[pyupgrade] Fix false positive for TypeVar with default on Python <3.13 (UP046,UP047) (#21045)

## Summary

Type default for Type parameter was added in Python 3.13 (PEP 696).

`typing_extensions.TypeVar` backports the default argument to earlier
versions.

`UP046` & `UP047` were getting triggered when
`typing_extensions.TypeVar` with `default` argument was used on python
version < 3.13

It shouldn't be triggered for python version < 3.13

This commit fixes the bug by adding a python version check before
triggering them.

Fixes #20929.

## Test Plan

### Manual testing 1

As the issue author pointed out in
#20929 (comment),
ran the following on `main` branch:
> % cargo run -p ruff -- check ../efax/ --target-version py312
--no-cache

<details><summary>Output</summary>

```zsh
   Compiling ruff_linter v0.14.1 (/Users/prakhar/ruff/crates/ruff_linter)
   Compiling ruff v0.14.1 (/Users/prakhar/ruff/crates/ruff)
   Compiling ruff_graph v0.1.0 (/Users/prakhar/ruff/crates/ruff_graph)
   Compiling ruff_workspace v0.0.0 (/Users/prakhar/ruff/crates/ruff_workspace)
   Compiling ruff_server v0.2.2 (/Users/prakhar/ruff/crates/ruff_server)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 6.72s
     Running `target/debug/ruff check ../efax/ --target-version py312 --no-cache`
UP046 Generic class `ExpectationParametrization` uses `Generic` subclass instead of type parameters
  --> /Users/prakhar/efax/efax/_src/expectation_parametrization.py:17:48
   |
17 | class ExpectationParametrization(Distribution, Generic[NP]):
   |                                                ^^^^^^^^^^^
18 |     """The expectation parametrization of an exponential family distribution.
   |
help: Use type parameters

UP046 Generic class `ExpToNat` uses `Generic` subclass instead of type parameters
  --> /Users/prakhar/efax/efax/_src/mixins/exp_to_nat/exp_to_nat.py:27:68
   |
26 | @DataClass
27 | class ExpToNat(ExpectationParametrization[NP], SimpleDistribution, Generic[NP]):
   |                                                                    ^^^^^^^^^^^
28 |     """This mixin implements the conversion from expectation to natural parameters.
   |
help: Use type parameters

UP046 Generic class `HasEntropyEP` uses `Generic` subclass instead of type parameters
  --> /Users/prakhar/efax/efax/_src/mixins/has_entropy.py:25:20
   |
23 |                    HasEntropy,
24 |                    JaxAbstractClass,
25 |                    Generic[NP]):
   |                    ^^^^^^^^^^^
26 |     @abstract_jit
27 |     @AbstractMethod
   |
help: Use type parameters

UP046 Generic class `HasEntropyNP` uses `Generic` subclass instead of type parameters
  --> /Users/prakhar/efax/efax/_src/mixins/has_entropy.py:64:20
   |
62 | class HasEntropyNP(NaturalParametrization[EP],
63 |                    HasEntropy,
64 |                    Generic[EP]):
   |                    ^^^^^^^^^^^
65 |     @jit
66 |     @Final
   |
help: Use type parameters

UP046 Generic class `NaturalParametrization` uses `Generic` subclass instead of type parameters
  --> /Users/prakhar/efax/efax/_src/natural_parametrization.py:43:30
   |
41 | class NaturalParametrization(Distribution,
42 |                              JaxAbstractClass,
43 |                              Generic[EP, Domain]):
   |                              ^^^^^^^^^^^^^^^^^^^
44 |     """The natural parametrization of an exponential family distribution.
   |
help: Use type parameters

UP046 Generic class `Structure` uses `Generic` subclass instead of type parameters
  --> /Users/prakhar/efax/efax/_src/structure/structure.py:31:17
   |
30 | @DataClass
31 | class Structure(Generic[P]):
   |                 ^^^^^^^^^^
32 |     """This class generalizes the notion of type for Distribution objects.
   |
help: Use type parameters

UP046 Generic class `DistributionInfo` uses `Generic` subclass instead of type parameters
  --> /Users/prakhar/efax/tests/distribution_info.py:20:24
   |
20 | class DistributionInfo(Generic[NP, EP, Domain]):
   |                        ^^^^^^^^^^^^^^^^^^^^^^^
21 |     def __init__(self, dimensions: int = 1, safety: float = 0.0) -> None:
22 |         super().__init__()
   |
help: Use type parameters

Found 7 errors.
No fixes available (7 hidden fixes can be enabled with the `--unsafe-fixes` option).
```
</details> 

