Sema: Undo changes in chronological order in SolverTrail::undo()

include/swift/Sema/ConstraintGraph.h

@@ -137,14 +137,12 @@ class ConstraintGraphNode {
   /// gets removed for a constraint graph.
   void retractFromInference(Constraint *constraint);
 
-  /// Re-evaluate the given constraint. This happens when there are changes
-  /// in associated type variables e.g. bound/unbound to/from a fixed type,
-  /// equivalence class changes.
-  void reintroduceToInference(Constraint *constraint);
-
-  /// Similar to \c introduceToInference(Constraint *, ...) this method is going
-  /// to notify inference that this type variable has been bound to a concrete
-  /// type.
+  /// Perform graph updates that must be undone after we bind a fixed type
+  /// to a type variable.
+  void retractFromInference(Type fixedType);
+
+  /// Perform graph updates that must be undone before we bind a fixed type
+  /// to a type variable.
   ///
   /// The reason why this can't simplify be a part of \c bindTypeVariable
   /// is related to the fact that it's sometimes expensive to re-compute
@@ -161,12 +159,18 @@ class ConstraintGraphNode {
   ///
   /// This is useful in situations when type variable gets bound and unbound,
   /// or equivalence class changes.
-  void notifyReferencingVars() const;
+  void notifyReferencingVars(
+      llvm::function_ref<void(ConstraintGraphNode &,
+                              Constraint *)> notification) const;
 
   /// Notify all of the type variables referenced by this one about a change.
   void notifyReferencedVars(
-      llvm::function_ref<void(ConstraintGraphNode &)> notification);
+      llvm::function_ref<void(ConstraintGraphNode &)> notification) const;
 
+  void updateFixedType(
+      Type fixedType,
+      llvm::function_ref<void (ConstraintGraphNode &,
+                               Constraint *)> notification) const;
   /// }
 
   /// The constraint graph this node belongs to.
@@ -261,16 +265,28 @@ class ConstraintGraph {
   /// Primitive form for SolverTrail::Change::undo().
   void removeConstraint(TypeVariableType *typeVar, Constraint *constraint);
 
+  /// Prepare to merge the given node into some other node.
+  ///
+  /// This records graph changes that must be undone after the merge has
+  /// been undone.
+  void mergeNodesPre(TypeVariableType *typeVar2);
+
   /// Merge the two nodes for the two given type variables.
   ///
   /// The type variables must actually have been merged already; this
-  /// operation merges the two nodes.
+  /// operation merges the two nodes. This also records graph changes
+  /// that must be undone before the merge can be undone.
   void mergeNodes(TypeVariableType *typeVar1, TypeVariableType *typeVar2);
 
   /// Bind the given type variable to the given fixed type.
   void bindTypeVariable(TypeVariableType *typeVar, Type fixedType);
 
-  /// Introduce the type variable's fixed type to inference.
+  /// Perform graph updates that must be undone after we bind a fixed type
+  /// to a type variable.
+  void retractFromInference(TypeVariableType *typeVar, Type fixedType);
+
+  /// Perform graph updates that must be undone before we bind a fixed type
+  /// to a type variable.
   void introduceToInference(TypeVariableType *typeVar, Type fixedType);
 
   /// Describes which constraints \c gatherConstraints should gather.

include/swift/Sema/ConstraintSystem.h

@@ -555,12 +555,7 @@ class TypeVariableType::Implementation {
   /// \param trail The record of state changes.
   void mergeEquivalenceClasses(TypeVariableType *other,
                                constraints::SolverTrail *trail) {
-    // Merge the equivalence classes corresponding to these two type
-    // variables. Always merge 'up' the constraint stack, because it is simpler.
-    if (getID() > other->getImpl().getID()) {
-      other->getImpl().mergeEquivalenceClasses(getTypeVariable(), trail);
-      return;
-    }
+    ASSERT(getID() < other->getImpl().getID());
 
     auto otherRep = other->getImpl().getRepresentative(trail);
     if (trail)

lib/Sema/CSTrail.cpp

@@ -730,21 +730,10 @@ void SolverTrail::undo(unsigned toIndex) {
   ASSERT(!UndoActive);
   UndoActive = true;
 
