Fix the error for unsupported objective

docs/src/changelog.md

@@ -16,6 +16,12 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   usually rejects them. The new `SolveWithoutVariables` and
   `NoVariablesFeasibilityTolerance` attributes control this behavior.
 
+### Changed
+
+- Setting a `MOI.ScalarNonlinearFunction` objective that is not a valid cubic
+  polynomial, or whose parametric form the inner optimizer does not support,
+  now throws `MOI.UnsupportedAttribute` instead of an `ErrorException`.
+
 ## Version 0.15.2 (March 15, 2026)
 
 ### Fixed

src/cubic_objective.jl

@@ -5,16 +5,23 @@
 
 function MOI.set(
     model::Optimizer{T},
-    ::MOI.ObjectiveFunction{MOI.ScalarNonlinearFunction},
+    attr::MOI.ObjectiveFunction{MOI.ScalarNonlinearFunction},
     f::MOI.ScalarNonlinearFunction,
 ) where {T}
-    # 1. Attempt to parse as cubic
+    # 1. Attempt to parse as cubic. A function that does not parse is an
+    # objective this optimizer does not support, so it is reported with the
+    # MOI exception for that: callers such as bridges or DiffOpt's choice of
+    # differentiation model can then catch it and try something else
     parsed = _parse_cubic_expression(f, T)
     if parsed === nothing
-        error(
-            "ScalarNonlinearFunction must be a valid cubic polynomial with " *
-            "parameters multiplying at most quadratic variable terms. " *
-            "Non-polynomial operations or degree > 3 are not supported.",
+        throw(
+            MOI.UnsupportedAttribute(
+                attr,
+                "ScalarNonlinearFunction must be a valid cubic polynomial " *
+                "with parameters multiplying at most quadratic variable " *
+                "terms. Non-polynomial operations or degree > 3 are not " *
+                "supported.",
+            ),
         )
     end
 
@@ -32,6 +39,15 @@ function MOI.set(
             current,
         )
     catch e
+        msg =
+            "Failed to set cubic objective function, f = $f, on inner " *
+            "optimizer. Original error: $(sprint(showerror, e))"
+        # the inner optimizer not supporting the parametric form means this
+        # optimizer does not support the objective either; any other error is
+        # a genuine failure and keeps its plain report
+        if e isa MOI.UnsupportedError
+            throw(MOI.UnsupportedAttribute(attr, msg))
+        end
         # rethrow the original error with the additional info of the objective function that caused it
         error(
             "Failed to set cubic objective function, f = $f, on inner " *

test/test_cubic.jl

@@ -30,13 +30,17 @@ end
 # ============================================================================
 
 """
-Mock optimizer that rejects ScalarQuadraticFunction objectives.
+Mock optimizer that rejects ScalarQuadraticFunction objectives, with a plain
+error or, if `unsupported`, with `MOI.UnsupportedAttribute`.
 """
 struct NoQuadObjModel <: MOI.ModelLike
     inner::MOI.Utilities.Model{Float64}
+    unsupported::Bool
 end
 
-NoQuadObjModel() = NoQuadObjModel(MOI.Utilities.Model{Float64}())
+function NoQuadObjModel(; unsupported::Bool = false)
+    return NoQuadObjModel(MOI.Utilities.Model{Float64}(), unsupported)
+end
 
 MOI.add_variable(m::NoQuadObjModel) = MOI.add_variable(m.inner)
 MOI.is_empty(m::NoQuadObjModel) = MOI.is_empty(m.inner)
@@ -46,10 +50,11 @@ function MOI.is_valid(m::NoQuadObjModel, vi::MOI.VariableIndex)
 end
 
 function MOI.set(
-    ::NoQuadObjModel,
-    ::MOI.ObjectiveFunction{<:MOI.ScalarQuadraticFunction},
+    m::NoQuadObjModel,
+    attr::MOI.ObjectiveFunction{<:MOI.ScalarQuadraticFunction},
     ::MOI.ScalarQuadraticFunction,
 )
+    m.unsupported && throw(MOI.UnsupportedAttribute(attr))
     return error("Quadratic objectives not supported")
 end
 
@@ -865,9 +870,9 @@ function test_cubic_objective_set_invalid_expression()
     y = MOI.add_variable(model)
     z = MOI.add_variable(model)
 
-    # x*y*z (3 variables, no parameter) should error
+    # x*y*z (3 variables, no parameter) is not a supported objective
     f = MOI.ScalarNonlinearFunction(:*, Any[x, y, z])
-    @test_throws ErrorException MOI.set(
+    @test_throws MOI.UnsupportedAttribute MOI.set(
         model,
         MOI.ObjectiveFunction{MOI.ScalarNonlinearFunction}(),
         f,
@@ -897,6 +902,27 @@ function test_cubic_objective_set_error_on_inner_optimizer()
     return
 end
 
+function test_cubic_objective_set_unsupported_by_inner_optimizer()
+    model = POI.Optimizer(NoQuadObjModel(; unsupported = true))
+    x = MOI.add_variable(model)
+    p, _ = MOI.add_constrained_variable(model, MOI.Parameter(2.0))
+    p_v = POI.v_idx(POI.p_idx(p))
+    # p * x^2 parses, but its parametric form is a quadratic objective, which
+    # the inner optimizer does not support, so neither does POI
+    f = MOI.ScalarNonlinearFunction(:*, Any[1.0, p_v, x, x])
+    attr = MOI.ObjectiveFunction{MOI.ScalarNonlinearFunction}()
+    err = try
+        MOI.set(model, attr, f)
+        nothing
+    catch e
+        e
+    end
+    @test err isa MOI.UnsupportedAttribute
+    @test err.attr == attr
+    @test occursin("Failed to set cubic objective function", err.message)
+    return
+end
+
 function test_cubic_objective_get_no_cache()
     model = POI.Optimizer(HiGHS.Optimizer())