[pyflakes] Visit forward annotations in TypeAliasType as types (F401) (#15829)

## Summary

Fixes https://github.com/astral-sh/ruff/issues/15812 by visiting the
second argument as a type definition.

## Test Plan

New F401 tests based on the report.

---------

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

crates/ruff_linter/resources/test/fixtures/pyflakes/F401_34.py

@@ -0,0 +1,67 @@
+"""Regression tests for https://github.com/astral-sh/ruff/issues/15812"""
+
+
+def f():
+    from typing import Union
+
+    from typing_extensions import TypeAliasType
+
+    Json = TypeAliasType(
+        "Json",
+        "Union[dict[str, Json], list[Json], str, int, float, bool, None]",
+    )
+
+
+def f():
+    from typing import Union
+
+    from typing_extensions import TypeAliasType, TypeVar
+
+    T = TypeVar("T")
+    V = TypeVar("V")
+    Json = TypeAliasType(
+        "Json",
+        "Union[dict[str, Json], list[Json], str, int, float, bool, T, V, None]",
+        type_params=(T, V),
+    )
+
+
+def f():
+    from typing import Union
+
+    from typing_extensions import TypeAliasType
+
+    Json = TypeAliasType(
+        value="Union[dict[str, Json], list[Json], str, int, float, bool, None]",
+        name="Json",
+    )
+
+
+# strictly speaking it's a false positive to emit F401 for both of these, but
+# we can't really be expected to understand that the strings here are type
+# expressions (and type checkers probably wouldn't understand them as type
+# expressions either!)
+def f():
+    from typing import Union
+
+    from typing_extensions import TypeAliasType
+
+    args = [
+        "Json",
+        "Union[dict[str, Json], list[Json], str, int, float, bool, None]",
+    ]
+
+    Json = TypeAliasType(*args)
+
+
+def f():
+    from typing import Union
+
+    from typing_extensions import TypeAliasType
+
+    kwargs = {
+        "name": "Json",
+        "value": "Union[dict[str, Json], list[Json], str, int, float, bool, None]",
+    }
+
+    Json = TypeAliasType(**kwargs)

crates/ruff_linter/src/checkers/ast/mod.rs

@@ -41,9 +41,9 @@ use ruff_python_ast::name::QualifiedName;
 use ruff_python_ast::str::Quote;
 use ruff_python_ast::visitor::{walk_except_handler, walk_pattern, Visitor};
 use ruff_python_ast::{
-    self as ast, AnyParameterRef, Comprehension, ElifElseClause, ExceptHandler, Expr, ExprContext,
-    FStringElement, Keyword, MatchCase, ModModule, Parameter, Parameters, Pattern, Stmt, Suite,
-    UnaryOp,
+    self as ast, AnyParameterRef, ArgOrKeyword, Comprehension, ElifElseClause, ExceptHandler, Expr,
+    ExprContext, FStringElement, Keyword, MatchCase, ModModule, Parameter, Parameters, Pattern,
+    Stmt, Suite, UnaryOp,
 };
 use ruff_python_ast::{helpers, str, visitor, PySourceType};
 use ruff_python_codegen::{Generator, Stylist};
@@ -1269,6 +1269,11 @@ impl<'a> Visitor<'a> for Checker<'a> {
                                 .match_typing_qualified_name(&qualified_name, "TypeVar")
                             {
                                 Some(typing::Callable::TypeVar)
+                            } else if self
+                                .semantic
+                                .match_typing_qualified_name(&qualified_name, "TypeAliasType")
+                            {
+                                Some(typing::Callable::TypeAliasType)
                             } else if self
                                 .semantic
                                 .match_typing_qualified_name(&qualified_name, "NamedTuple")
@@ -1354,6 +1359,24 @@ impl<'a> Visitor<'a> for Checker<'a> {
                             }
                         }
                     }
+                    Some(typing::Callable::TypeAliasType) => {
+                        // Ex) TypeAliasType("Json", "Union[dict[str, Json]]", type_params=())
+                        for (i, arg) in arguments.arguments_source_order().enumerate() {
+                            match (i, arg) {
+                                (1, ArgOrKeyword::Arg(arg)) => self.visit_type_definition(arg),
+                                (_, ArgOrKeyword::Arg(arg)) => self.visit_non_type_definition(arg),
+                                (_, ArgOrKeyword::Keyword(Keyword { arg, value, .. })) => {
+                                    if let Some(id) = arg {
+                                        if matches!(&**id, "value" | "type_params") {
+                                            self.visit_type_definition(value);
+                                        } else {
+                                            self.visit_non_type_definition(value);
+                                        }
+                                    }
+                                }
+                            }
+                        }
+                    }
                     Some(typing::Callable::NamedTuple) => {
                         // Ex) NamedTuple("a", [("a", int)])
                         let mut args = arguments.args.iter();

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

@@ -56,6 +56,7 @@ mod tests {
     #[test_case(Rule::UnusedImport, Path::new("F401_22.py"))]
     #[test_case(Rule::UnusedImport, Path::new("F401_23.py"))]
     #[test_case(Rule::UnusedImport, Path::new("F401_32.py"))]
+    #[test_case(Rule::UnusedImport, Path::new("F401_34.py"))]
     #[test_case(Rule::ImportShadowedByLoopVar, Path::new("F402.py"))]
     #[test_case(Rule::ImportShadowedByLoopVar, Path::new("F402.ipynb"))]
     #[test_case(Rule::UndefinedLocalWithImportStar, Path::new("F403.py"))]

crates/ruff_linter/src/rules/pyflakes/snapshots/ruff_linter__rules__pyflakes__tests__F401_F401_34.py.snap

@@ -0,0 +1,41 @@
+---
+source: crates/ruff_linter/src/rules/pyflakes/mod.rs
+---
+F401_34.py:45:24: F401 [*] `typing.Union` imported but unused
+   |
+43 | # expressions either!)
+44 | def f():
+45 |     from typing import Union
+   |                        ^^^^^ F401
+46 |
+47 |     from typing_extensions import TypeAliasType
+   |
+   = help: Remove unused import: `typing.Union`
+
+ℹ Safe fix
+42 42 | # expressions (and type checkers probably wouldn't understand them as type
+43 43 | # expressions either!)
+44 44 | def f():
+45    |-    from typing import Union
+46 45 | 
+47 46 |     from typing_extensions import TypeAliasType
+48 47 | 
+
+F401_34.py:58:24: F401 [*] `typing.Union` imported but unused
+   |
+57 | def f():
+58 |     from typing import Union
+   |                        ^^^^^ F401
+59 |
+60 |     from typing_extensions import TypeAliasType
+   |
+   = help: Remove unused import: `typing.Union`
+
+ℹ Safe fix
+55 55 | 
+56 56 | 
+57 57 | def f():
+58    |-    from typing import Union
+59 58 | 
+60 59 |     from typing_extensions import TypeAliasType
+61 60 |

crates/ruff_python_semantic/src/analyze/typing.rs

@@ -28,6 +28,7 @@ pub enum Callable {
     NamedTuple,
     TypedDict,
     MypyExtension,
+    TypeAliasType,
 }
 
 #[derive(Debug, Copy, Clone)]