[pyupgrade] Skip fix when a defaulted TypeVar precedes a non-defaulted one (UP040, UP046, UP047)

crates/ruff_linter/resources/test/fixtures/pyupgrade/UP040.py

@@ -132,3 +132,23 @@ class Foo:
 # Test case for TypeVar with default - should be converted when preview mode is enabled
 T_default = TypeVar("T_default", default=int)
 DefaultList: TypeAlias = list[T_default]
+
+# https://github.com/astral-sh/ruff/issues/27021
+# No fix if a defaulted TypeVar precedes a non-defaulted TypeVar: a non-default
+# type parameter can't follow a default type parameter in a PEP 695 type
+# parameter list, and reordering would change the meaning of subscriptions.
+T_with_default = TypeVar("T_with_default", default=int)
+S_no_default = TypeVar("S_no_default")
+BadPair: TypeAlias = tuple[T_with_default, S_no_default]
+BadPairAliasType = TypeAliasType(
+    "BadPairAliasType",
+    tuple[T_with_default, S_no_default],
+    type_params=(T_with_default, S_no_default),
+)
+
+# OK (in preview): the non-defaulted TypeVar comes first
+GoodPair: TypeAlias = tuple[S_no_default, T_with_default]
+
+# OK (in preview): all TypeVars have defaults
+S_with_default = TypeVar("S_with_default", default=str)
+AllDefaults: TypeAlias = tuple[T_with_default, S_with_default]

crates/ruff_linter/resources/test/fixtures/pyupgrade/UP046_0.py

@@ -142,3 +142,22 @@ class DefaultOnlyTypeVar(Generic[W]):  # -> [W = int]
 class Outer:
     class Inner(Generic[T]):
         var: T
+
+
+# https://github.com/astral-sh/ruff/issues/27021
+# No diagnostic if a defaulted TypeVar precedes a non-defaulted TypeVar: a
+# non-default type parameter can't follow a default type parameter in a PEP 695
+# type parameter list, and reordering would change the meaning of subscriptions.
+X = TypeVar("X", default=int)
+Y = TypeVar("Y")
+
+
+class DefaultBeforeNonDefault(Generic[X, Y]):
+    x: X
+    y: Y
+
+
+# OK (in preview): the non-defaulted TypeVar comes first
+class NonDefaultBeforeDefault(Generic[Y, X]):
+    x: X
+    y: Y

crates/ruff_linter/resources/test/fixtures/pyupgrade/UP047_0.py

@@ -58,6 +58,23 @@ def multi_param(t: list[T], c: Callable[[T], None]) -> T:
     return t[1]
 
 
+# https://github.com/astral-sh/ruff/issues/27021
+# No diagnostic if a defaulted TypeVar precedes a non-defaulted TypeVar: a
+# non-default type parameter can't follow a default type parameter in a PEP 695
+# type parameter list, and reordering would change the meaning of subscriptions.
+X = TypeVar("X", default=int)
+Y = TypeVar("Y")
+
+
+def default_before_non_default(x: X, y: Y) -> tuple[X, Y]:
+    return (x, y)
+
+
+# OK (in preview): the non-defaulted TypeVar comes first
+def non_default_before_default(y: Y, x: X) -> tuple[Y, X]:
+    return (y, x)
+
+
 # these cases are not handled
 
 def outer():

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

@@ -367,6 +367,24 @@ fn in_nested_context(checker: &Checker) -> bool {
         .any(|stmt| matches!(stmt, Stmt::ClassDef(_) | Stmt::FunctionDef(_)))
 }
 
+/// Returns `true` if a type variable without a default follows a type variable with a default.
+///
+/// In a PEP 695 type parameter list this is a syntax error:
+///
+/// ```python
+/// type Pair[T = int, S] = tuple[T, S]  # non-default type parameter `S` follows default type parameter
+/// ```
+///
+/// Reordering the parameters to make the list valid would not be an equivalent fix because the
+/// parameter order determines how positional arguments are bound when subscripting the alias,
+/// class, or function.
+fn non_default_follows_default(type_vars: &[TypeVar]) -> bool {
+    type_vars
+        .iter()
+        .skip_while(|tv| tv.default.is_none())
+        .any(|tv| tv.default.is_none())
+}
+
 /// Deduplicate `vars`, returning `None` if `vars` is empty or any duplicates are found.
 /// 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
@@ -385,6 +403,13 @@ fn check_type_vars<'a>(vars: Vec<TypeVar<'a>>, checker: &Checker) -> Option<Vec<
         return None;
     }
 
+    // If a type variable without a default follows one with a default, there is no valid,
+    // equivalent PEP 695 type parameter list, so bail out instead of emitting an invalid fix.
+    // See https://github.com/astral-sh/ruff/issues/27021.
+    if non_default_follows_default(&vars) {
+        return None;
+    }
+
     // If any type variables were not unique, just bail out here. this is a runtime error and we
     // can't predict what the user wanted.
     (vars.iter().unique_by(|tvar| tvar.name).count() == vars.len()).then_some(vars)

crates/ruff_linter/src/rules/pyupgrade/rules/pep695/non_pep695_type_alias.rs

@@ -14,6 +14,7 @@ use ruff_python_ast::PythonVersion;
 
 use super::{
     DisplayTypeVars, TypeParamKind, TypeVar, TypeVarReferenceVisitor, expr_name_to_type_var,
+    non_default_follows_default,
 };
 
 /// ## What it does
@@ -266,6 +267,13 @@ fn create_diagnostic(
         return;
     }
 
