Sema: Split off ConstraintGraph::addTypeVariable() from ::operator[]

Also, since we add a vertex immediately after introducing a new type
variable, the code path to handle the case where the type variable
had a parent or fixed type was actually dead.

Author: slavapestov

include/swift/Sema/ConstraintGraph.h

@@ -244,7 +244,10 @@ class ConstraintGraph {
   /// Retrieve the constraint system this graph describes.
   ConstraintSystem &getConstraintSystem() const { return CS; }
 
-  /// Access the node corresponding to the given type variable.
+  /// Add a new vertex to the graph.
+  void addTypeVariable(TypeVariableType *typeVar);
+
+  /// Look up the vertex associated with the given type variable.
   ConstraintGraphNode &operator[](TypeVariableType *typeVar);
 
   /// Add a new constraint to the graph.

lib/Sema/ConstraintGraph.cpp

@@ -50,18 +50,16 @@ ConstraintGraph::~ConstraintGraph() {
 
 #pragma mark Graph accessors
 
-ConstraintGraphNode &
-ConstraintGraph::operator[](TypeVariableType *typeVar) {
+void ConstraintGraph::addTypeVariable(TypeVariableType *typeVar) {
   // Check whether we've already created a node for this type variable.
   auto &impl = typeVar->getImpl();
-  if (auto nodePtr = impl.getGraphNode()) {
-    assert(impl.getGraphIndex() < TypeVariables.size() && "Out-of-bounds index");
-    assert(TypeVariables[impl.getGraphIndex()] == typeVar && 
-           "Type variable mismatch");
-    ASSERT(nodePtr->TypeVar == typeVar &&
-           "Use-after-free");
-    return *nodePtr;
-  }
+
+  // ComponentStep::Scope re-introduces type variables that are already
+  // in the graph, but not in ConstraintSystem::TypeVariables.
+  if (impl.getGraphNode())
+    return;
+
+  ASSERT(!impl.hasRepresentativeOrFixed());
 
   // Allocate the new node.
   ConstraintGraphNode *nodePtr;
@@ -85,19 +83,21 @@ ConstraintGraph::operator[](TypeVariableType *typeVar) {
   // create new nodes during an undo.
   if (CS.solverState)
     CS.recordChange(SolverTrail::Change::AddedTypeVariable(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) {
-    mergeNodesPre(typeVar);
-    mergeNodes(typeVarRep, typeVar);
-  }
-  else if (auto fixed = CS.getFixedType(typeVarRep)) {
-    // Bind the type variable.
-    bindTypeVariable(typeVar, fixed);
+ConstraintGraphNode &
+ConstraintGraph::operator[](TypeVariableType *typeVar) {
+  // Check whether we've already created a node for this type variable.
+  auto &impl = typeVar->getImpl();
+  auto *nodePtr = impl.getGraphNode();
+  if (!nodePtr) {
+    llvm::errs() << "Type variable $T" << impl.getID() << " not in constraint graph\n";
+    abort();
   }
-
+  ASSERT(nodePtr->TypeVar == typeVar && "Use-after-free");
+  DEBUG_ASSERT(impl.getGraphIndex() < TypeVariables.size() && "Out-of-bounds index");
+  DEBUG_ASSERT(TypeVariables[impl.getGraphIndex()] == typeVar &&
+               "Type variable mismatch");
   return *nodePtr;
 }
 

lib/Sema/ConstraintSystem.cpp

@@ -162,7 +162,7 @@ void ConstraintSystem::addTypeVariable(TypeVariableType *typeVar) {
   TypeVariables.insert(typeVar);
 
   // Notify the constraint graph.
-  (void)CG[typeVar];
+  CG.addTypeVariable(typeVar);
 }
 
 void ConstraintSystem::mergeEquivalenceClasses(TypeVariableType *typeVar1,