From a5b223cc2f30c3fd0114221cd7d5a19c71d11496 Mon Sep 17 00:00:00 2001
From: Erik Faulhaber <44124897+efaulhaber@users.noreply.github.com>
Date: Mon, 24 Jun 2024 09:36:38 +0200
Subject: [PATCH] Add a comment to coupled boundary condition

---
 src/semidiscretization/semidiscretization_coupled.jl | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/src/semidiscretization/semidiscretization_coupled.jl b/src/semidiscretization/semidiscretization_coupled.jl
index 44aae912c27..0a604c2b1f8 100644
--- a/src/semidiscretization/semidiscretization_coupled.jl
+++ b/src/semidiscretization/semidiscretization_coupled.jl
@@ -429,7 +429,11 @@ BoundaryConditionCoupled(2, (:j, :i_backwards, :end), Float64, fun)
 !!! warning "Experimental code"
     This is an experimental feature and can change any time.
 """
-mutable struct BoundaryConditionCoupled{NDIMS, other_semi_index, NDIMST2M1,
+mutable struct BoundaryConditionCoupled{NDIMS,
+                                        # Store the other semi index as type parameter,
+                                        # so that retrieving the other semidiscretization
+                                        # is type-stable.
+                                        other_semi_index, NDIMST2M1,
                                         uEltype <: Real, Indices, CouplingConverter}
     # NDIMST2M1 == NDIMS * 2 - 1
     # Buffer for boundary values: [variable, nodes_i, nodes_j, cell_i, cell_j]