-  // FIXME: Undo all changes in the correct order!
   for (unsigned i = Changes.size(); i > toIndex; i--) {
     auto change = Changes[i - 1];
-    if (change.Kind == ChangeKind::UpdatedTypeVariable) {
-      LLVM_DEBUG(llvm::dbgs() << "- "; change.dump(llvm::dbgs(), CS, 0));
-      change.undo(CS);
-    }
-  }
-
-  for (unsigned i = Changes.size(); i > toIndex; i--) {
-    auto change = Changes[i - 1];
-    if (change.Kind != ChangeKind::UpdatedTypeVariable) {
-      LLVM_DEBUG(llvm::dbgs() << "- "; change.dump(llvm::dbgs(), CS, 0));
-      change.undo(CS);
-    }
+    LLVM_DEBUG(llvm::dbgs() << "- "; change.dump(llvm::dbgs(), CS, 0));
+    change.undo(CS);
   }
 
   Changes.resize(toIndex);

lib/Sema/ConstraintGraph.cpp

@@ -89,8 +89,10 @@ ConstraintGraph::lookupNode(TypeVariableType *typeVar) {
   // If this type variable is not the representative of its equivalence class,
   // add it to its representative's set of equivalences.
   auto typeVarRep = CS.getRepresentative(typeVar);
-  if (typeVar != typeVarRep)
-    mergeNodes(typeVar, typeVarRep);
+  if (typeVar != typeVarRep) {
+    mergeNodesPre(typeVar);
+    mergeNodes(typeVarRep, typeVar);
+  }
   else if (auto fixed = CS.getFixedType(typeVarRep)) {
     // Bind the type variable.
     bindTypeVariable(typeVar, fixed);
@@ -177,7 +179,9 @@ void ConstraintGraphNode::removeConstraint(Constraint *constraint) {
   Constraints.pop_back();
 }
 
-void ConstraintGraphNode::notifyReferencingVars() const {
+void ConstraintGraphNode::notifyReferencingVars(
+    llvm::function_ref<void(ConstraintGraphNode &,
+                            Constraint *)> notification) const {
   SmallVector<TypeVariableType *, 4> stack;
 
   stack.push_back(TypeVar);
@@ -199,7 +203,7 @@ void ConstraintGraphNode::notifyReferencingVars() const {
             affectedVar->getImpl().getRepresentative(/*record=*/nullptr);
 
         if (!repr->getImpl().getFixedType(/*record=*/nullptr))
-          CG[repr].reintroduceToInference(constraint);
+          notification(CG[repr], constraint);
       }
     }
   };
@@ -236,7 +240,7 @@ void ConstraintGraphNode::notifyReferencingVars() const {
 }
 
 void ConstraintGraphNode::notifyReferencedVars(
-    llvm::function_ref<void(ConstraintGraphNode &)> notification) {
+    llvm::function_ref<void(ConstraintGraphNode &)> notification) const {
   for (auto *fixedBinding : getReferencedVars()) {
     notification(CG[fixedBinding]);
   }
@@ -249,25 +253,6 @@ void ConstraintGraphNode::addToEquivalenceClass(
   if (EquivalenceClass.empty())
     EquivalenceClass.push_back(getTypeVariable());
   EquivalenceClass.append(typeVars.begin(), typeVars.end());
-
-  {
-    for (auto *newMember : typeVars) {
-      auto &node = CG[newMember];
-
-      for (auto *constraint : node.getConstraints()) {
-        introduceToInference(constraint);
-
-        if (!isUsefulForReferencedVars(constraint))
-          continue;
-
-        notifyReferencedVars([&](ConstraintGraphNode &referencedVar) {
-          referencedVar.introduceToInference(constraint);
-        });
-      }
-
-      node.notifyReferencingVars();
-    }
-  }
 }
 
 void ConstraintGraphNode::truncateEquivalenceClass(unsigned prevSize) {
@@ -343,19 +328,17 @@ void ConstraintGraphNode::retractFromInference(Constraint *constraint) {
   }
 }
 
