From 0221d153cbe80750305a0216a60c83e009d1d5ca Mon Sep 17 00:00:00 2001
From: Giorgos Mousa <gmousa@proton.me>
Date: Sat, 8 Nov 2025 13:00:10 +0200
Subject: [PATCH] `Matroid._max_independent`: Fix call of `_rank`

Bug reported in https://github.com/passagemath/passagemath/issues/1783.
---
 src/sage/matroids/matroid.pyx | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/src/sage/matroids/matroid.pyx b/src/sage/matroids/matroid.pyx
index b2516a81cde..54265ca5452 100644
--- a/src/sage/matroids/matroid.pyx
+++ b/src/sage/matroids/matroid.pyx
@@ -637,12 +637,19 @@ cdef class Matroid(SageObject):
             True
             sage: all(M.is_dependent(X.union([y])) for y in M.groundset() if y not in X)
             True
+
+        TESTS::
+
+            sage: M = matroids.catalog.R10()
+            sage: M1M = M.direct_sum(M)
+            sage: Matroid(M1M, regular=True)  # indirect doctest
+            Regular matroid of rank 10 on 20 elements with 26244 bases
         """
         cdef list res = []
         cdef int r = 0
         for e in X:
             res.append(e)
-            if self._rank(res) > r:
+            if self._rank(frozenset(res)) > r:
                 r += 1
             else:
                 res.pop()