[GSB] Diagnose redundant same-type constraints.

include/swift/AST/DiagnosticsSema.def

@@ -1601,6 +1601,13 @@ NOTE(previous_layout_constraint, none,
      "layout constraint constraint %1 : %2 %select{written here|implied here}0",
      (bool, Type, LayoutConstraint))
 
+WARNING(redundant_same_type_constraint,none,
+        "redundant same-type constraint %0 == %1", (Type, Type))
+NOTE(previous_same_type_constraint, none,
+     "previous same-type constraint %1 == %2 "
+     "%select{written here|implied here}0",
+     (bool, Type, Type))
+
 ERROR(generic_param_access,none,
       "%0 %select{must be declared %select{"
       "%select{private|fileprivate|internal|%error|%error}3|private or fileprivate}4"

include/swift/AST/GenericSignatureBuilder.h

@@ -137,6 +137,22 @@ class GenericSignatureBuilder {
     /// The members of the equivalence class.
     TinyPtrVector<PotentialArchetype *> members;
 
+    /// Describes a component within the graph of same-type constraints within
+    /// the equivalence class that is held together by derived constraints.
+    struct DerivedSameTypeComponent {
+      /// The potential archetype that acts as the anchor for this component.
+      PotentialArchetype *anchor;
+
+      /// The (best) requirement source within the component that makes the
+      /// potential archetypes in this component equivalent to the concrete
+      /// type.
+      const RequirementSource *concreteTypeSource;
+    };
+
+    /// The set of connected components within this equivalence class, using
+    /// only the derived same-type constraints in the graph.
+    std::vector<DerivedSameTypeComponent> derivedSameTypeComponents;
+
     /// Construct a new equivalence class containing only the given
     /// potential archetype (which represents itself).
     EquivalenceClass(PotentialArchetype *representative);
@@ -462,6 +478,32 @@ class GenericSignatureBuilder {
                            Diag<Type, T> redundancyDiag,
                            Diag<bool, Type, T> otherNoteDiag);
 
+  /// Check a list of constraints, removing self-derived constraints
+  /// and diagnosing redundant constraints.
+  ///
+  /// \param isSuitableRepresentative Determines whether the given constraint
+  /// is a suitable representative.
+  ///
+  /// \param checkConstraint Checks the given constraint against the
+  /// canonical constraint to determine which diagnostics (if any) should be
+  /// emitted.
+  ///
+  /// \returns the representative constraint.
+  template<typename T, typename DiagT>
+  Constraint<T> checkConstraintList(
+                           ArrayRef<GenericTypeParamType *> genericParams,
+                           std::vector<Constraint<T>> &constraints,
+                           llvm::function_ref<bool(const Constraint<T> &)>
+                             isSuitableRepresentative,
+                           llvm::function_ref<ConstraintRelation(const T&)>
+                             checkConstraint,
+                           Optional<Diag<unsigned, Type, DiagT, DiagT>>
+                             conflictingDiag,
+                           Diag<Type, DiagT> redundancyDiag,
+                           Diag<bool, Type, DiagT> otherNoteDiag,
+                           llvm::function_ref<DiagT(const T&)> diagValue,
+                           bool removeSelfDerived);
+
   /// Check the concrete type constraints within the equivalence
   /// class of the given potential archetype.
   void checkConcreteTypeConstraints(