Running it after the changes:
```zsh
ruff % cargo run -p ruff -- check ../efax/ --target-version py312 --no-cache
   Compiling ruff_linter v0.14.1 (/Users/prakhar/ruff/crates/ruff_linter)
   Compiling ruff v0.14.1 (/Users/prakhar/ruff/crates/ruff)
   Compiling ruff_graph v0.1.0 (/Users/prakhar/ruff/crates/ruff_graph)
   Compiling ruff_workspace v0.0.0 (/Users/prakhar/ruff/crates/ruff_workspace)
   Compiling ruff_server v0.2.2 (/Users/prakhar/ruff/crates/ruff_server)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 7.86s
     Running `target/debug/ruff check ../efax/ --target-version py312 --no-cache`
All checks passed!
```

---

### Manual testing 2

Ran the check on the following script (mainly to verify `UP047`):
```py
from __future__ import annotations                                                                                                                                                    

from typing import Generic

from typing_extensions import TypeVar

T = TypeVar("T", default=int)


def generic_function(var: T) -> T:
    return var


Q = TypeVar("Q", default=str)


class GenericClass(Generic[Q]):
    var: Q
```

On `main` branch:
> ruff % cargo run -p ruff -- check ~/up046.py --target-version py312
--preview --no-cache

<details><summary>Output</summary>

```zsh
   Compiling ruff_linter v0.14.1 (/Users/prakhar/ruff/crates/ruff_linter)
   Compiling ruff v0.14.1 (/Users/prakhar/ruff/crates/ruff)
   Compiling ruff_graph v0.1.0 (/Users/prakhar/ruff/crates/ruff_graph)
   Compiling ruff_workspace v0.0.0 (/Users/prakhar/ruff/crates/ruff_workspace)
   Compiling ruff_server v0.2.2 (/Users/prakhar/ruff/crates/ruff_server)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 7.43s
     Running `target/debug/ruff check /Users/prakhar/up046.py --target-version py312 --preview --no-cache`
UP047 Generic function `generic_function` should use type parameters
  --> /Users/prakhar/up046.py:10:5
   |
10 | def generic_function(var: T) -> T:
   |     ^^^^^^^^^^^^^^^^^^^^^^^^
11 |     return var
   |
help: Use type parameters

UP046 Generic class `GenericClass` uses `Generic` subclass instead of type parameters
  --> /Users/prakhar/up046.py:17:20
   |
17 | class GenericClass(Generic[Q]):
   |                    ^^^^^^^^^^
18 |     var: Q
   |
help: Use type parameters

Found 2 errors.
No fixes available (2 hidden fixes can be enabled with the `--unsafe-fixes` option).
```

</details> 

After the fix (this branch):
```zsh
ruff % cargo run -p ruff -- check ~/up046.py --target-version py312 --preview --no-cache
   Compiling ruff_linter v0.14.1 (/Users/prakhar/ruff/crates/ruff_linter)
   Compiling ruff v0.14.1 (/Users/prakhar/ruff/crates/ruff)
   Compiling ruff_graph v0.1.0 (/Users/prakhar/ruff/crates/ruff_graph)
   Compiling ruff_workspace v0.0.0 (/Users/prakhar/ruff/crates/ruff_workspace)
   Compiling ruff_server v0.2.2 (/Users/prakhar/ruff/crates/ruff_server)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 7.40s
     Running `target/debug/ruff check /Users/prakhar/up046.py --target-version py312 --preview --no-cache`
All checks passed!
```

Signed-off-by: Prakhar Pratyush <[email protected]>

Author: prakhar1144

crates/ruff_linter/resources/test/fixtures/pyupgrade/UP046_2.py

@@ -0,0 +1,13 @@
+"""This is placed in a separate fixture as `TypeVar` needs to be imported
+from `typing_extensions` to support default arguments in Python version < 3.13.
+We verify that UP046 doesn't apply in this case.
+"""
+
+from typing import Generic
+from typing_extensions import TypeVar
+
+T = TypeVar("T", default=str)
+
+
+class DefaultTypeVar(Generic[T]):
+    var: T

crates/ruff_linter/resources/test/fixtures/pyupgrade/UP047.py renamed to crates/ruff_linter/resources/test/fixtures/pyupgrade/UP047_0.py

