From 55dbbdc75020e3ffead5359244c67a6103013ed2 Mon Sep 17 00:00:00 2001
From: Fredrik Bagge Carlson <baggepinnen@gmail.com>
Date: Fri, 15 May 2026 19:35:20 +0200
Subject: [PATCH] Fix UndefVarError in merge_nonunique_inputs warning
 interpolation

The warning strings interpolated `$(u[i])` but `u` is not defined inside
`merge_nonunique_inputs(sys)`. When the warning condition fires (overlapping
non-zero entries between B/D columns merged under the same input name) the
@warn would raise UndefVarError instead of issuing the documented warning.

Use the local `inputnames[i]` instead, and add a regression test that
constructs a system with overlapping B columns under a duplicated input name
and asserts the warning is emitted.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
---
 src/named_systems2.jl       |  4 ++--
 test/test_named_systems2.jl | 17 ++++++++++++++++-
 2 files changed, 18 insertions(+), 3 deletions(-)

diff --git a/src/named_systems2.jl b/src/named_systems2.jl
index 2e71041e..631f6de2 100644
--- a/src/named_systems2.jl
+++ b/src/named_systems2.jl
@@ -610,8 +610,8 @@ function merge_nonunique_inputs(sys)
         # Check that the B-matrix entries are non-overlapping
         Bi = sys.B[:, inds]
         Di = sys.D[:, inds]
-        any(>(1), sum(.! iszero.(Bi), dims=2)) && @warn("Input names are not unique and the multiple B-matrix columns associated with the name $(u[i]) have a non-empty intersection of non-zero entries.")
-        any(>(1), sum(.! iszero.(Di), dims=2)) && @warn("Input names are not unique and the multiple D-matrix columns associated with the name $(u[i]) have a non-empty intersection of non-zero entries.")
+        any(>(1), sum(.! iszero.(Bi), dims=2)) && @warn("Input names are not unique and the multiple B-matrix columns associated with the name $(inputnames[i]) have a non-empty intersection of non-zero entries.")
+        any(>(1), sum(.! iszero.(Di), dims=2)) && @warn("Input names are not unique and the multiple D-matrix columns associated with the name $(inputnames[i]) have a non-empty intersection of non-zero entries.")
         B = copy(sys.B)
         D = copy(sys.D)
         B[:, inds[1]] = sum(Bi, dims=2)
diff --git a/test/test_named_systems2.jl b/test/test_named_systems2.jl
index fc55b537..6a4f84e4 100644
--- a/test/test_named_systems2.jl
+++ b/test/test_named_systems2.jl
@@ -543,4 +543,19 @@ isys = 2/s1
 # Test https://github.com/JuliaControl/RobustAndOptimalControl.jl/issues/130
 P = named_ss(ssrand(1,1,2, Ts=1.0))
 C = named_ss(ssrand(1,1,2, Ts=1.0))
-@test gangoffour(P, C) isa NTuple{4, NamedStateSpace{Discrete{Float64}, StateSpace{Discrete{Float64}, Float64}}}
\ No newline at end of file
+@test gangoffour(P, C) isa NTuple{4, NamedStateSpace{Discrete{Float64}, StateSpace{Discrete{Float64}, Float64}}}
+
+## merge_nonunique_inputs: warning path must not crash on overlapping B/D columns
+# Regression: previous code interpolated an undefined variable `u` and raised
+# UndefVarError instead of issuing the documented warning.
+let
+    A = -I(2)
+    B = [1.0  1.0; 0.0  1.0]   # both columns nonzero in row 1 -> overlap
+    C = [1.0 0.0]
+    D = [0.0 0.0]
+    nsys = named_ss(ss(A, B, C, D); u=[:u_dup, :u_dup], y=:y, unique=false)
+    merged = @test_logs (:warn, r"B-matrix columns") RobustAndOptimalControl.merge_nonunique_inputs(nsys)
+    @test merged.nu == 1
+    @test merged.u == [:u_dup]
+    @test merged.B == sum(B, dims=2)
+end
\ No newline at end of file