use Never type instead of Unknown for unbound public types

Uses `Type::Never` instead of `Type::Unknown` for the case where
a publicly available variable is unbound. In the case where it is
unbound, we want a union of its actual type with `Never` instead of
`Unbound`, because the `Unbound` case will cause runtime error.

crates/red_knot_python_semantic/src/types.rs

@@ -58,9 +58,7 @@ fn symbol_ty_by_id<'db>(db: &'db dyn Db, scope: ScopeId<'db>, symbol: ScopedSymb
             Some(bindings_ty(
                 db,
                 use_def.public_bindings(symbol),
-                use_def
-                    .public_may_be_unbound(symbol)
-                    .then_some(Type::Unknown),
+                use_def.public_may_be_unbound(symbol).then_some(Type::Never),
             ))
         } else {
             None
@@ -72,9 +70,7 @@ fn symbol_ty_by_id<'db>(db: &'db dyn Db, scope: ScopeId<'db>, symbol: ScopedSymb
         bindings_ty(
             db,
             use_def.public_bindings(symbol),
-            use_def
-                .public_may_be_unbound(symbol)
-                .then_some(Type::Unbound),
+            use_def.public_may_be_unbound(symbol).then_some(Type::Never),
         )
     }
 }

crates/red_knot_python_semantic/src/types/infer.rs

@@ -3620,7 +3620,7 @@ mod tests {
         )?;
 
         // TODO: sys.version_info, and need to understand @final and @type_check_only
-        assert_public_ty(&db, "src/a.py", "x", "Unknown | EllipsisType");
+        assert_public_ty(&db, "src/a.py", "x", "EllipsisType | Unknown");
 
         Ok(())
     }
@@ -3952,6 +3952,40 @@ mod tests {
         Ok(())
     }
 
+    #[test]
+    fn resolve_unbound_public() -> anyhow::Result<()> {
+        let mut db = setup_db();
+
+        db.write_dedented(
+            "src/a.py",
+            "
+            if flag:
+                x: int = 1
+            ",
+        )?;
+
+        assert_public_ty(&db, "src/a.py", "x", "int");
+
+        Ok(())
+    }
+
+    #[test]
+    fn resolve_unannotated_unbound_public() -> anyhow::Result<()> {
+        let mut db = setup_db();
+
+        db.write_dedented(
+            "src/a.py",
+            "
+            if flag:
+                x = 1
+            ",
+        )?;
+
+        assert_public_ty(&db, "src/a.py", "x", "Literal[1]");
+
+        Ok(())
+    }
+
     #[test]
     fn literal_int_arithmetic() -> anyhow::Result<()> {
         let mut db = setup_db();
@@ -5452,7 +5486,7 @@ mod tests {
         // TODO: once we support `sys.version_info` branches,
         // we can set `--target-version=py311` in this test
         // and the inferred type will just be `BaseExceptionGroup` --Alex
-        assert_public_ty(&db, "src/a.py", "e", "Unknown | BaseExceptionGroup");
+        assert_public_ty(&db, "src/a.py", "e", "BaseExceptionGroup");
 
         Ok(())
     }
@@ -5478,7 +5512,7 @@ mod tests {
         // and the inferred type will just be `BaseExceptionGroup` --Alex
         //
         // TODO more precise would be `ExceptionGroup[OSError]` --Alex
-        assert_public_ty(&db, "src/a.py", "e", "Unknown | BaseExceptionGroup");
+        assert_public_ty(&db, "src/a.py", "e", "BaseExceptionGroup");
 
         Ok(())
     }
@@ -5504,7 +5538,7 @@ mod tests {
         // and the inferred type will just be `BaseExceptionGroup` --Alex
         //
         // TODO more precise would be `ExceptionGroup[TypeError | AttributeError]` --Alex
-        assert_public_ty(&db, "src/a.py", "e", "Unknown | BaseExceptionGroup");
+        assert_public_ty(&db, "src/a.py", "e", "BaseExceptionGroup");
 
         Ok(())
     }