Update

src/nlp_expr.jl

@@ -673,25 +673,26 @@ function _evaluate_expr(
     return convert(Float64, expr)
 end
 
-struct _ConcreteExpression{T}
+struct _ConcreteExpression
     head::Symbol
-    args::Vector{T}
+    args::Vector{Real}
+    _ConcreteExpression(head::Symbol) = new(head, Real[])
 end
 
 function _evaluate_expr(
     registry::MOI.Nonlinear.OperatorRegistry,
     f::Function,
     expr::GenericNonlinearExpr,
 )
-    ret = _ConcreteExpression{Float64}(expr.head, Float64[])
+    ret = _ConcreteExpression(expr.head)
     stack = Any[]
     for arg in reverse(expr.args)
         push!(stack, (ret, arg))
     end
     while !isempty(stack)
         parent, arg = pop!(stack)
         if arg isa MOI.ScalarNonlinearFunction
-            new_ret = _ConcreteExpression{Float64}(arg.head, Float64[])
+            new_ret = _ConcreteExpression(arg.head)
             push!(parent.args, new_ret)
             for child in reverse(arg.args)
                 push!(stack, (new_ret, child))

test/test_nlp_expr.jl

@@ -382,6 +382,22 @@ function test_evaluate_expr_stackoverflow()
     return
 end
 
+function test_evaluate_expr_stackoverflow_user_defined_function()
+    N = 10_000
+    f(x, y) = *(x, y)
+    model = Model()
+    @variable(model, x[1:N], start = 0)
+    @operator(model, op_f, 2, f)
+    y = x[1]
+    for i in 2:N
+        y = op_f(x[i], y)
+    end
+    @test value(start_value, y) == 0.0
+    set_start_value.(x, 1.0)
+    @test value(start_value, y) == 1.0
+    return
+end
+
 function test_nlobjective_with_nlexpr()
     model = Model()
     @variable(model, x)