fixes crashes for left matmul

Author: Transurgeon

cvxpy/cvxcore/python/canonInterface.py

@@ -292,6 +292,13 @@ def get_problem_matrix(linOps,
     default_canon_backend = get_default_canon_backend()
     canon_backend = default_canon_backend if not canon_backend else canon_backend
 
+    # DIFFENGINE is a reduction-layer replacement for ConeMatrixStuffing and
+    # has no lin_ops backend of its own. When code paths reach this matrix
+    # builder directly (e.g. tests that bypass the stuffing reduction), fall
+    # through to the CPP backend so the lin_ops pipeline still works.
+    if canon_backend == s.DIFFENGINE_BACKEND:
+        canon_backend = s.CPP_CANON_BACKEND
+
     if canon_backend == s.CPP_CANON_BACKEND:
         from cvxpy.cvxcore.python.cppbackend import build_matrix
         return build_matrix(id_to_col, param_to_size, param_to_col, var_length, constr_length, linOps)

cvxpy/reductions/solvers/nlp_solvers/diff_engine/converters.py

@@ -65,14 +65,24 @@ def convert_matmul(expr, children, var_dict, n_vars, param_dict):
 
     if left_arg.is_constant():
         A = _matmul_normalize_1d(left_arg.value, 'left')
-        param_node = param_dict[left_arg.id] if isinstance(left_arg, cp.Parameter) else None
+        if isinstance(left_arg, cp.Parameter):
+            param_node = param_dict[left_arg.id]
+        elif left_arg.parameters():
+            param_node = left_child
+        else:
+            param_node = None
         if sparse.issparse(A):
             return make_sparse_left_matmul(param_node, right_child, A)
         return make_dense_left_matmul(param_node, right_child, A)
 
     elif right_arg.is_constant():
         A = _matmul_normalize_1d(right_arg.value, 'right')
-        param_node = param_dict[right_arg.id] if isinstance(right_arg, cp.Parameter) else None
+        if isinstance(right_arg, cp.Parameter):
+            param_node = param_dict[right_arg.id]
+        elif right_arg.parameters():
+            param_node = right_child
+        else:
+            param_node = None
         if sparse.issparse(A):
             return make_sparse_right_matmul(param_node, left_child, A)
         return make_dense_right_matmul(param_node, left_child, A)

pyproject.toml

@@ -59,6 +59,13 @@ testpaths = [
     "cvxpy/tests/"
 ]
 
+[tool.uv.sources]
+# Local dev override: build sparsediffpy from the adjacent source tree so we
+# pick up bindings (vstack / diag_mat / upper_tri / parameter support) that
+# haven't shipped a PyPI release yet. Remove once sparsediffpy >= 0.3.0 is
+# published.
+sparsediffpy = { path = "../SparseDiffPy", editable = true }
+
 [build-system]
 requires = [
     "numpy >= 2.0.0",