When unifying two type variables, make sure the new representative type

variable has the must-be-materializable bit set if the old one does.
When assigning a fixed type to a type variable that must be
materializable, transfer the bit to any type variables within the fixed
type, as appropriate.

Add Options field to SavedTypeVariableBinding to save/restore type
variable options during solution.

<rdar://problem/21026806> Propagate MustBeMaterializable bit among type variables appropriately

Swift SVN r28883

lib/Sema/CSSimplify.cpp

@@ -1373,14 +1373,17 @@ ConstraintSystem::matchTypes(Type type1, Type type2, TypeMatchKind kind,
 
         // Check whether the type variable must be bound to a materializable
         // type.
-        if (typeVar1->getImpl().mustBeMaterializable() &&
-            !type2->isMaterializable()) {
-          if (shouldRecordFailures()) {
-            // TODO: customize error message for closure vs. generic param
-            recordFailure(getConstraintLocator(locator),  
-                          Failure::IsNotMaterializable, type2);
+        if (typeVar1->getImpl().mustBeMaterializable()) {
+          if (!type2->isMaterializable()) {
+            if (shouldRecordFailures()) {
+              // TODO: customize error message for closure vs. generic param
+              recordFailure(getConstraintLocator(locator),  
+                            Failure::IsNotMaterializable, type2);
+            }
+            return SolutionKind::Error;
+          } else {
+            setMustBeMaterializableRecursive(type2);
           }
-          return SolutionKind::Error;
         }
 
         // A constraint that binds any pointer to a void pointer is

lib/Sema/ConstraintSystem.cpp

@@ -18,6 +18,7 @@
 #include "ConstraintSystem.h"
 #include "ConstraintGraph.h"
 #include "swift/AST/ArchetypeBuilder.h"
+#include "swift/AST/TypeWalker.h"
 #include "llvm/ADT/SmallString.h"
 
 using namespace swift;
@@ -127,6 +128,36 @@ void ConstraintSystem::assignFixedType(TypeVariableType *typeVar, Type type,
   addTypeVariableConstraintsToWorkList(typeVar);
 }
 
+void ConstraintSystem::setMustBeMaterializableRecursive(Type type)
+{
+  assert(type->isMaterializable() &&
+         "argument to setMustBeMateriziable may not be inherently "
+         "non-materializable");
+  class Walker : public TypeWalker {
+    ConstraintSystem &CS;
+  public:
+    Walker(ConstraintSystem &CS) : CS(CS) {}
+
+    virtual Action walkToTypePre(Type ty) override {
+      if (auto varTy = dyn_cast<TypeVariableType>(ty.getPointer())) {
+        varTy->getImpl().setMustBeMaterializable(CS.getSavedBindings());
+        return Action::Continue;
+      } else if (isa<FunctionType>(ty.getPointer()) ||
+                 isa<PolymorphicFunctionType>(ty.getPointer()) ||
+                 isa<DynamicSelfType>(ty.getPointer())) {
+        return Action::SkipChildren;
+      } else if (isa<LValueType>(ty.getPointer()) ||
+                 isa<InOutType>(ty.getPointer())) {
+        llvm_unreachable("attempt to setMustBeMaterializableRecursive on "
+                         "non-materializable type");
+      } else {
+        return Action::Continue;
+      }
+    }
+  };
+  simplifyType(type).walk(Walker(*this));
+}
+
 void ConstraintSystem::addTypeVariableConstraintsToWorkList(
        TypeVariableType *typeVar) {
   // Gather the constraints affected by a change to this type variable.

lib/Sema/ConstraintSystem.h

@@ -70,6 +70,9 @@ class SavedTypeVariableBinding {
   /// \brief The parent or fixed type.
   llvm::PointerUnion<TypeVariableType *, TypeBase *> ParentOrFixed;
 
+  /// \brief The type variable options.
+  unsigned Options : 3;
+
 public:
   explicit SavedTypeVariableBinding(TypeVariableType *typeVar);
 
@@ -287,19 +290,20 @@ public:
                                constraints::SavedTypeVariableBindings *record) {
     // 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()) {
-      auto rep = other->getImpl().getRepresentative(record);
+    if (getID() > other->getImpl().getID()) {
+      other->getImpl().mergeEquivalenceClasses(getTypeVariable(), record);
+      return;
+    }
+
+    auto otherRep = other->getImpl().getRepresentative(record);
+    if (record)
+      otherRep->getImpl().recordBinding(*record);
+    otherRep->getImpl().ParentOrFixed = getTypeVariable();
+    assert(otherRep->getImpl().canBindToLValue() == canBindToLValue());
+    if (!mustBeMaterializable() && otherRep->getImpl().mustBeMaterializable()) {
       if (record)
-        rep->getImpl().recordBinding(*record);
-      rep->getImpl().ParentOrFixed = getTypeVariable();
-      assert(rep->getImpl().canBindToLValue() == canBindToLValue());
-    } else {
-      auto rep = getRepresentative(record);
-      if (record)
-        rep->getImpl().recordBinding(*record);
-      rep->getImpl().ParentOrFixed = other;
-      assert(rep->getImpl().canBindToLValue()
-               == other->getImpl().canBindToLValue());
+        recordBinding(*record);
+      Options |= TVO_MustBeMaterializable;
     }
   }
 
@@ -335,6 +339,15 @@ public:
     rep->getImpl().ParentOrFixed = type.getPointer();
   }
 
+  void setMustBeMaterializable(constraints::SavedTypeVariableBindings *record) {
+    auto rep = getRepresentative(record);
+    if (!rep->getImpl().mustBeMaterializable()) {
+      if (record)
+        rep->getImpl().recordBinding(*record);
+      rep->getImpl().Options |= TVO_MustBeMaterializable;
+    }
+  }
+
   /// \brief Print the type variable to the given output stream.
   void print(llvm::raw_ostream &OS);
 };
@@ -1909,6 +1922,11 @@ public:
   void assignFixedType(TypeVariableType *typeVar, Type type,
                        bool updateState = true);
 
+  // \brief Set the TVO_MustBeMaterializable bit on all type variables
+  // necessary to ensure that the type in question is materializable in a
+  // viable solution.
+  void setMustBeMaterializableRecursive(Type type);
+  
   /// \brief Determine if the type in question is an Array<T>.
   bool isArrayType(Type t);
 

lib/Sema/TypeCheckConstraints.cpp

@@ -79,9 +79,11 @@ TypeBase *TypeVariableType::getBaseBeingSubstituted() {
 }
 
 SavedTypeVariableBinding::SavedTypeVariableBinding(TypeVariableType *typeVar)
-  : TypeVar(typeVar), ParentOrFixed(typeVar->getImpl().ParentOrFixed) { }
+  : TypeVar(typeVar), ParentOrFixed(typeVar->getImpl().ParentOrFixed),
+    Options(typeVar->getImpl().Options) { }
 
 void SavedTypeVariableBinding::restore() {
+  TypeVar->getImpl().Options = Options;
   TypeVar->getImpl().ParentOrFixed = ParentOrFixed;
 }
 
@@ -2170,6 +2172,8 @@ void ConstraintSystem::print(raw_ostream &out) {
     tv->getImpl().print(out);
     if (tv->getImpl().canBindToLValue())
       out << " [lvalue allowed]";
+    if (tv->getImpl().mustBeMaterializable())
+      out << " [must be materializable]";
     auto rep = getRepresentative(tv);
     if (rep == tv) {
       if (auto fixed = getFixedType(tv)) {