Fix bug which mutates user expressions in constraint macro (#3883)

src/macros.jl

@@ -253,6 +253,16 @@ function _rewrite_expression(expr::Expr)
     new_expr = MacroTools.postwalk(_rewrite_to_jump_logic, expr)
     new_aff, parse_aff = _MA.rewrite(new_expr; move_factors_into_sums = false)
     ret = gensym()
+    has_copy_if_mutable = Ref(false)
+    MacroTools.postwalk(parse_aff) do x
+        if x === MutableArithmetics.copy_if_mutable
+            has_copy_if_mutable[] = true
+        end
+        return x
+    end
+    if !has_copy_if_mutable[]
+        new_aff = :($_MA.copy_if_mutable($new_aff))
+    end
     code = quote
         $parse_aff
         $ret = $flatten!($new_aff)

src/macros/@constraint.jl

@@ -1020,6 +1020,9 @@ function _clear_constant!(α::Number)
     return zero(α), α
 end
 
+# !!! warning
+#     This method assumes that we can mutate `expr`. Ensure that this is the
+#     case upstream of this call site.
 function build_constraint(
     ::Function,
     expr::Union{Number,GenericAffExpr,GenericQuadExpr},

test/test_macros.jl

@@ -2487,4 +2487,31 @@ function test_array_scalar_sets()
     return
 end
 
+function test_do_not_mutate_expression_double_sided_comparison()
+    model = Model()
+    @variable(model, x)
+    @expression(model, a[1:1], x + 1)
+    @constraint(model, -1 <= a[1] <= 1)
+    @test isequal_canonical(a[1], x + 1)
+    return
+end
+
+function test_do_not_mutate_expression_single_sided_comparison()
+    model = Model()
+    @variable(model, x)
+    @expression(model, a[1:1], x + 1)
+    @constraint(model, a[1] >= 1)
+    @test isequal_canonical(a[1], x + 1)
+    return
+end
+
+function test_do_not_mutate_expression_in_set()
+    model = Model()
+    @variable(model, x)
+    @expression(model, a[1:1], x + 1)
+    @constraint(model, a[1] in MOI.Interval(-1, 1))
+    @test isequal_canonical(a[1], x + 1)
+    return
+end
+
 end  # module