+    // If a type variable without a default follows one with a default, there is no valid,
+    // equivalent PEP 695 type parameter list, so bail out instead of emitting an invalid fix.
+    // See https://github.com/astral-sh/ruff/issues/27021.
+    if non_default_follows_default(type_vars) {
+        return;
+    }
+
     let source = checker.source();
     let tokens = checker.tokens();
     let comment_ranges = checker.comment_ranges();

crates/ruff_linter/src/rules/pyupgrade/snapshots/ruff_linter__rules__pyupgrade__tests__UP040.py__preview_diff.snap

@@ -7,7 +7,7 @@ source: crates/ruff_linter/src/rules/pyupgrade/mod.rs
 
 --- Summary ---
 Removed: 0
-Added: 3
+Added: 5
 
 --- Added ---
 UP040 [*] Type alias `x` uses `TypeAlias` annotation instead of the `type` keyword
@@ -56,11 +56,50 @@ UP040 [*] Type alias `DefaultList` uses `TypeAlias` annotation instead of the `t
 133 | T_default = TypeVar("T_default", default=int)
 134 | DefaultList: TypeAlias = list[T_default]
     | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+135 |
+136 | # https://github.com/astral-sh/ruff/issues/27021
     |
 help: Use the `type` keyword
     |
 133 | T_default = TypeVar("T_default", default=int)
     - DefaultList: TypeAlias = list[T_default]
 134 + type DefaultList[T_default = int] = list[T_default]
+135 |
+    |
+note: This is an unsafe fix and may change runtime behavior
+
+
+UP040 [*] Type alias `GoodPair` uses `TypeAlias` annotation instead of the `type` keyword
+   --> UP040.py:150:1
+    |
+149 | # OK (in preview): the non-defaulted TypeVar comes first
+150 | GoodPair: TypeAlias = tuple[S_no_default, T_with_default]
+    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+151 |
+152 | # OK (in preview): all TypeVars have defaults
+    |
+help: Use the `type` keyword
+    |
+149 | # OK (in preview): the non-defaulted TypeVar comes first
+    - GoodPair: TypeAlias = tuple[S_no_default, T_with_default]
+150 + type GoodPair[S_no_default, T_with_default = int] = tuple[S_no_default, T_with_default]
+151 |
+    |
+note: This is an unsafe fix and may change runtime behavior
+
+
+UP040 [*] Type alias `AllDefaults` uses `TypeAlias` annotation instead of the `type` keyword
+   --> UP040.py:154:1
+    |
+152 | # OK (in preview): all TypeVars have defaults
+153 | S_with_default = TypeVar("S_with_default", default=str)
+154 | AllDefaults: TypeAlias = tuple[T_with_default, S_with_default]
+    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+    |
+help: Use the `type` keyword
+    |
+153 | S_with_default = TypeVar("S_with_default", default=str)
+    - AllDefaults: TypeAlias = tuple[T_with_default, S_with_default]
+154 + type AllDefaults[T_with_default = int, S_with_default = str] = tuple[T_with_default, S_with_default]
     |
 note: This is an unsafe fix and may change runtime behavior

crates/ruff_linter/src/rules/pyupgrade/snapshots/ruff_linter__rules__pyupgrade__tests__UP046_0.py__preview_diff.snap

@@ -7,7 +7,7 @@ source: crates/ruff_linter/src/rules/pyupgrade/mod.rs
 
 --- Summary ---
 Removed: 0
-Added: 2
+Added: 3
 
 --- Added ---
 UP046 [*] Generic class `DefaultTypeVar` uses `Generic` subclass instead of type parameters
@@ -42,3 +42,22 @@ help: Use type parameters
 138 |     var: W
     |
 note: This is an unsafe fix and may change runtime behavior
+
+
+UP046 [*] Generic class `NonDefaultBeforeDefault` uses `Generic` subclass instead of type parameters
+   --> UP046_0.py:161:31
+    |
+160 | # OK (in preview): the non-defaulted TypeVar comes first
+161 | class NonDefaultBeforeDefault(Generic[Y, X]):
+    |                               ^^^^^^^^^^^^^
+162 |     x: X
+163 |     y: Y
+    |
+help: Use type parameters
+    |
+160 | # OK (in preview): the non-defaulted TypeVar comes first
+    - class NonDefaultBeforeDefault(Generic[Y, X]):
+161 + class NonDefaultBeforeDefault[Y, X = int]:
+162 |     x: X
+    |
+note: This is an unsafe fix and may change runtime behavior

crates/ruff_linter/src/rules/pyupgrade/snapshots/ruff_linter__rules__pyupgrade__tests__UP047_0.py__preview_diff.snap

@@ -7,7 +7,7 @@ source: crates/ruff_linter/src/rules/pyupgrade/mod.rs
 
 --- Summary ---
 Removed: 0
-Added: 1
+Added: 2
 
 --- Added ---
 UP047 [*] Generic function `default_var` should use type parameters
@@ -25,3 +25,21 @@ help: Use type parameters
 52 |     return v
    |
 note: This is an unsafe fix and may change runtime behavior
+
+
+UP047 [*] Generic function `non_default_before_default` should use type parameters
+  --> UP047_0.py:74:5
+   |
+73 | # OK (in preview): the non-defaulted TypeVar comes first
+74 | def non_default_before_default(y: Y, x: X) -> tuple[Y, X]:
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+75 |     return (y, x)
+   |
+help: Use type parameters
+   |
+73 | # OK (in preview): the non-defaulted TypeVar comes first
+   - def non_default_before_default(y: Y, x: X) -> tuple[Y, X]:
+74 + def non_default_before_default[Y, X = int](y: Y, x: X) -> tuple[Y, X]:
+75 |     return (y, x)
+   |
+note: This is an unsafe fix and may change runtime behavior