[Bridges] fix IndicatorToMILPBridge when z is not binary (#2857)

Author: odow

src/Bridges/Constraint/bridges/IndicatorToMILPBridge.jl

@@ -188,6 +188,18 @@ end
 
 MOI.Bridges.needs_final_touch(::IndicatorToMILPBridge) = true
 
+function _is_binary(model::MOI.ModelLike, f::MOI.AbstractScalarFunction)
+    x = convert(MOI.VariableIndex, f)
+    return _is_binary(model, x)
+end
+
+function _is_binary(model::MOI.ModelLike, x::MOI.VariableIndex)
+    return MOI.is_valid(
+        model,
+        MOI.ConstraintIndex{MOI.VariableIndex,MOI.ZeroOne}(x.value),
+    )
+end
+
 function MOI.Bridges.final_touch(
     bridge::IndicatorToMILPBridge{T,F},
     model::MOI.ModelLike,
@@ -198,6 +210,10 @@ function MOI.Bridges.final_touch(
     ret = MOI.Utilities.get_bounds(model, bounds, fi)
     if ret === nothing
         throw(MOI.Bridges.BridgeRequiresFiniteDomainError(bridge, fi))
+    elseif !_is_binary(model, scalars[1])
+        error(
+            "Unable to reformulate indicator constraint to a MILP. The indicator variable must be binary.",
+        )
     end
     if bridge.slack === nothing
         # This is the first time calling final_touch

test/Bridges/Constraint/IndicatorToMILPBridge.jl

@@ -301,6 +301,25 @@ function test_delete_before_final_touch()
     return
 end
 
+function test_runtests_error_not_binary()
+    inner = MOI.Utilities.Model{Int}()
+    model = MOI.Bridges.Constraint.IndicatorToMILP{Int}(inner)
+    x = MOI.add_variables(model, 2)
+    MOI.add_constraint(model, x[2], MOI.Interval(0, 4))
+    c = MOI.add_constraint(
+        model,
+        MOI.VectorOfVariables(x),
+        MOI.Indicator{MOI.ACTIVATE_ON_ZERO}(MOI.GreaterThan(2)),
+    )
+    @test_throws(
+        ErrorException(
+            "Unable to reformulate indicator constraint to a MILP. The indicator variable must be binary.",
+        ),
+        MOI.Bridges.final_touch(model),
+    )
+    return
+end
+
 end  # module
 
 TestConstraintIndicatorToMILP.runtests()