned None")
E           hydra.errors.ConfigCompositionException

../omegaconf/omegaconf/nodes.py:55: ConfigCompositionException

Checklist

• I checked on the latest version of Hydra
• I created a minimal repro (See this for tips).

To reproduce

Use master branch of Hydra with OmegaConf's commit 2dd15f9

Additional context

This might actually be an OmegaConf bug (I'm not sure).