From 4355014291a528e46e7f0f1083acfeffe2d78ee8 Mon Sep 17 00:00:00 2001
From: odow <o.dowson@gmail.com>
Date: Thu, 26 Aug 2021 16:53:56 +1200
Subject: [PATCH 1/2] Add operator fallbacks for pure SingleVariable operations

---
 src/Utilities/functions.jl  | 17 +++++++++++++++++
 test/Utilities/functions.jl | 13 +++++++++++++
 2 files changed, 30 insertions(+)

diff --git a/src/Utilities/functions.jl b/src/Utilities/functions.jl
index 2603c899ad..8c8392c5d8 100644
--- a/src/Utilities/functions.jl
+++ b/src/Utilities/functions.jl
@@ -1922,6 +1922,23 @@ function Base.:-(α::Number, f::MOI.SingleVariable)
     return operate(-, typeof(α), α, f)
 end
 
+# SingleVariable defaults to Float64
+function Base.:+(arg::MOI.SingleVariable, args::MOI.SingleVariable...)
+    return operate(+, Float64, arg, args...)
+end
+
+function Base.:-(arg::MOI.SingleVariable, args::MOI.SingleVariable...)
+    return operate(-, Float64, arg, args...)
+end
+
+function Base.:*(
+    arg::MOI.SingleVariable,
+    arg2::MOI.SingleVariable,
+    args::MOI.SingleVariable...,
+)
+    return operate(*, Float64, arg, arg2, args...)
+end
+
 # Vector +/-
 ###############################################################################
 
diff --git a/test/Utilities/functions.jl b/test/Utilities/functions.jl
index 314482dcb0..8ff4dfc12d 100644
--- a/test/Utilities/functions.jl
+++ b/test/Utilities/functions.jl
@@ -1781,6 +1781,19 @@ function test_zero_with_output_dimension()
     return
 end
 
+"""
+    test_SingleVariable_operators()
+
+Test the three Base.:(op) fallbacks for pure SingleVariable operations.
+"""
+function test_SingleVariable_operators()
+    x = MOI.SingleVariable(MOI.VariableIndex(1))
+    @test x + x ≈ 1.0 * x + 1.0 * x
+    @test x - x ≈ 1.0 * x - 1.0 * x
+    @test x * x ≈ 1.0 * x * 1.0 * x
+    return
+end
+
 end  # module
 
 TestFunctions.runtests()

From 8efc46f5fb6bbb7fff43802abe2cd2a135699e26 Mon Sep 17 00:00:00 2001
From: odow <o.dowson@gmail.com>
Date: Fri, 27 Aug 2021 10:02:01 +1200
Subject: [PATCH 2/2] Throw error instead

---
 src/Utilities/functions.jl  | 32 +++++++++++++++++++++++---------
 test/Utilities/functions.jl |  6 +++---
 2 files changed, 26 insertions(+), 12 deletions(-)

diff --git a/src/Utilities/functions.jl b/src/Utilities/functions.jl
index 8c8392c5d8..6e28be93ca 100644
--- a/src/Utilities/functions.jl
+++ b/src/Utilities/functions.jl
@@ -1922,21 +1922,35 @@ function Base.:-(α::Number, f::MOI.SingleVariable)
     return operate(-, typeof(α), α, f)
 end
 
-# SingleVariable defaults to Float64
-function Base.:+(arg::MOI.SingleVariable, args::MOI.SingleVariable...)
-    return operate(+, Float64, arg, args...)
+function Base.:+(::MOI.SingleVariable, ::MOI.SingleVariable...)
+    return error(
+        "Unable to add SingleVariables together because no coefficient type " *
+        "is specified. Instead of `x + y`, convert one of the terms to a " *
+        "`ScalarAffineFunction` first by left-multiplying by `one(T)` where " *
+        "`T` is the coefficient type For example: `1.0 * x + y`.",
+    )
 end
 
-function Base.:-(arg::MOI.SingleVariable, args::MOI.SingleVariable...)
-    return operate(-, Float64, arg, args...)
+function Base.:-(::MOI.SingleVariable, ::MOI.SingleVariable...)
+    return error(
+        "Unable to subtract SingleVariables together because no coefficient " *
+        "type is specified. Instead of `x - y`, convert one of the terms to a " *
+        "`ScalarAffineFunction` first by left-multiplying by `one(T)` where " *
+        "`T` is the coefficient type For example: `1.0 * x - y`.",
+    )
 end
 
 function Base.:*(
-    arg::MOI.SingleVariable,
-    arg2::MOI.SingleVariable,
-    args::MOI.SingleVariable...,
+    ::MOI.SingleVariable,
+    ::MOI.SingleVariable,
+    ::MOI.SingleVariable...,
 )
-    return operate(*, Float64, arg, arg2, args...)
+    return error(
+        "Unable to multiply SingleVariables together because no coefficient " *
+        "type is specified. Instead of `x * y`, convert one of the terms to a " *
+        "`ScalarAffineFunction` first by left-multiplying by `one(T)` where " *
+        "`T` is the coefficient type For example: `1.0 * x * y`.",
+    )
 end
 
 # Vector +/-
diff --git a/test/Utilities/functions.jl b/test/Utilities/functions.jl
index 8ff4dfc12d..7137fddbc8 100644
--- a/test/Utilities/functions.jl
+++ b/test/Utilities/functions.jl
@@ -1788,9 +1788,9 @@ Test the three Base.:(op) fallbacks for pure SingleVariable operations.
 """
 function test_SingleVariable_operators()
     x = MOI.SingleVariable(MOI.VariableIndex(1))
-    @test x + x ≈ 1.0 * x + 1.0 * x
-    @test x - x ≈ 1.0 * x - 1.0 * x
-    @test x * x ≈ 1.0 * x * 1.0 * x
+    @test_throws ErrorException x + x
+    @test_throws ErrorException x - x
+    @test_throws ErrorException x * x
     return
 end