Add operator fallbacks for pure SingleVariable operations

src/Utilities/functions.jl

@@ -1922,6 +1922,37 @@ function Base.:-(α::Number, f::MOI.SingleVariable)
     return operate(-, typeof(α), α, f)
 end
 
+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.:-(::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.:*(
+    ::MOI.SingleVariable,
+    ::MOI.SingleVariable,
+    ::MOI.SingleVariable...,
+)
+    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 +/-
 ###############################################################################
 

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_throws ErrorException x + x
+    @test_throws ErrorException x - x
+    @test_throws ErrorException x * x
+    return
+end
+
 end  # module
 
 TestFunctions.runtests()