[Utilities] use ConstraintPrimal fallback in CachingOptimizer

src/Utilities/cachingoptimizer.jl

@@ -853,6 +853,7 @@ function MOI.get(
         return MOI.get(model.model_cache, attr, index)
     end
 end
+
 function MOI.get(
     model::CachingOptimizer,
     attr::Union{MOI.AbstractVariableAttribute,MOI.AbstractConstraintAttribute},
@@ -878,6 +879,58 @@ function MOI.get(
     end
 end
 
+###
+### MOI.ConstraintPrimal
+###
+
+# ConstraintPrimal is slightly unique for CachingOptimizer because if the solver
+# doesn't support the attribute directly, we can use the fallback to query the
+# function from the cache and the variable value from the optimizer.
+
+function MOI.get(
+    model::CachingOptimizer,
+    attr::MOI.ConstraintPrimal,
+    index::MOI.ConstraintIndex,
+)
+    if state(model) == NO_OPTIMIZER
+        error(
+            "Cannot query $(attr) from caching optimizer because no " *
+            "optimizer is attached.",
+        )
+    end
+    try
+        return MOI.get(
+            model.optimizer,
+            attr,
+            model.model_to_optimizer_map[index],
+        )
+    catch
+        return get_fallback(model, attr, index)
+    end
+end
+
+function MOI.get(
+    model::CachingOptimizer,
+    attr::MOI.ConstraintPrimal,
+    indices::Vector{<:MOI.ConstraintIndex},
+)
+    if state(model) == NO_OPTIMIZER
+        error(
+            "Cannot query $(attr) from caching optimizer because no " *
+            "optimizer is attached.",
+        )
+    end
+    try
+        return MOI.get(
+            model.optimizer,
+            attr,
+            [model.model_to_optimizer_map[i] for i in indices],
+        )
+    catch
+        return [get_fallback(model, attr, i) for i in indices]
+    end
+end
+
 #####
 ##### Names
 #####

test/Utilities/cachingoptimizer.jl

@@ -797,6 +797,68 @@ function test_copy_to_and_optimize!()
     return
 end
 
+###
+### Test ConstraintPrimal fallback in CachingOptimizer.
+###
+### It's a bit complicated because we need an optimizer that errors on
+### ConstraintPrimal. This is the minimal set of methods needed to test the
+### fallback.
+###
+
+struct _ConstraintPrimal1310 <: MOI.AbstractOptimizer end
+
+MOI.is_empty(::_ConstraintPrimal1310) = true
+
+MOI.get(::_ConstraintPrimal1310, ::MOI.ListOfModelAttributesSet) = []
+
+MOI.get(::_ConstraintPrimal1310, ::MOI.PrimalStatus) = MOI.FEASIBLE_POINT
+
+function MOI.optimize!(::_ConstraintPrimal1310, model::MOI.ModelLike)
+    index_map = MOI.IndexMap()
+    for x in MOI.get(model, MOI.ListOfVariableIndices())
+        index_map[x] = x
+    end
+    for (F, S) in MOI.get(model, MOI.ListOfConstraintTypesPresent())
+        for ci in MOI.get(model, MOI.ListOfConstraintIndices{F,S}())
+            index_map[ci] = ci
+        end
+    end
+    return index_map, false
+end
+
+function MOI.get(
+    ::_ConstraintPrimal1310,
+    ::MOI.VariablePrimal,
+    ::MOI.VariableIndex,
+)
+    return 1.2
+end
+
+function test_ConstraintPrimal_fallback()
+    model = MOI.Utilities.CachingOptimizer(
+        MOI.Utilities.Model{Float64}(),
+        _ConstraintPrimal1310(),
+    )
+    x = MOI.add_variable(model)
+    c = MOI.add_constraint(model, x, MOI.GreaterThan(1.0))
+    MOI.optimize!(model)
+    @test MOI.get(model, MOI.ConstraintPrimal(), c) == 1.2
+    @test MOI.get(model, MOI.ConstraintPrimal(), [c]) == [1.2]
+    return
+end
+
+function test_ConstraintPrimal_fallback_error()
+    model = MOI.Utilities.CachingOptimizer(
+        MOI.Utilities.Model{Float64}(),
+        MOI.Utilities.AUTOMATIC,
+    )
+    x = MOI.add_variable(model)
+    c = MOI.add_constraint(model, x, MOI.GreaterThan(1.0))
+    @test_throws(ErrorException, MOI.get(model, MOI.ConstraintPrimal(), c))
+    @test_throws(ErrorException, MOI.get(model, MOI.ConstraintPrimal(), [c]))
+    return
+end
+
 end  # module
 
 TestCachingOptimizer.runtests()