[WIP] New conflict attributes

Author: joaquimg

src/attributes.jl

@@ -1844,6 +1844,16 @@ struct ConflictStatus <: AbstractModelAttribute end
 
 attribute_value_type(::ConflictStatus) = ConflictStatusCode
 
+"""
+    ConflictCount()
+
+An [`AbstractModelAttribute`](@ref) for the number of conflicts found by the
+solver in the most recent call to [`compute_conflict!`](@ref).
+"""
+struct ConflictCount <: AbstractModelAttribute end
+
+attribute_value_type(::ConflictCount) = Int
+
 """
     ListOfVariableAttributesSet()
 
@@ -2677,13 +2687,78 @@ end
 
 A constraint attribute to query the [`ConflictParticipationStatusCode`](@ref)
 indicating whether the constraint participates in the conflict.
+
+## `conflict_index`
+
+The optimizer may return multiple conflicts. See [`ConflictCount`](@ref)
+for querying the number of conflicts found.
+
+## Implementation
+
+Optimizers should implement the following methods:
+```julia
+MOI.get(::Optimizer, ::MOI.ConstraintConflictStatus, ::MOI.ConstraintIndex)::T
+```
+They should not implement [`set`](@ref) or [`supports`](@ref).
 """
-struct ConstraintConflictStatus <: AbstractConstraintAttribute end
+struct ConstraintConflictStatus <: AbstractConstraintAttribute
+    conflict_index::Int
+    ConstraintConflictStatus(conflict_index = 1) = new(conflict_index)
+end
 
 function attribute_value_type(::ConstraintConflictStatus)
     return ConflictParticipationStatusCode
 end
 
+"""
+    ListOfConstraintIndicesInConflict(conflict_index::Int = 1)
+
+An [`AbstractModelAttribute`](@ref) for the `Vector{ConstraintIndex}` of all
+constraints that participate in the conflict with index `conflict_index`.
+
+The `conflict_index` is 1 by default, but it can be set to any value between
+1 and the number of conflicts found by the solver, as indicated by
+[`ConflictCount`](@ref).
+
+## Implementation
+Optimizers may implement the following methods:
+```julia
+MOI.get(
+    ::Optimizer,
+    ::MOI.ListOfConstraintIndicesInConflict,
+)::Vector{MOI.ConstraintIndex}
+```
+A ineficient fallback is available for `get`.
+They should not implement [`set`](@ref) or [`supports`](@ref).
+"""
+struct ListOfConstraintIndicesInConflict <: MOI.AbstractModelAttribute
+    conflict_index::Int
+    ListOfConstraintIndicesInConflict(conflict_index = 1) = new(conflict_index)
+end
+
+function attribute_value_type(::ListOfConstraintIndicesInConflict)
+    return Vector{ConstraintIndex}
+end
+
+function get(
+    model::ModelLike,
+    attr::ListOfConstraintIndicesInConflict,
+)::Vector{ConstraintIndex}
+    result = Vector{ConstraintIndex}()
+    conflict_index = attr.conflict_index
+    constraint_types = get(model, ListOfConstraintTypesPresent())
+    for (F, S) in constraint_types
+        constraints = MOI.get(model, ListOfConstraintIndices{F,S}())
+        for con in constraints
+            status = get(model, ConstraintConflictStatus(conflict_index), con)
+            if status == NOT_IN_CONFLICT
+                push!(result, con)
+            end
+        end
+    end
+    return result
+end
+
 """
     UserDefinedFunction(name::Symbol, arity::Int) <: AbstractModelAttribute