Update

src/operators.jl

@@ -333,11 +333,9 @@ end
 function Base.:+(a::GenericAffExpr, q::GenericQuadExpr)
     return GenericQuadExpr(a + q.aff, copy(q.terms))
 end
-function Base.:-(a::GenericAffExpr, q::GenericQuadExpr)
-    result = -q
-    # This makes an unnecessary copy of aff, but it's important for a to appear
-    # first.
-    result.aff = a + result.aff
+function Base.:-(a::GenericAffExpr{S}, q::GenericQuadExpr{T}) where {S,T}
+    result = -_copy_convert_coef(_MA.promote_operation(-, S, T), q)
+    add_to_expression!(result.aff, a)
     return result
 end
 

test/test_operator.jl

@@ -689,6 +689,18 @@ function test_base_complex()
     return
 end
 
+function test_aff_minus_quad()
+    model = Model()
+    @variable(model, x)
+    a, b = 1.0 * x, (2 + 3im) * x^2
+    @test a - b == -(b - a)
+    @test b - a == -(a - b)
+    a, b = (1.0 + 2im) * x, 3 * x^2 + 4 * x
+    @test a - b == -(b - a)
+    @test b - a == -(a - b)
+    return
+end
+
 function test_hermitian_and_symmetric()
     model = Model()
     @variable(model, A[1:2, 1:2], Symmetric)