From 6139a4383c268c21216b76c2426e1923a24d4dd0 Mon Sep 17 00:00:00 2001
From: 40% <Zeroto521@gmail.com>
Date: Tue, 19 Aug 2025 13:40:03 +0800
Subject: [PATCH 1/5] Add __matmul__ support to MatrixExpr

Implements the __matmul__ method for MatrixExpr, enabling matrix multiplication using the @ operator and ensuring the result is returned as a MatrixExpr instance.
---
 src/pyscipopt/matrix.pxi | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/src/pyscipopt/matrix.pxi b/src/pyscipopt/matrix.pxi
index 0548fbd10..9eb0349f2 100644
--- a/src/pyscipopt/matrix.pxi
+++ b/src/pyscipopt/matrix.pxi
@@ -98,7 +98,10 @@ class MatrixExpr(np.ndarray):
     
     def __rsub__(self, other):
         return super().__rsub__(other).view(MatrixExpr)
-    
+
+    def __matmul__(self, other):
+        return super().__matmul__(other).view(MatrixExpr)
+
 class MatrixGenExpr(MatrixExpr):
     pass
 

From b38481ac2bb720529ff0e6c084d64b3725b6ca52 Mon Sep 17 00:00:00 2001
From: 40% <Zeroto521@gmail.com>
Date: Tue, 19 Aug 2025 13:45:56 +0800
Subject: [PATCH 2/5] Add tests for matrix matmul return types

Adds tests to verify that matrix multiplication returns MatrixExpr instead of MatrixVariable for various input shapes.
---
 tests/test_matrix_variable.py | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/tests/test_matrix_variable.py b/tests/test_matrix_variable.py
index 0308bb694..e2ed2f383 100644
--- a/tests/test_matrix_variable.py
+++ b/tests/test_matrix_variable.py
@@ -392,3 +392,20 @@ def test_matrix_cons_indicator():
     assert m.getVal(is_equal).sum() == 2
     assert (m.getVal(x) == m.getVal(y)).all().all()
     assert (m.getVal(x) == np.array([[5, 5, 5], [5, 5, 5]])).all().all()
+
+
+def test_matrix_matmul_return_type():
+    # test #1058, require returning type is MatrixExpr not MatrixVariable
+    m = Model()
+
+    # test 1D @ 1D → 0D
+    x = m.addMatrixVar(3)
+    assert isinstance(x @ x, MatrixExpr)
+
+    # test 1D @ 1D → 2D
+    assert isinstance(x[:, None] @ x[None, :], MatrixExpr)
+
+    # test 2D @ 2D → 2D
+    y = m.addMatrixVar((3, 4))
+    z = m.addMatrixVar((2, 3))
+    assert isinstance(y @ z, MatrixExpr)

From ad513776614f3026c3e85b753b1d780a1310db4d Mon Sep 17 00:00:00 2001
From: 40% <Zeroto521@gmail.com>
Date: Tue, 19 Aug 2025 13:50:02 +0800
Subject: [PATCH 3/5] Use type() checks instead of isinstance in matmul tests

Replaces isinstance checks with type() comparisons for MatrixExpr in matrix matmul return type tests to ensure exact type matching.
---
 tests/test_matrix_variable.py | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/tests/test_matrix_variable.py b/tests/test_matrix_variable.py
index e2ed2f383..44da4c04c 100644
--- a/tests/test_matrix_variable.py
+++ b/tests/test_matrix_variable.py
@@ -400,12 +400,12 @@ def test_matrix_matmul_return_type():
 
     # test 1D @ 1D → 0D
     x = m.addMatrixVar(3)
-    assert isinstance(x @ x, MatrixExpr)
+    assert type(x @ x) is MatrixExpr
 
     # test 1D @ 1D → 2D
-    assert isinstance(x[:, None] @ x[None, :], MatrixExpr)
+    assert type(x[:, None] @ x[None, :]) is MatrixExpr
 
     # test 2D @ 2D → 2D
     y = m.addMatrixVar((3, 4))
     z = m.addMatrixVar((2, 3))
-    assert isinstance(y @ z, MatrixExpr)
+    assert type(y @ z) is MatrixExpr

From a9051835c8bb5c2b48ce2e4a697b1e0e6530c330 Mon Sep 17 00:00:00 2001
From: 40% <Zeroto521@gmail.com>
Date: Tue, 19 Aug 2025 13:56:42 +0800
Subject: [PATCH 4/5] Fix matrix variable shapes in matmul test

Corrects the shapes of matrix variables in test_matrix_matmul_return_type to ensure proper 2D matrix multiplication and type assertion.
---
 tests/test_matrix_variable.py | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/tests/test_matrix_variable.py b/tests/test_matrix_variable.py
index 44da4c04c..c7db949b3 100644
--- a/tests/test_matrix_variable.py
+++ b/tests/test_matrix_variable.py
@@ -406,6 +406,6 @@ def test_matrix_matmul_return_type():
     assert type(x[:, None] @ x[None, :]) is MatrixExpr
 
     # test 2D @ 2D → 2D
-    y = m.addMatrixVar((3, 4))
-    z = m.addMatrixVar((2, 3))
+    y = m.addMatrixVar((2, 3))
+    z = m.addMatrixVar((3, 4))
     assert type(y @ z) is MatrixExpr

From fe1773dd91248890d32736945786942ed745ff54 Mon Sep 17 00:00:00 2001
From: 40% <Zeroto521@gmail.com>
Date: Tue, 19 Aug 2025 13:56:51 +0800
Subject: [PATCH 5/5] Update CHANGELOG.md

---
 CHANGELOG.md | 1 +
 1 file changed, 1 insertion(+)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index addf21992..868656025 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -18,6 +18,7 @@
 ### Fixed
 - Raised an error when an expression is used when a variable is required
 - Fixed some compile warnings
+- Fixed the type of @ matrix operation result from MatrixVariable to MatrixExpr.
 ### Changed
 - MatrixExpr.sum() now supports axis arguments and can return either a scalar or MatrixExpr, depending on the result dimensions.
 - AddMatrixCons() also accepts ExprCons.