[red-knot] Fallback for typing._NoDefaultType

crates/red_knot_python_semantic/src/types.rs

@@ -28,7 +28,7 @@ use crate::symbol::{Boundness, Symbol};
 use crate::types::diagnostic::TypeCheckDiagnosticsBuilder;
 use crate::types::mro::{ClassBase, Mro, MroError, MroIterator};
 use crate::types::narrow::narrowing_constraint;
-use crate::{Db, FxOrderSet, Module, Program};
+use crate::{Db, FxOrderSet, Module, Program, PythonVersion};
 
 mod builder;
 mod diagnostic;
@@ -1897,13 +1897,13 @@ impl<'db> KnownClass {
     }
 
     pub fn to_class_literal(self, db: &'db dyn Db) -> Type<'db> {
-        core_module_symbol(db, self.canonical_module(), self.as_str())
+        core_module_symbol(db, self.canonical_module(db), self.as_str())
             .ignore_possibly_unbound()
             .unwrap_or(Type::Unknown)
     }
 
     /// Return the module in which we should look up the definition for this class
-    pub(crate) const fn canonical_module(self) -> CoreStdlibModule {
+    pub(crate) fn canonical_module(self, db: &'db dyn Db) -> CoreStdlibModule {
         match self {
             Self::Bool
             | Self::Object
@@ -1921,10 +1921,18 @@ impl<'db> KnownClass {
             Self::GenericAlias | Self::ModuleType | Self::FunctionType => CoreStdlibModule::Types,
             Self::NoneType => CoreStdlibModule::Typeshed,
             Self::SpecialForm | Self::TypeVar | Self::TypeAliasType => CoreStdlibModule::Typing,
-            // TODO when we understand sys.version_info, we will need an explicit fallback here,
-            // because typing_extensions has a 3.13+ re-export for the `typing.NoDefault`
-            // singleton, but not for `typing._NoDefaultType`
-            Self::NoDefaultType => CoreStdlibModule::TypingExtensions,
+            Self::NoDefaultType => {
+                let python_version = Program::get(db).target_version(db);
+
+                // typing_extensions has a 3.13+ re-export for the `typing.NoDefault`
+                // singleton, but not for `typing._NoDefaultType`. So we need to switch
+                // to `typing._NoDefaultType` for newer versions:
+                if python_version >= PythonVersion::PY313 {
+                    CoreStdlibModule::Typing
+                } else {
+                    CoreStdlibModule::TypingExtensions
+                }
+            }
         }
     }
 
@@ -1984,11 +1992,11 @@ impl<'db> KnownClass {
         };
 
         let module = file_to_module(db, file)?;
-        candidate.check_module(&module).then_some(candidate)
+        candidate.check_module(db, &module).then_some(candidate)
     }
 
     /// Return `true` if the module of `self` matches `module_name`
-    fn check_module(self, module: &Module) -> bool {
+    fn check_module(self, db: &'db dyn Db, module: &Module) -> bool {
         if !module.search_path().is_standard_library() {
             return false;
         }
@@ -2008,7 +2016,7 @@ impl<'db> KnownClass {
             | Self::GenericAlias
             | Self::ModuleType
             | Self::VersionInfo
-            | Self::FunctionType => module.name() == self.canonical_module().as_str(),
+            | Self::FunctionType => module.name() == self.canonical_module(db).as_str(),
             Self::NoneType => matches!(module.name().as_str(), "_typeshed" | "types"),
             Self::SpecialForm | Self::TypeVar | Self::TypeAliasType | Self::NoDefaultType => {
                 matches!(module.name().as_str(), "typing" | "typing_extensions")
@@ -3683,13 +3691,28 @@ pub(crate) mod tests {
     #[test_case(Ty::None)]
     #[test_case(Ty::BooleanLiteral(true))]
     #[test_case(Ty::BooleanLiteral(false))]
-    #[test_case(Ty::KnownClassInstance(KnownClass::NoDefaultType))]
     fn is_singleton(from: Ty) {
         let db = setup_db();
 
         assert!(from.into_type(&db).is_singleton(&db));
     }
 
+    /// Explicitly test for Python version <3.13 and >=3.13, to ensure that
+    /// the fallback to `typing_extensions` is working correctly.
+    /// See [`KnownClass::canonical_module`] for more information.
+    #[test_case(PythonVersion::PY312)]
+    #[test_case(PythonVersion::PY313)]
+    fn no_default_type_is_singleton(python_version: PythonVersion) {
+        let db = TestDbBuilder::new()
+            .with_python_version(python_version)
+            .build()
+            .unwrap();
+
+        let no_default = Ty::KnownClassInstance(KnownClass::NoDefaultType).into_type(&db);
+
+        assert!(no_default.is_singleton(&db));
+    }
+
     #[test_case(Ty::None)]
     #[test_case(Ty::BooleanLiteral(true))]
     #[test_case(Ty::IntLiteral(1))]