-void ConstraintGraphNode::reintroduceToInference(Constraint *constraint) {
-  retractFromInference(constraint);
-  introduceToInference(constraint);
-}
-
-void ConstraintGraphNode::introduceToInference(Type fixedType) {
+void ConstraintGraphNode::updateFixedType(
+    Type fixedType,
+    llvm::function_ref<void (ConstraintGraphNode &,
+                             Constraint *)> notification) const {
   // Notify all of the type variables that reference this one.
   //
   // Since this type variable has been replaced with a fixed type
   // all of the concrete types that reference it are going to change,
   // which means that all of the not-yet-attempted bindings should
   // change as well.
-  notifyReferencingVars();
+  notifyReferencingVars(notification);
 
   if (!fixedType->hasTypeVariable())
     return;
@@ -371,11 +354,27 @@ void ConstraintGraphNode::introduceToInference(Type fixedType) {
     // all of the constraints that reference bound type variable.
     for (auto *constraint : getConstraints()) {
       if (isUsefulForReferencedVars(constraint))
-        node.reintroduceToInference(constraint);
+        notification(node, constraint);
     }
   }
 }
 
+void ConstraintGraphNode::retractFromInference(Type fixedType) {
+  return updateFixedType(
+      fixedType,
+      [](ConstraintGraphNode &node, Constraint *constraint) {
+        node.retractFromInference(constraint);
+      });
+}
+
+void ConstraintGraphNode::introduceToInference(Type fixedType) {
+  return updateFixedType(
+      fixedType,
+      [](ConstraintGraphNode &node, Constraint *constraint) {
+        node.introduceToInference(constraint);
+      });
+}
+
 #pragma mark Graph mutation
 
 void ConstraintGraph::removeNode(TypeVariableType *typeVar) {
@@ -486,31 +485,60 @@ void ConstraintGraph::removeConstraint(TypeVariableType *typeVar,
   OrphanedConstraints.pop_back();
 }
 
+void ConstraintGraph::mergeNodesPre(TypeVariableType *typeVar2) {
+  // Merge equivalence class from the non-representative type variable.
+  auto &nonRepNode = (*this)[typeVar2];
+
+  for (auto *newMember : nonRepNode.getEquivalenceClassUnsafe()) {
+    auto &node = (*this)[newMember];
+
+    node.notifyReferencingVars(
+      [&](ConstraintGraphNode &node, Constraint *constraint) {
+        node.retractFromInference(constraint);
+      });
+  }
+}
+
 void ConstraintGraph::mergeNodes(TypeVariableType *typeVar1, 
                                  TypeVariableType *typeVar2) {
-  assert(CS.getRepresentative(typeVar1) == CS.getRepresentative(typeVar2) &&
-         "type representatives don't match");
-
   // Retrieve the node for the representative that we're merging into.
-  auto typeVarRep = CS.getRepresentative(typeVar1);
-  auto &repNode = (*this)[typeVarRep];
+  ASSERT(CS.getRepresentative(typeVar1) == typeVar1);
 
-  // Retrieve the node for the non-representative.
-  assert((typeVar1 == typeVarRep || typeVar2 == typeVarRep) &&
-         "neither type variable is the new representative?");
-  auto typeVarNonRep = typeVar1 == typeVarRep? typeVar2 : typeVar1;
+  auto &repNode = (*this)[typeVar1];
 
   // Record the change, if there are active scopes.
   if (CS.isRecordingChanges()) {
     CS.recordChange(
       SolverTrail::Change::ExtendedEquivalenceClass(
-                        typeVarRep,
+                        typeVar1,
                         repNode.getEquivalenceClass().size()));
   }
 
   // Merge equivalence class from the non-representative type variable.
-  auto &nonRepNode = (*this)[typeVarNonRep];
-  repNode.addToEquivalenceClass(nonRepNode.getEquivalenceClassUnsafe());
+  auto &nonRepNode = (*this)[typeVar2];
+
+  auto typeVars = nonRepNode.getEquivalenceClassUnsafe();
+  repNode.addToEquivalenceClass(typeVars);
+
+  for (auto *newMember : typeVars) {
+    auto &node = (*this)[newMember];
+
+    for (auto *constraint : node.getConstraints()) {
+      repNode.introduceToInference(constraint);
+
+      if (!isUsefulForReferencedVars(constraint))
+        continue;
+
+      repNode.notifyReferencedVars([&](ConstraintGraphNode &referencedVar) {
+        referencedVar.introduceToInference(constraint);
+      });
+    }
+
+    node.notifyReferencingVars(
+      [&](ConstraintGraphNode &node, Constraint *constraint) {
+        node.introduceToInference(constraint);
+      });
+  }
 }
 
 void ConstraintGraph::bindTypeVariable(TypeVariableType *typeVar, Type fixed) {
@@ -537,6 +565,10 @@ void ConstraintGraph::bindTypeVariable(TypeVariableType *typeVar, Type fixed) {
   }
 }
 