@@ -0,0 +1,12 @@
+"""This is placed in a separate fixture as `TypeVar` needs to be imported
+from `typing_extensions` to support default arguments in Python version < 3.13.
+We verify that UP047 doesn't apply in this case.
+"""
+
+from typing_extensions import TypeVar
+
+T = TypeVar("T", default=int)
+
+
+def default_var(var: T) -> T:
+    return var

crates/ruff_linter/resources/test/fixtures/pyupgrade/UP047_1.py

@@ -111,7 +111,7 @@ mod tests {
     #[test_case(Rule::NonPEP695TypeAlias, Path::new("UP040.pyi"))]
     #[test_case(Rule::NonPEP695GenericClass, Path::new("UP046_0.py"))]
     #[test_case(Rule::NonPEP695GenericClass, Path::new("UP046_1.py"))]
-    #[test_case(Rule::NonPEP695GenericFunction, Path::new("UP047.py"))]
+    #[test_case(Rule::NonPEP695GenericFunction, Path::new("UP047_0.py"))]
     #[test_case(Rule::PrivateTypeParameter, Path::new("UP049_0.py"))]
     #[test_case(Rule::PrivateTypeParameter, Path::new("UP049_1.py"))]
     #[test_case(Rule::UselessClassMetaclassType, Path::new("UP050.py"))]
@@ -125,6 +125,22 @@ mod tests {
         Ok(())
     }
 
+    #[test_case(Rule::NonPEP695GenericClass, Path::new("UP046_2.py"))]
+    #[test_case(Rule::NonPEP695GenericFunction, Path::new("UP047_1.py"))]
+    fn rules_not_applied_default_typevar_backported(rule_code: Rule, path: &Path) -> Result<()> {
+        let snapshot = path.to_string_lossy().to_string();
+        let diagnostics = test_path(
+            Path::new("pyupgrade").join(path).as_path(),
+            &settings::LinterSettings {
+                preview: PreviewMode::Enabled,
+                unresolved_target_version: PythonVersion::PY312.into(),
+                ..settings::LinterSettings::for_rule(rule_code)
+            },
+        )?;
+        assert_diagnostics!(snapshot, diagnostics);
+        Ok(())
+    }
+
     #[test_case(Rule::SuperCallWithParameters, Path::new("UP008.py"))]
     #[test_case(Rule::TypingTextStrAlias, Path::new("UP019.py"))]
     fn rules_preview(rule_code: Rule, path: &Path) -> Result<()> {
@@ -144,7 +160,7 @@ mod tests {
     #[test_case(Rule::NonPEP695TypeAlias, Path::new("UP040.pyi"))]
     #[test_case(Rule::NonPEP695GenericClass, Path::new("UP046_0.py"))]
     #[test_case(Rule::NonPEP695GenericClass, Path::new("UP046_1.py"))]
-    #[test_case(Rule::NonPEP695GenericFunction, Path::new("UP047.py"))]
+    #[test_case(Rule::NonPEP695GenericFunction, Path::new("UP047_0.py"))]
     fn type_var_default_preview(rule_code: Rule, path: &Path) -> Result<()> {
         let snapshot = format!("{}__preview_diff", path.to_string_lossy());
         assert_diagnostics_diff!(

crates/ruff_linter/src/rules/pyupgrade/mod.rs

@@ -6,8 +6,8 @@ use std::fmt::Display;
 
 use itertools::Itertools;
 use ruff_python_ast::{
-    self as ast, Arguments, Expr, ExprCall, ExprName, ExprSubscript, Identifier, Stmt, StmtAssign,
-    TypeParam, TypeParamParamSpec, TypeParamTypeVar, TypeParamTypeVarTuple,
+    self as ast, Arguments, Expr, ExprCall, ExprName, ExprSubscript, Identifier, PythonVersion,
+    Stmt, StmtAssign, TypeParam, TypeParamParamSpec, TypeParamTypeVar, TypeParamTypeVarTuple,
     name::Name,
     visitor::{self, Visitor},
 };
@@ -369,15 +369,19 @@ fn in_nested_context(checker: &Checker) -> bool {
 }
 
