Fix F811 false positive with match (#4161)

crates/ruff/resources/test/fixtures/pyflakes/F811_22.py

@@ -0,0 +1,13 @@
+def redef(value):
+    match value:
+        case True:
+
+            def fun(x, y):
+                return x
+
+        case False:
+
+            def fun(x, y):
+                return y
+
+    return fun

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

@@ -96,6 +96,7 @@ mod tests {
     #[test_case(Rule::RedefinedWhileUnused, Path::new("F811_19.py"); "F811_19")]
     #[test_case(Rule::RedefinedWhileUnused, Path::new("F811_20.py"); "F811_20")]
     #[test_case(Rule::RedefinedWhileUnused, Path::new("F811_21.py"); "F811_21")]
+    #[test_case(Rule::RedefinedWhileUnused, Path::new("F811_22.py"); "F811_22")]
     #[test_case(Rule::UndefinedName, Path::new("F821_0.py"); "F821_0")]
     #[test_case(Rule::UndefinedName, Path::new("F821_1.py"); "F821_1")]
     #[test_case(Rule::UndefinedName, Path::new("F821_2.py"); "F821_2")]

crates/ruff/src/rules/pyflakes/snapshots/ruff__rules__pyflakes__tests__F811_F811_22.py.snap

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

crates/ruff_python_ast/src/branch_detection.rs

@@ -71,6 +71,10 @@ fn alternatives(stmt: RefEquality<Stmt>) -> Vec<Vec<RefEquality<Stmt>>> {
                 body.iter().map(RefEquality).collect()
             }))
             .collect(),
+        StmtKind::Match { cases, .. } => cases
+            .iter()
+            .map(|case| case.body.iter().map(RefEquality).collect())
+            .collect(),
         _ => vec![],
     }
 }