BoundaryElemIntegrityCheckThread Bug Fix for Boundary Integrity Check on SideUserObject Using Lower-d Variable

framework/src/loops/BoundaryElemIntegrityCheckThread.C

@@ -77,7 +77,16 @@ BoundaryElemIntegrityCheckThread::operator()(const ConstBndElemRange & range)
         .queryInto(objs);
     for (const auto & uo : objs)
       if (uo->checkVariableBoundaryIntegrity())
-        boundaryIntegrityCheckError(*uo, uo->checkAllVariables(*elem), bnd_name);
+      {
+        auto leftover_vars = uo->checkAllVariables(*elem);
+        if (!leftover_vars.empty())
+        {
+          const Elem * const lower_d_elem = mesh.getLowerDElem(elem, side);
+          if (lower_d_elem)
+            leftover_vars = uo->checkVariables(*lower_d_elem, leftover_vars);
+        }
+        boundaryIntegrityCheckError(*uo, leftover_vars, bnd_name);
+      }
 
     auto check = [elem, boundary_id, &bnd_name, tid, &mesh, side](const auto & warehouse)
     {

test/tests/userobjects/side_user_object_no_boundary_error/lower_d_side_boundary.i

@@ -0,0 +1,106 @@
+[Mesh]
+  [square]
+    type = GeneratedMeshGenerator
+    nx = 3
+    ny = 3
+    dim = 2
+  []
+  build_all_side_lowerd_mesh = true
+[]
+
+[Variables]
+  [u]
+    order = THIRD
+    family = MONOMIAL
+    block = 0
+  []
+  [uhat]
+    order = CONSTANT
+    family = MONOMIAL
+    block = BOUNDARY_SIDE_LOWERD_SUBDOMAIN
+  []
+  [lambda]
+    order = CONSTANT
+    family = MONOMIAL
+    block = INTERNAL_SIDE_LOWERD_SUBDOMAIN
+  []
+  [lambdab]
+    order = CONSTANT
+    family = MONOMIAL
+    block = BOUNDARY_SIDE_LOWERD_SUBDOMAIN
+  []
+[]
+
+[AuxVariables]
+  [v]
+    order = CONSTANT
+    family = MONOMIAL
+    block = 0
+    initial_condition = '1'
+  []
+[]
+
+[Kernels]
+  [diff]
+    type = MatDiffusion
+    variable = u
+    diffusivity = '1'
+    block = 0
+  []
+  [source]
+    type = CoupledForce
+    variable = u
+    v = v
+    coef = '1'
+    block = 0
+  []
+  [reaction]
+    type = Reaction
+    variable = uhat
+    rate = '1'
+    block = BOUNDARY_SIDE_LOWERD_SUBDOMAIN
+  []
+  [uhat_coupled]
+    type = CoupledForce
+    variable = uhat
+    block = BOUNDARY_SIDE_LOWERD_SUBDOMAIN
+    v = lambdab
+    coef = '1'
+  []
+[]
+
+[DGKernels]
+  [surface]
+    type = HFEMDiffusion
+    variable = u
+    lowerd_variable = lambda
+  []
+[]
+
+[BCs]
+  [all]
+    type = HFEMDirichletBC
+    boundary = 'left right top bottom'
+    variable = u
+    lowerd_variable = lambdab
+    uhat = uhat
+  []
+[]
+
+[Problem]
+  solve = false
+[]
+
+[Executioner]
+  type = Steady
+  solve_type = 'NEWTON'
+  petsc_options_iname = '-pc_type -snes_linesearch_type -pc_factor_mat_solver_type'
+  petsc_options_value = 'lu       basic                 mumps'
+[]
+
+[Postprocessors]
+  [avg]
+    type = SideAverageValue
+    boundary = 'left right top bottom'
+  []
+[]

test/tests/userobjects/side_user_object_no_boundary_error/tests

@@ -1,12 +1,29 @@
 [Tests]
   design = 'SideAverageValue.md'
-  issues = '#9103'
+  issues = '#9103 #21566'
 
-  [./test]
+  [test]
     type = RunException
     input = side_no_boundary.i
     expect_err = "missing required parameter 'Postprocessors/avg/boundary'"
 
     requirement = "The system shall error if the boundary is not specified when computing the average value of a variable on a boundary."
-  [../]
+  []
+
+  [lower_d_test]
+    type = RunApp
+    input = lower_d_side_boundary.i
+    cli_args = "Postprocessors/avg/variable='lambdab'"
+    mesh_mode = replicated
+    requirement = "The system shall check if the lower-d variable is properly defined on a boundary."
+  []
+
+  [lower_d_test_error]
+    type = RunException
+    input = lower_d_side_boundary.i
+    cli_args = "Postprocessors/avg/variable='lambda'"
+    mesh_mode = replicated
+    expect_err="'avg' of type 'SideAverageValue' depends on variable\(s\) 'lambda'."
+    requirement = "The system shall error if the lower-d variable is not properly defined on a boundary."
+  []
 []