 /// Deduplicate `vars`, returning `None` if `vars` is empty or any duplicates are found.
-/// Also returns `None` if any `TypeVar` has a default value and preview mode is not enabled.
+/// Also returns `None` if any `TypeVar` has a default value and the target Python version
+/// is below 3.13 or preview mode is not enabled. Note that `typing_extensions` backports
+/// the default argument, but the rule should be skipped in that case.
 fn check_type_vars<'a>(vars: Vec<TypeVar<'a>>, checker: &Checker) -> Option<Vec<TypeVar<'a>>> {
     if vars.is_empty() {
         return None;
     }
 
-    // If any type variables have defaults and preview mode is not enabled, skip the rule
+    // If any type variables have defaults, skip the rule unless
+    // running with preview mode enabled and targeting Python 3.13+.
     if vars.iter().any(|tv| tv.default.is_some())
-        && !is_type_var_default_enabled(checker.settings())
+        && (checker.target_version() < PythonVersion::PY313
+            || !is_type_var_default_enabled(checker.settings()))
     {
         return None;
     }

crates/ruff_linter/src/rules/pyupgrade/rules/pep695/mod.rs

@@ -0,0 +1,4 @@
+---
+source: crates/ruff_linter/src/rules/pyupgrade/mod.rs
+---
+

crates/ruff_linter/src/rules/pyupgrade/snapshots/ruff_linter__rules__pyupgrade__tests__UP046_2.py.snap

@@ -2,7 +2,7 @@
 source: crates/ruff_linter/src/rules/pyupgrade/mod.rs
 ---
 UP047 [*] Generic function `f` should use type parameters
-  --> UP047.py:12:5
+  --> UP047_0.py:12:5
    |
 12 | def f(t: T) -> T:
    |     ^^^^^^^
@@ -20,7 +20,7 @@ help: Use type parameters
 note: This is an unsafe fix and may change runtime behavior
 
 UP047 [*] Generic function `g` should use type parameters
-  --> UP047.py:16:5
+  --> UP047_0.py:16:5
    |
 16 | def g(ts: tuple[*Ts]) -> tuple[*Ts]:
    |     ^^^^^^^^^^^^^^^^^
@@ -38,7 +38,7 @@ help: Use type parameters
 note: This is an unsafe fix and may change runtime behavior
 
 UP047 [*] Generic function `h` should use type parameters
-  --> UP047.py:20:5
+  --> UP047_0.py:20:5
    |
 20 |   def h(
    |  _____^
@@ -62,7 +62,7 @@ help: Use type parameters
 note: This is an unsafe fix and may change runtime behavior
 
 UP047 [*] Generic function `i` should use type parameters
-  --> UP047.py:29:5
+  --> UP047_0.py:29:5
    |
 29 | def i(s: S) -> S:
    |     ^^^^^^^
@@ -80,7 +80,7 @@ help: Use type parameters
 note: This is an unsafe fix and may change runtime behavior
 
 UP047 [*] Generic function `broken_fix` should use type parameters
-  --> UP047.py:39:5
+  --> UP047_0.py:39:5
    |
 37 | # TypeVars with the new-style generic syntax and will be rejected by type
 38 | # checkers
@@ -100,7 +100,7 @@ help: Use type parameters
 note: This is an unsafe fix and may change runtime behavior
 
 UP047 [*] Generic function `any_str_param` should use type parameters
-  --> UP047.py:43:5
+  --> UP047_0.py:43:5
    |
 43 | def any_str_param(s: AnyStr) -> AnyStr:
    |     ^^^^^^^^^^^^^^^^^^^^^^^^

crates/ruff_linter/src/rules/pyupgrade/snapshots/ruff_linter__rules__pyupgrade__tests__UP047.py.snap renamed to crates/ruff_linter/src/rules/pyupgrade/snapshots/ruff_linter__rules__pyupgrade__tests__UP047_0.py.snap

@@ -11,7 +11,7 @@ Added: 1
 
 --- Added ---
 UP047 [*] Generic function `default_var` should use type parameters
-  --> UP047.py:51:5
+  --> UP047_0.py:51:5
    |
 51 | def default_var(v: V) -> V:
    |     ^^^^^^^^^^^^^^^^^

crates/ruff_linter/src/rules/pyupgrade/snapshots/ruff_linter__rules__pyupgrade__tests__UP047.py__preview_diff.snap renamed to crates/ruff_linter/src/rules/pyupgrade/snapshots/ruff_linter__rules__pyupgrade__tests__UP047_0.py__preview_diff.snap

@@ -0,0 +1,4 @@
+---
+source: crates/ruff_linter/src/rules/pyupgrade/mod.rs
+---
+