From 944e5c5d3d6a035380fcd6d40c330af30675125b Mon Sep 17 00:00:00 2001
From: James Foster <38274066+jd-foster@users.noreply.github.com>
Date: Fri, 3 Nov 2023 10:24:41 +1100
Subject: [PATCH] =?UTF-8?q?Add=20error=20message=20for=20Matrix=20=C2=B1?=
 =?UTF-8?q?=20AbstractJuMPScalar=20(#3557)?=
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

---
 src/operators.jl      | 20 ++++++++++++++++++++
 test/test_operator.jl | 39 +++++++++++++++++++++++++++++++++++++++
 2 files changed, 59 insertions(+)

diff --git a/src/operators.jl b/src/operators.jl
index 5310234ddc0..608dacc5028 100644
--- a/src/operators.jl
+++ b/src/operators.jl
@@ -451,3 +451,23 @@ function LinearAlgebra.issymmetric(x::Matrix{T}) where {T<:_JuMPTypes}
     end
     return true
 end
+
+function Base.:+(A::AbstractMatrix, x::AbstractJuMPScalar)
+    return error(
+        "Addition between a Matrix and a JuMP variable is not supported: instead of `A + x`, " *
+        "do `A .+ x` for element-wise addition, or if you are modifying the diagonal entries of the matrix " *
+        "do `A + x * LinearAlgebra.I(n)`, where `n` is the diagonal length.",
+    )
+end
+
+Base.:+(x::AbstractJuMPScalar, A::AbstractMatrix) = A + x
+
+function Base.:-(A::AbstractMatrix, x::AbstractJuMPScalar)
+    return error(
+        "Subtraction between a Matrix and a JuMP variable is not supported: instead of `A - x`, " *
+        "do `A .- x` for element-wise subtraction, or if you are modifying the diagonal entries of the matrix " *
+        "do `A - x * LinearAlgebra.I(n)`, where `n` is the diagonal length.",
+    )
+end
+
+Base.:-(x::AbstractJuMPScalar, A::AbstractMatrix) = A - x
diff --git a/test/test_operator.jl b/test/test_operator.jl
index 967fb8b666d..4f43a05891a 100644
--- a/test/test_operator.jl
+++ b/test/test_operator.jl
@@ -625,4 +625,43 @@ function test_complex_pow()
     return
 end
 
+function test_matrix_abstractscalar_add()
+    model = Model()
+    @variable(model, x)
+    A = rand(Float64, 2, 2)
+    @test_throws(
+        ErrorException(
+            "Addition between a Matrix and a JuMP variable is not supported: instead of `A + x`, " *
+            "do `A .+ x` for element-wise addition, or if you are modifying the diagonal entries of the matrix " *
+            "do `A + x * LinearAlgebra.I(n)`, where `n` is the diagonal length.",
+        ),
+        A + x
+    ),
+    @test_throws(
+        ErrorException(
+            "Addition between a Matrix and a JuMP variable is not supported: instead of `A + x`, " *
+            "do `A .+ x` for element-wise addition, or if you are modifying the diagonal entries of the matrix " *
+            "do `A + x * LinearAlgebra.I(n)`, where `n` is the diagonal length.",
+        ),
+        x + A
+    ),
+    @test_throws(
+        ErrorException(
+            "Subtraction between a Matrix and a JuMP variable is not supported: instead of `A - x`, " *
+            "do `A .- x` for element-wise subtraction, or if you are modifying the diagonal entries of the matrix " *
+            "do `A - x * LinearAlgebra.I(n)`, where `n` is the diagonal length.",
+        ),
+        A - x
+    ),
+    @test_throws(
+        ErrorException(
+            "Subtraction between a Matrix and a JuMP variable is not supported: instead of `A - x`, " *
+            "do `A .- x` for element-wise subtraction, or if you are modifying the diagonal entries of the matrix " *
+            "do `A - x * LinearAlgebra.I(n)`, where `n` is the diagonal length.",
+        ),
+        x - A
+    ),
+    return
+end
+
 end