From 51c4299433f4ca3e018edcf4665282cfc5537c60 Mon Sep 17 00:00:00 2001
From: Vastargazing <vebohr@gmail.com>
Date: Fri, 3 Apr 2026 18:37:13 +0300
Subject: [PATCH] coretests: add argument order regression tests for
 min_by/max_by/minmax_by
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A recent regression swapped the argument order passed to the compare
closure in min_by, max_by and minmax_by (compare(&v2, &v1) instead of
compare(&v1, &v2)). This was fixed, but no regression test was added.

Add tests that record the arguments the compare closure receives and
assert they match (v1, v2) — the documented contract.
---
 library/coretests/tests/cmp.rs | 31 +++++++++++++++++++++++++++++++
 library/coretests/tests/lib.rs |  1 +
 2 files changed, 32 insertions(+)

diff --git a/library/coretests/tests/cmp.rs b/library/coretests/tests/cmp.rs
index 0a14470060c3d..888a3cfd57a85 100644
--- a/library/coretests/tests/cmp.rs
+++ b/library/coretests/tests/cmp.rs
@@ -48,6 +48,37 @@ fn test_ord_min_max_by() {
     assert_eq!(cmp::max_by(2, -1, f), 2);
 }
 
+// Regression test for #136307 / #139357: ensure compare() receives (v1, v2), not (v2, v1).
+#[test]
+fn min_by_compare_argument_order() {
+    let mut order = vec![];
+    let _ = cmp::min_by(1i32, 2, |a, b| {
+        order.push((*a, *b));
+        a.cmp(b)
+    });
+    assert_eq!(order, [(1, 2)]);
+}
+
+#[test]
+fn max_by_compare_argument_order() {
+    let mut order = vec![];
+    let _ = cmp::max_by(1i32, 2, |a, b| {
+        order.push((*a, *b));
+        a.cmp(b)
+    });
+    assert_eq!(order, [(1, 2)]);
+}
+
+#[test]
+fn minmax_by_compare_argument_order() {
+    let mut order = vec![];
+    let _ = cmp::minmax_by(1i32, 2, |a, b| {
+        order.push((*a, *b));
+        a.cmp(b)
+    });
+    assert_eq!(order, [(1, 2)]);
+}
+
 #[test]
 fn test_ord_min_max_by_key() {
     let f = |x: &i32| x.abs();
diff --git a/library/coretests/tests/lib.rs b/library/coretests/tests/lib.rs
index 90a33aeead150..851eb12eb65c3 100644
--- a/library/coretests/tests/lib.rs
+++ b/library/coretests/tests/lib.rs
@@ -13,6 +13,7 @@
 #![feature(char_internals)]
 #![feature(char_max_len)]
 #![feature(clone_to_uninit)]
+#![feature(cmp_minmax)]
 #![feature(const_array)]
 #![feature(const_bool)]
 #![feature(const_cell_traits)]