Sema: Collect defaults in PotentialBindings::infer()

Author: slavapestov

include/swift/Sema/CSBindings.h

@@ -248,6 +248,9 @@ struct PotentialBindings {
   /// The set of protocol conformance requirements imposed on this type variable.
   llvm::SmallVector<Constraint *, 4> Protocols;
 
+  /// The set of fallback constraints imposed on this type variable.
+  llvm::SmallVector<Constraint *, 2> Defaults;
+
   ASTNode AssociatedCodeCompletionToken = ASTNode();
 
   /// Add a potential binding to the list of bindings,

include/swift/Sema/CSTrail.def

@@ -78,6 +78,7 @@ GRAPH_NODE_CHANGE(InferredBindings)
 GRAPH_NODE_CHANGE(RetractedBindings)
 GRAPH_NODE_CHANGE(RetractedDelayedBy)
 GRAPH_NODE_CHANGE(RetractedProtocol)
+GRAPH_NODE_CHANGE(RetractedDefault)
 
 BINDING_RELATION_CHANGE(RetractedAdjacentVar)
 BINDING_RELATION_CHANGE(RetractedSubtypeOf)

lib/Sema/CSBindings.cpp

@@ -61,18 +61,17 @@ BindingSet::BindingSet(ConstraintSystem &CS, TypeVariableType *TypeVar,
       addLiteralRequirement(constraint);
       break;
 
-    case ConstraintKind::Defaultable:
-    case ConstraintKind::FallbackType:
-      // Do these in a separate pass.
-      if (isDirectRequirement(CS, TypeVar, constraint))
-        addDefault(constraint);
-      break;
-
     default:
       break;
     }
   }
 
+  for (auto *constraint : info.Defaults) {
+    // Do these in a separate pass.
+    if (isDirectRequirement(CS, TypeVar, constraint))
+      addDefault(constraint);
+  }
+
   for (auto &entry : info.AdjacentVars)
     AdjacentVars.insert(entry.first);
 }
@@ -2090,9 +2089,12 @@ void PotentialBindings::infer(ConstraintSystem &CS,
   case ConstraintKind::SameShape:
   case ConstraintKind::MaterializePackExpansion:
   case ConstraintKind::LiteralConformsTo:
+    // Constraints from which we can't do anything.
+    break;
+
   case ConstraintKind::Defaultable:
   case ConstraintKind::FallbackType:
-    // Constraints from which we can't do anything.
+    Defaults.push_back(constraint);
     break;
 
   // For now let's avoid inferring protocol requirements from
@@ -2237,6 +2239,10 @@ void PotentialBindings::retract(ConstraintSystem &CS,
       llvm::remove_if(Protocols, CALLBACK(RetractedProtocol)),
       Protocols.end());
 
+  Defaults.erase(
+      llvm::remove_if(Defaults, CALLBACK(RetractedDefault)),
+      Defaults.end());
+
 #define PAIR_CALLBACK(ChangeKind)                                              \
   [&](std::pair<TypeVariableType *, Constraint *> pair) {                      \
     if (pair.second == constraint) {                                           \

lib/Sema/CSTrail.cpp

@@ -543,6 +543,11 @@ void SolverTrail::Change::undo(ConstraintSystem &cs) const {
         .Protocols.push_back(TheConstraint.Constraint);
     break;
 
+  case ChangeKind::RetractedDefault:
+    cg[TheConstraint.TypeVar].getPotentialBindings()
+        .Defaults.push_back(TheConstraint.Constraint);
+    break;
+
   case ChangeKind::RetractedAdjacentVar:
     cg[BindingRelation.TypeVar].getPotentialBindings()
         .AdjacentVars.emplace_back(BindingRelation.OtherTypeVar,