+void ConstraintGraph::retractFromInference(TypeVariableType *typeVar, Type fixed) {
+  (*this)[typeVar].retractFromInference(fixed);
+}
+
 void ConstraintGraph::introduceToInference(TypeVariableType *typeVar, Type fixed) {
   (*this)[typeVar].introduceToInference(fixed);
 }

lib/Sema/ConstraintSystem.cpp

@@ -173,9 +173,13 @@ void ConstraintSystem::mergeEquivalenceClasses(TypeVariableType *typeVar1,
   assert(typeVar2 == getRepresentative(typeVar2) &&
          "typeVar2 is not the representative");
   assert(typeVar1 != typeVar2 && "cannot merge type with itself");
-  typeVar1->getImpl().mergeEquivalenceClasses(typeVar2, getTrail());
 
-  // Merge nodes in the constraint graph.
+  // Always merge 'up' the constraint stack, because it is simpler.
+  if (typeVar1->getImpl().getID() > typeVar2->getImpl().getID())
+    std::swap(typeVar1, typeVar2);
+
+  CG.mergeNodesPre(typeVar2);
+  typeVar1->getImpl().mergeEquivalenceClasses(typeVar2, getTrail());
   CG.mergeNodes(typeVar1, typeVar2);
 
   if (updateWorkList) {
@@ -205,6 +209,7 @@ void ConstraintSystem::assignFixedType(TypeVariableType *typeVar, Type type,
   assert(!type->hasError() &&
          "Should not be assigning a type involving ErrorType!");
 
+  CG.retractFromInference(typeVar, type);
   typeVar->getImpl().assignFixedType(type, getTrail());
 
   if (!updateState)

test/Sema/issue-46000.swift

@@ -11,7 +11,7 @@ struct Data {}
 extension DispatchData {
   func asFoundationData<T>(execute: (Data) throws -> T) rethrows -> T {
     return try withUnsafeBytes { (ptr: UnsafePointer<Int8>) -> Void in
-      // expected-error@-1 {{cannot convert return expression of type 'Void' to return type 'T'}}
+      // expected-error@-1 {{declared closure result 'Void' is incompatible with contextual type 'T'}}
       let data = Data()
       return try execute(data) // expected-error {{cannot convert value of type 'T' to closure result type 'Void'}}
     }

test/type/opaque.swift

@@ -273,11 +273,8 @@ func associatedTypeIdentity() {
   sameType(cr, dr) // expected-error {{conflicting arguments to generic parameter 'T' ('(some R).S' (result type of 'candace') vs. '(some R).S' (result type of 'doug'))}}
   sameType(gary(candace()).r_out(), gary(candace()).r_out())
   sameType(gary(doug()).r_out(), gary(doug()).r_out())
-  // TODO(diagnostics): This is not great but the problem comes from the way solver discovers and attempts bindings, if we could detect that
-  // `(some R).S` from first reference to `gary()` in inconsistent with the second one based on the parent type of `S` it would be much easier to diagnose.
   sameType(gary(doug()).r_out(), gary(candace()).r_out())
-  // expected-error@-1:12 {{conflicting arguments to generic parameter 'T' ('some R' (result type of 'doug') vs. 'some R' (result type of 'candace'))}}
-  // expected-error@-2:34 {{conflicting arguments to generic parameter 'T' ('some R' (result type of 'doug') vs. 'some R' (result type of 'candace'))}}
+  // expected-error@-1:39 {{cannot convert value of type 'some R' (result of 'candace()') to expected argument type 'some R' (result of 'doug()')}}
 }
 
 func redeclaration() -> some P { return 0 } // expected-note 2{{previously declared}}