Avoid suggesting InitVar for __post_init__ that references PEP 695 type parameters (#23226)

Closes #19628.

Author: charliermarsh

crates/ruff_linter/resources/test/fixtures/ruff/RUF033.py

@@ -140,3 +140,12 @@ class D:
     def __post_init__(self, x: int = """
     """) -> None:
         self.x = x
+
+
+# https://github.com/astral-sh/ruff/issues/19628
+# No fix: annotation references a type variable scoped to `__post_init__`
+@dataclass
+class E:
+    def __post_init__[T: (str, bytes)](self, a: T | None = None, b: T | None = None) -> None:
+        self.a = str(a)
+        self.b = str(b)

crates/ruff_linter/src/rules/ruff/rules/post_init_default.rs

@@ -2,6 +2,7 @@ use anyhow::Context;
 
 use ruff_macros::{ViolationMetadata, derive_message_formats};
 use ruff_python_ast as ast;
+use ruff_python_ast::helpers::any_over_expr;
 use ruff_python_ast::token::parenthesized_range;
 use ruff_python_semantic::{Scope, ScopeKind};
 use ruff_python_trivia::{indentation_at_offset, textwrap};
@@ -146,6 +147,21 @@ fn use_initvar(
         ));
     }
 
+    // If the annotation references a type variable scoped to `__post_init__`
+    // (PEP 695), moving it to the class body would produce a `NameError`.
+    if let Some(annotation) = parameter.annotation() {
+        if let Some(type_params) = &post_init_def.type_params {
+            if any_over_expr(annotation, &|expr| {
+                expr.as_name_expr()
+                    .is_some_and(|name| type_params.iter().any(|tp| tp.name().id == name.id))
+            }) {
+                return Err(anyhow::anyhow!(
+                    "Annotation references a type variable scoped to `__post_init__`"
+                ));
+            }
+        }
+    }
+
     // Ensure that `dataclasses.InitVar` is accessible. For example,
     // + `from dataclasses import InitVar`
     let (import_edit, initvar_binding) = checker.importer().get_or_import_symbol(

crates/ruff_linter/src/rules/ruff/snapshots/ruff_linter__rules__ruff__tests__RUF033_RUF033.py.snap

@@ -489,4 +489,30 @@ help: Use `dataclasses.InitVar` instead
 141 +     """
 142 +     def __post_init__(self, x: int) -> None:
 143 |         self.x = x
+144 | 
+145 | 
 note: This is an unsafe fix and may change runtime behavior
+
+RUF033 `__post_init__` method with argument defaults
+   --> RUF033.py:149:60
+    |
+147 | @dataclass
+148 | class E:
+149 |     def __post_init__[T: (str, bytes)](self, a: T | None = None, b: T | None = None) -> None:
+    |                                                            ^^^^
+150 |         self.a = str(a)
+151 |         self.b = str(b)
+    |
+help: Use `dataclasses.InitVar` instead
+
+RUF033 `__post_init__` method with argument defaults
+   --> RUF033.py:149:80
+    |
+147 | @dataclass
+148 | class E:
+149 |     def __post_init__[T: (str, bytes)](self, a: T | None = None, b: T | None = None) -> None:
+    |                                                                                ^^^^
+150 |         self.a = str(a)
+151 |         self.b = str(b)
+    |
+help: Use `dataclasses.InitVar` instead