[Bridges] Fix ListOfVariablesWithAttributeSet (#2380)

Author: blegat

src/Bridges/bridge_optimizer.jl

@@ -894,6 +894,22 @@ function MOI.get(
     return unbridged_function(b, MOI.get(b.model, attr))
 end
 
+function MOI.get(
+    b::AbstractBridgeOptimizer,
+    attr::MOI.ListOfVariablesWithAttributeSet,
+)
+    if Variable.has_bridges(Variable.bridges(b))
+        # If there are variable bridges, `MOI.get(b.model, attr)`
+        # will return a list containing solver variables that do not
+        # correspond to any user variables.
+        # We choose the easy option of simply returning all variables
+        # for now.
+        return MOI.get(b, MOI.ListOfVariableIndices())
+    else
+        return unbridged_function(b, MOI.get(b.model, attr))
+    end
+end
+
 function MOI.get(
     b::AbstractBridgeOptimizer,
     attr::MOI.ListOfConstraintsWithAttributeSet{F,S,MOI.ConstraintName},

test/Bridges/bridge_optimizer.jl

@@ -1158,6 +1158,24 @@ function test_variable_bridge_constraint_attribute()
     return
 end
 
+function test_ListOfVariablesWithAttributeSet(T = Float64)
+    uf = MOI.Utilities.UniversalFallback(MOI.Utilities.Model{T}())
+    model = MOI.Bridges.full_bridge_optimizer(uf, T)
+    x = MOI.add_variables(model, 2)
+    MOI.set(model, MOI.VariableName(), x[1], "x")
+    # Passed through to Model with no special support
+    attr = MOI.ListOfVariablesWithAttributeSet(MOI.VariableName())
+    @test MOI.get(model, attr) == x
+    # Handled by UniversalFallback
+    attr = MOI.ListOfVariablesWithAttributeSet(MOI.VariablePrimalStart())
+    # ... no attributes set
+    @test MOI.get(model, attr) == MOI.VariableIndex[]
+    # ... one attribute set
+    MOI.set(model, MOI.VariablePrimalStart(), x[2], 1.0)
+    @test MOI.get(model, attr) == [x[2]]
+    return
+end
+
 end  # module
 
 TestBridgeOptimizer.runtests()

test/Bridges/lazy_bridge_optimizer.jl

@@ -328,11 +328,6 @@ function test_MOI_runtests_StandardSDPAModel()
             # fix would require that a bridge optimizer has a try-catch for this
             # error.
             "test_model_ScalarFunctionConstantNotZero",
-            # The error is:
-            # Cannot substitute `MOI.VariableIndex(1)` as it is bridged into `0.0 + 1.0 MOI.VariableIndex(-1)`.
-            # This seems okay. We can't get a list of variables if they are
-            # bridged.
-            "test_model_ListOfVariablesWithAttributeSet",
         ],
     )
     return