Sema: Introduce TVO_CanBindToPack type variable flag

Author: slavapestov

include/swift/AST/Types.h

@@ -384,9 +384,9 @@ class alignas(1 << TypeAlignInBits) TypeBase
     NumProtocols : 16
   );
 
-  SWIFT_INLINE_BITFIELD_FULL(TypeVariableType, TypeBase, 5+32,
+  SWIFT_INLINE_BITFIELD_FULL(TypeVariableType, TypeBase, 6+32,
     /// Type variable options.
-    Options : 5,
+    Options : 6,
     : NumPadBits,
     /// The unique number assigned to this type variable.
     ID : 32

include/swift/Sema/ConstraintSystem.h

@@ -330,6 +330,9 @@ enum TypeVariableOptions {
   /// Whether a more specific deduction for this type variable implies a
   /// better solution to the constraint system.
   TVO_PrefersSubtypeBinding = 0x10,
+
+  /// Whether the type variable can be bound to a pack type or not.
+  TVO_CanBindToPack = 0x20,
 };
 
 /// The implementation object for a type variable used within the
@@ -387,18 +390,21 @@ class TypeVariableType::Implementation {
            && "Truncation");
   }
 
-  /// Whether this type variable can bind to an lvalue type.
+  /// Whether this type variable can bind to an LValueType.
   bool canBindToLValue() const { return getRawOptions() & TVO_CanBindToLValue; }
 
-  /// Whether this type variable can bind to an inout type.
+  /// Whether this type variable can bind to an InOutType.
   bool canBindToInOut() const { return getRawOptions() & TVO_CanBindToInOut; }
 
-  /// Whether this type variable can bind to an inout type.
+  /// Whether this type variable can bind to a noescape FunctionType.
   bool canBindToNoEscape() const { return getRawOptions() & TVO_CanBindToNoEscape; }
 
-  /// Whether this type variable can bind to a hole.
+  /// Whether this type variable can bind to a PlaceholderType.
   bool canBindToHole() const { return getRawOptions() & TVO_CanBindToHole; }
 
+  /// Whether this type variable can bind to a PackType.
+  bool canBindToPack() const { return getRawOptions() & TVO_CanBindToPack; }
+
   /// Whether this type variable prefers a subtype binding over a supertype
   /// binding.
   bool prefersSubtypeBinding() const {
@@ -638,6 +644,7 @@ class TypeVariableType::Implementation {
     ENTRY(TVO_CanBindToNoEscape, "noescape");
     ENTRY(TVO_CanBindToHole, "hole");
     ENTRY(TVO_PrefersSubtypeBinding, "");
+    ENTRY(TVO_CanBindToPack, "pack");
     }
   #undef ENTRY
   }

lib/Sema/ConstraintSystem.cpp

@@ -1735,8 +1735,12 @@ TypeVariableType *ConstraintSystem::openGenericParameter(
   auto *paramLocator = getConstraintLocator(
       locator.withPathElement(LocatorPathElt::GenericParameter(parameter)));
 
-  auto typeVar = createTypeVariable(paramLocator, TVO_PrefersSubtypeBinding |
-                                                      TVO_CanBindToHole);
+  unsigned options = (TVO_PrefersSubtypeBinding |
+                      TVO_CanBindToHole);
+  if (parameter->isParameterPack())
+    options |= TVO_CanBindToPack;
+
+  auto typeVar = createTypeVariable(paramLocator, options);
   auto result = replacements.insert(std::make_pair(
       cast<GenericTypeParamType>(parameter->getCanonicalType()), typeVar));