Validate BCs for Petrov-Galerkin matrix assembly

python/dolfinx/fem/assemble.py

@@ -92,6 +92,25 @@ def _pack(form):
     return _pack(form)
 
 
+def _check_bcs_for_different_trial_test_spaces(a: Form, bcs: Sequence[DirichletBC]) -> None:
+    """Reject DirichletBCs for unsupported Petrov-Galerkin assembly."""
+    if not bcs or len(a.function_spaces) != 2:
+        return
+
+    test_space, trial_space = a.function_spaces
+    if test_space is trial_space:
+        return
+    if test_space.mesh is not trial_space.mesh:
+        return
+
+    for bc in bcs:
+        if test_space.contains(bc.function_space) or trial_space.contains(bc.function_space):
+            raise RuntimeError(
+                "Dirichlet boundary conditions for bilinear forms with different test "
+                "and trial spaces are not supported by this matrix assembly path."
+            )
+
+
 # -- Vector and matrix instantiation --------------------------------------
 
 
@@ -325,6 +344,7 @@ def _assemble_matrix_csr(
         The returned matrix is not finalised, i.e. ghost values are not
         accumulated.
     """
+    _check_bcs_for_different_trial_test_spaces(a, [] if bcs is None else bcs)
     bcs = [] if bcs is None else [bc._cpp_object for bc in bcs]
     constants = pack_constants(a) if constants is None else constants  # type: ignore[assignment]
     coeffs = pack_coefficients(a) if coeffs is None else coeffs  # type: ignore[assignment]

python/dolfinx/fem/petsc.py

@@ -49,7 +49,10 @@
 from dolfinx.cpp.fem.petsc import discrete_gradient as _discrete_gradient
 from dolfinx.cpp.fem.petsc import interpolation_matrix as _interpolation_matrix
 from dolfinx.fem import IntegralType, pack_coefficients, pack_constants
-from dolfinx.fem.assemble import _assemble_vector_array
+from dolfinx.fem.assemble import (
+    _assemble_vector_array,
+    _check_bcs_for_different_trial_test_spaces,
+)
 from dolfinx.fem.assemble import apply_lifting as _apply_lifting
 from dolfinx.fem.bcs import DirichletBC
 from dolfinx.fem.bcs import bcs_by_block as _bcs_by_block
@@ -422,6 +425,10 @@ def _(
     The returned matrix is not finalised, i.e. ghost values are not
     accumulated.
     """
+    bcs = [] if bcs is None else bcs
+    if not isinstance(a, Sequence):
+        _check_bcs_for_different_trial_test_spaces(a, bcs)
+
     if A.getType() == PETSc.Mat.Type.NEST:  # type: ignore[attr-defined]
         if not isinstance(a, Sequence):
             raise ValueError("Must provide a sequence of forms when assembling a nest matrix")

python/test/unit/fem/test_assembler.py

@@ -178,6 +178,26 @@ def nest_matrix_norm(A):
     return math.sqrt(norm)
 
 
+def _different_trial_test_space_form_and_bc():
+    msh = create_unit_square(MPI.COMM_WORLD, 4, 4)
+    U = functionspace(msh, ("Lagrange", 1))
+    V = functionspace(msh, ("Lagrange", 2))
+    u = ufl.TrialFunction(U)
+    v = ufl.TestFunction(V)
+    a = form(inner(u, v) * dx)
+
+    facets = locate_entities_boundary(msh, msh.topology.dim - 1, lambda x: np.isclose(x[0], 0.0))
+    dofs = locate_dofs_topological(V, msh.topology.dim - 1, facets)
+    bc = dirichletbc(default_scalar_type(0), dofs, V)
+    return a, bc
+
+
+def test_assembly_rejects_bcs_for_different_trial_test_spaces():
+    a, bc = _different_trial_test_space_form_and_bc()
+    with pytest.raises(RuntimeError, match="different test and trial spaces"):
+        assemble_matrix(a, bcs=[bc])
+
+
 @pytest.mark.petsc4py
 def test_vector_single_space_as_block():
     from dolfinx.fem.petsc import create_vector as petsc_create_vector
@@ -193,6 +213,14 @@ def test_vector_single_space_as_block():
 class TestPETScAssemblers:
     """Test PETSc-based assemblers for matrices and vectors."""
 
+    def test_assembly_rejects_bcs_for_different_trial_test_spaces(self):
+        """Test that PETSc assembly rejects same-domain Petrov-Galerkin BCs."""
+        from dolfinx.fem.petsc import assemble_matrix as petsc_assemble_matrix
+
+        a, bc = _different_trial_test_space_form_and_bc()
+        with pytest.raises(RuntimeError, match="different test and trial spaces"):
+            petsc_assemble_matrix(a, bcs=[bc])
+
     @pytest.mark.parametrize("mode", [GhostMode.none, GhostMode.shared_facet])
     def test_basic_assembly_petsc_matrixcsr(self, mode):
         """Test basic assembly of PETSc Mat and compare with MatrixCSR assembly."""