Fix map_indices for AbstractAutomaticDifferentiation (#2394)

Author: odow

src/Nonlinear/types.jl

@@ -184,6 +184,13 @@ An abstract type for extending [`Evaluator`](@ref).
 """
 abstract type AbstractAutomaticDifferentiation end
 
+function MOI.Utilities.map_indices(
+    ::F,
+    backend::AbstractAutomaticDifferentiation,
+) where {F<:Function}
+    return backend
+end
+
 """
     Evaluator(
         model::Model,

test/Nonlinear/Nonlinear.jl

@@ -1147,6 +1147,22 @@ function test_unsupported_features_expr_graph_only()
     return
 end
 
+function test_automatic_differentiation_backend()
+    inner = MOI.Utilities.UniversalFallback(MOI.Utilities.Model{Float64}())
+    model = MOI.Utilities.CachingOptimizer(
+        MOI.Utilities.UniversalFallback(MOI.Utilities.Model{Float64}()),
+        MOI.Utilities.MockOptimizer(inner),
+    )
+    MOI.set(
+        model,
+        MOI.AutomaticDifferentiationBackend(),
+        MOI.Nonlinear.ExprGraphOnly(),
+    )
+    @test MOI.get(inner, MOI.AutomaticDifferentiationBackend()) ==
+          MOI.Nonlinear.ExprGraphOnly()
+    return
+end
+
 end  # TestNonlinear
 
 TestNonlinear.runtests()