[ConstraintSystem] Introduce TVO_PackExpansion

This flag indicates that a type variable could only be bound
to PackExpansionType and nothing else.

Author: xedin

include/swift/AST/Types.h

@@ -400,12 +400,12 @@ class alignas(1 << TypeAlignInBits) TypeBase
     NumProtocols : 16
   );
 
-  SWIFT_INLINE_BITFIELD_FULL(TypeVariableType, TypeBase, 6+32,
+  SWIFT_INLINE_BITFIELD_FULL(TypeVariableType, TypeBase, 7+31,
     /// Type variable options.
-    Options : 6,
+    Options : 7,
     : NumPadBits,
     /// The unique number assigned to this type variable.
-    ID : 32
+    ID : 31
   );
 
   SWIFT_INLINE_BITFIELD(SILFunctionType, TypeBase, NumSILExtInfoBits+1+4+1+2+1+1,

include/swift/Sema/ConstraintSystem.h

@@ -335,6 +335,9 @@ enum TypeVariableOptions {
 
   /// Whether the type variable can be bound to a pack type or not.
   TVO_CanBindToPack = 0x20,
+
+  /// Whether the type variable can be bound only to a pack expansion type.
+  TVO_PackExpansion = 0x40,
 };
 
 /// The implementation object for a type variable used within the
@@ -407,6 +410,9 @@ class TypeVariableType::Implementation {
   /// Whether this type variable can bind to a PackType.
   bool canBindToPack() const { return getRawOptions() & TVO_CanBindToPack; }
 
+  /// Whether this type variable can bind only to PackExpansionType.
+  bool isPackExpansion() const { return getRawOptions() & TVO_PackExpansion; }
+
   /// Whether this type variable prefers a subtype binding over a supertype
   /// binding.
   bool prefersSubtypeBinding() const {
@@ -650,6 +656,7 @@ class TypeVariableType::Implementation {
     ENTRY(TVO_CanBindToHole, "hole");
     ENTRY(TVO_PrefersSubtypeBinding, "");
     ENTRY(TVO_CanBindToPack, "pack");
+    ENTRY(TVO_PackExpansion, "pack expansion");
     }
   #undef ENTRY
   }

lib/Sema/CSSimplify.cpp

@@ -2086,9 +2086,7 @@ static bool isPackExpansionType(Type type) {
     return true;
 
   if (auto *typeVar = type->getAs<TypeVariableType>())
-    return typeVar->getImpl()
-        .getLocator()
-        ->isLastElement<LocatorPathElt::PackExpansionType>();
+    return typeVar->getImpl().isPackExpansion();
 
   return false;
 }
@@ -4311,6 +4309,11 @@ ConstraintSystem::matchTypesBindTypeVar(
     type = PlaceholderType::get(typeVar->getASTContext(), typeVar);
   }
 
+  // FIXME: Add a fix for this.
+  if (typeVar->getImpl().isPackExpansion() && !type->is<PackExpansionType>()) {
+    return getTypeMatchFailure(locator);
+  }
+
   // Binding to a pack expansion type is always an error in Swift 6 mode.
   // This indicates that a pack expansion expression was used in a context
   // that doesn't support it.
@@ -4322,7 +4325,7 @@ ConstraintSystem::matchTypesBindTypeVar(
   // generic parameter - `init(_ data: repeat each T)`.
   //
   // See BindTupleOfFunctionParams constraint for more details.
-  if (type->is<PackExpansionType>()) {
+  if (!typeVar->getImpl().isPackExpansion() && type->is<PackExpansionType>()) {
     bool representsParameterList =
         typeVar->getImpl()
             .getLocator()

lib/Sema/TypeCheckConstraints.cpp

@@ -69,6 +69,8 @@ void TypeVariableType::Implementation::print(llvm::raw_ostream &OS) {
     bindingOptions.push_back(TypeVariableOptions::TVO_CanBindToHole);
   if (canBindToPack())
     bindingOptions.push_back(TypeVariableOptions::TVO_CanBindToPack);
+  if (isPackExpansion())
+    bindingOptions.push_back(TypeVariableOptions::TVO_PackExpansion);
   if (!bindingOptions.empty()) {
     OS << " [allows bindings to: ";
     interleave(bindingOptions, OS,