Fix undefined-loop-variable with NoReturn and Never (#7476)

Co-authored-by: detachhead <detachhead@users.noreply.github.com>
Co-authored-by: Jacob Walls <jacobtylerwalls@gmail.com>

doc/whatsnew/fragments/7311.false_positive

@@ -0,0 +1,4 @@
+Fix false positive for ``undefined-loop-variable`` in ``for-else`` loops that use a function
+having a return type annotation of ``NoReturn`` or ``Never``.
+
+Closes #7311

pylint/checkers/variables.py

@@ -19,15 +19,20 @@
 from typing import TYPE_CHECKING, Any, NamedTuple
 
 import astroid
-from astroid import extract_node, nodes
+from astroid import bases, extract_node, nodes
 from astroid.typing import InferenceResult
 
 from pylint.checkers import BaseChecker, utils
 from pylint.checkers.utils import (
     in_type_checking_block,
     is_postponed_evaluation_enabled,
 )
-from pylint.constants import PY39_PLUS, TYPING_TYPE_CHECKS_GUARDS
+from pylint.constants import (
+    PY39_PLUS,
+    TYPING_NEVER,
+    TYPING_NORETURN,
+    TYPING_TYPE_CHECKS_GUARDS,
+)
 from pylint.interfaces import CONTROL_FLOW, HIGH, INFERENCE, INFERENCE_FAILURE
 from pylint.typing import MessageDefinitionTuple
 
@@ -2245,13 +2250,35 @@ def _loopvar_name(self, node: astroid.Name) -> None:
         if not isinstance(assign, nodes.For):
             self.add_message("undefined-loop-variable", args=node.name, node=node)
             return
-        if any(
-            isinstance(
+        for else_stmt in assign.orelse:
+            if isinstance(
                 else_stmt, (nodes.Return, nodes.Raise, nodes.Break, nodes.Continue)
-            )
-            for else_stmt in assign.orelse
-        ):
-            return
+            ):
+                return
+            # TODO: 2.16: Consider using RefactoringChecker._is_function_def_never_returning
+            if isinstance(else_stmt, nodes.Expr) and isinstance(
+                else_stmt.value, nodes.Call
+            ):
+                inferred_func = utils.safe_infer(else_stmt.value.func)
+                if (
+                    isinstance(inferred_func, nodes.FunctionDef)
+                    and inferred_func.returns
+                ):
+                    inferred_return = utils.safe_infer(inferred_func.returns)
+                    if isinstance(
+                        inferred_return, nodes.FunctionDef
+                    ) and inferred_return.qname() in {
+                        *TYPING_NORETURN,
+                        *TYPING_NEVER,
+                        "typing._SpecialForm",
+                    }:
+                        return
+                    # typing_extensions.NoReturn returns a _SpecialForm
+                    if (
+                        isinstance(inferred_return, bases.Instance)
+                        and inferred_return.qname() == "typing._SpecialForm"
+                    ):
+                        return
 
         maybe_walrus = utils.get_node_first_ancestor_of_type(node, nodes.NamedExpr)
         if maybe_walrus:

pylint/constants.py

@@ -155,3 +155,16 @@ def _get_pylint_home() -> str:
 
 
 PYLINT_HOME = _get_pylint_home()
+
+TYPING_NORETURN = frozenset(
+    (
+        "typing.NoReturn",
+        "typing_extensions.NoReturn",
+    )
+)
+TYPING_NEVER = frozenset(
+    (
+        "typing.Never",
+        "typing_extensions.Never",
+    )
+)

pylint/extensions/typing.py

@@ -17,6 +17,7 @@
     only_required_for_messages,
     safe_infer,
 )
+from pylint.constants import TYPING_NORETURN
 from pylint.interfaces import INFERENCE
 
 if TYPE_CHECKING:
@@ -75,12 +76,6 @@ class TypingAlias(NamedTuple):
 
 ALIAS_NAMES = frozenset(key.split(".")[1] for key in DEPRECATED_TYPING_ALIASES)
 UNION_NAMES = ("Optional", "Union")
-TYPING_NORETURN = frozenset(
-    (
-        "typing.NoReturn",
-        "typing_extensions.NoReturn",
-    )
-)
 
 
 class DeprecatedTypingAliasMsg(NamedTuple):

tests/functional/u/undefined/undefined_loop_variable.py

@@ -1,5 +1,13 @@
 # pylint: disable=missing-docstring,redefined-builtin, consider-using-f-string, unnecessary-direct-lambda-call
 
+import sys
+
+if sys.version_info >= (3, 8):
+    from typing import NoReturn
+else:
+    from typing_extensions import NoReturn
+
+
 def do_stuff(some_random_list):
     for var in some_random_list:
         pass
@@ -125,6 +133,18 @@ def for_else_continue(iterable):
         print(thing)
 
 
+def for_else_no_return(iterable):
+    def fail() -> NoReturn:
+        ...
+
+    while True:
+        for thing in iterable:
+            break
+        else:
+            fail()
+        print(thing)
+
+
 lst = []
 lst2 = [1, 2, 3]
 

tests/functional/u/undefined/undefined_loop_variable.txt

@@ -1,4 +1,4 @@
-undefined-loop-variable:6:11:6:14:do_stuff:Using possibly undefined loop variable 'var':UNDEFINED
-undefined-loop-variable:25:7:25:11::Using possibly undefined loop variable 'var1':UNDEFINED
-undefined-loop-variable:75:11:75:14:do_stuff_with_redefined_range:Using possibly undefined loop variable 'var':UNDEFINED
-undefined-loop-variable:181:11:181:20:find_even_number:Using possibly undefined loop variable 'something':UNDEFINED
+undefined-loop-variable:14:11:14:14:do_stuff:Using possibly undefined loop variable 'var':UNDEFINED
+undefined-loop-variable:33:7:33:11::Using possibly undefined loop variable 'var1':UNDEFINED
+undefined-loop-variable:83:11:83:14:do_stuff_with_redefined_range:Using possibly undefined loop variable 'var':UNDEFINED
+undefined-loop-variable:201:11:201:20:find_even_number:Using possibly undefined loop variable 'something':UNDEFINED

tests/functional/u/undefined/undefined_loop_variable_py311.py

@@ -0,0 +1,17 @@
+"""Tests for undefined-loop-variable using Python 3.11 syntax."""
+
+from typing import Never
+
+
+def for_else_never(iterable):
+    """Test for-else with Never type."""
+
+    def idontreturn() -> Never:
+        """This function never returns."""
+
+    while True:
+        for thing in iterable:
+            break
+        else:
+            idontreturn()
+        print(thing)

tests/functional/u/undefined/undefined_loop_variable_py311.rc

@@ -0,0 +1,2 @@
+[testoptions]
+min_pyver=3.11