From 3f3d0c57ac66b4fa42a3f10209dd1fde29c5ce57 Mon Sep 17 00:00:00 2001
From: David Tolnay <dtolnay@gmail.com>
Date: Fri, 27 Sep 2024 10:29:10 -0700
Subject: [PATCH 1/2] Add regression test for issue 1738

---
 tests/test_expr.rs | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/tests/test_expr.rs b/tests/test_expr.rs
index d20cce8d6a..f79e4fafda 100644
--- a/tests/test_expr.rs
+++ b/tests/test_expr.rs
@@ -642,6 +642,12 @@ fn test_assign_range_precedence() {
     syn::parse_str::<Expr>("() .. () += ()").unwrap_err();
 }
 
+#[test]
+fn test_chained_comparison() {
+    // https://github.com/dtolnay/syn/issues/1738
+    let _ = syn::parse_str::<Expr>("a = a < a <");
+}
+
 #[test]
 fn test_fixup() {
     struct FlattenParens;

From b3d2886fc9bbff5eb45995c72beec0463a8cec2a Mon Sep 17 00:00:00 2001
From: David Tolnay <dtolnay@gmail.com>
Date: Fri, 27 Sep 2024 10:29:31 -0700
Subject: [PATCH 2/2] Fix infinite loop on chained comparison

---
 src/expr.rs        | 4 ++--
 tests/test_expr.rs | 3 +++
 2 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/src/expr.rs b/src/expr.rs
index db8d5b9211..8216191c2f 100644
--- a/src/expr.rs
+++ b/src/expr.rs
@@ -1284,7 +1284,7 @@ pub(crate) mod parsing {
                 if precedence == Precedence::Compare {
                     if let Expr::Binary(lhs) = &lhs {
                         if Precedence::of_binop(&lhs.op) == Precedence::Compare {
-                            break;
+                            return Err(input.error("comparison operators cannot be chained"));
                         }
                     }
                 }
@@ -1346,7 +1346,7 @@ pub(crate) mod parsing {
                 if precedence == Precedence::Compare {
                     if let Expr::Binary(lhs) = &lhs {
                         if Precedence::of_binop(&lhs.op) == Precedence::Compare {
-                            break;
+                            return Err(input.error("comparison operators cannot be chained"));
                         }
                     }
                 }
diff --git a/tests/test_expr.rs b/tests/test_expr.rs
index f79e4fafda..399972c424 100644
--- a/tests/test_expr.rs
+++ b/tests/test_expr.rs
@@ -646,6 +646,9 @@ fn test_assign_range_precedence() {
 fn test_chained_comparison() {
     // https://github.com/dtolnay/syn/issues/1738
     let _ = syn::parse_str::<Expr>("a = a < a <");
+
+    let err = syn::parse_str::<Expr>("a < a < a").unwrap_err();
+    assert_eq!("comparison operators cannot be chained", err.to_string());
 }
 
 #[test]