interpreter: Rework extract_required_kwarg to have better typing

This allows us to avoid making `feature` a union, therefore if we write
code like:
```python
disabled, required, feature = extract_required_kwarg(...)
if disabled:
   ...
```
and now mypy knows that feature is a `str` inside the `if disabled`
block

2 files changed, 24 insertions, 9 deletions

diff --git a/mesonbuild/interpreter/interpreter.py b/mesonbuild/interpreter/interpreter.py
index 69d04bd61..43433d591 100644
--- a/mesonbuild/interpreter/interpreter.py
+++ b/mesonbuild/interpreter/interpreter.py
@@ -868,7 +868,6 @@ class Interpreter(InterpreterBase, HoldableObject):
             pass
         disabled, required, feature = extract_required_kwarg(kwargs, self.subproject)
         if disabled:
-            assert feature, 'for mypy'
             mlog.log('Subproject', mlog.bold(subp_name), ':', 'skipped: feature', mlog.bold(feature), 'disabled')
             return self.disabled_subproject(subp_name, disabled_feature=feature)
 
@@ -1299,7 +1298,6 @@ class Interpreter(InterpreterBase, HoldableObject):
         native = kwargs['native']
 
         if disabled:
-            assert feature, 'for mypy'
             for lang in sorted(langs, key=compilers.sort_clink):
                 mlog.log('Compiler for language', mlog.bold(lang), 'skipped: feature', mlog.bold(feature), 'disabled')
             return False
@@ -1754,7 +1752,6 @@ class Interpreter(InterpreterBase, HoldableObject):
                           ) -> T.Union['build.Executable', ExternalProgram, 'OverrideProgram']:
         disabled, required, feature = extract_required_kwarg(kwargs, self.subproject)
         if disabled:
-            assert feature, 'for mypy'
             mlog.log('Program', mlog.bold(' '.join(args[0])), 'skipped: feature', mlog.bold(feature), 'disabled')
             return self.notfound_program(args[0])
 
diff --git a/mesonbuild/interpreter/interpreterobjects.py b/mesonbuild/interpreter/interpreterobjects.py
index b6137d175..697c4a0f0 100644
--- a/mesonbuild/interpreter/interpreterobjects.py
+++ b/mesonbuild/interpreter/interpreterobjects.py
@@ -36,7 +36,7 @@ if T.TYPE_CHECKING:
     from ..interpreterbase import FeatureCheckBase, SubProject, TYPE_var, TYPE_kwargs, TYPE_nvar, TYPE_nkwargs
     from .interpreter import Interpreter
 
-    from typing_extensions import TypedDict
+    from typing_extensions import Literal, TypedDict
 
     class EnvironmentSeparatorKW(TypedDict):
 
@@ -52,18 +52,36 @@ _ERROR_MSG_KW: KwargInfo[T.Optional[str]] = KwargInfo('error_message', (str, Non
 def extract_required_kwarg(kwargs: 'kwargs.ExtractRequired',
                            subproject: 'SubProject',
                            feature_check: T.Optional[FeatureCheckBase] = None,
-                           default: bool = True) -> T.Tuple[bool, bool, T.Optional[str]]:
+                           default: bool = True
+                           ) -> T.Union[T.Tuple[Literal[True], bool, str],
+                                        T.Tuple[Literal[False], bool, None]]:
+    """Check common keyword arguments for required status.
+
+    This handles booleans vs feature option.
+
+    :param kwargs:
+      keyword arguments from the Interpreter, containing a `required` argument
+    :param subproject: The subproject this is
+    :param feature_check:
+        A custom feature check for this use of `required` with a
+        `UserFeatureOption`, defaults to None.
+    :param default:
+        The default value is `required` is not set in  `kwargs`, defaults to
+        True
+    :raises InterpreterException: If the type of `kwargs['required']` is invalid
+    :return:
+        a tuple of `disabled, required, feature_name`. If `disabled` is `True`
+        `feature_name` will be a string, otherwise it is `None`
+    """
     val = kwargs.get('required', default)
-    disabled = False
     required = False
-    feature: T.Optional[str] = None
     if isinstance(val, options.UserFeatureOption):
         if not feature_check:
             feature_check = FeatureNew('User option "feature"', '0.47.0')
         feature_check.use(subproject)
         feature = val.name
         if val.is_disabled():
-            disabled = True
+            return True, required, feature
         elif val.is_enabled():
             required = True
     elif isinstance(val, bool):
@@ -76,7 +94,7 @@ def extract_required_kwarg(kwargs: 'kwargs.ExtractRequired',
     # TODO: this should be removed, and those callers should learn about FeatureOptions
     kwargs['required'] = required
 
-    return disabled, required, feature
+    return False, required, None
 
 def extract_search_dirs(kwargs: 'kwargs.ExtractSearchDirs') -> T.List[str]:
     search_dirs_str = mesonlib.stringlistify(kwargs.get('dirs', []))