[WIP] Always use PackTypes as the substitutions for type parameter packs

This simplifies the representation and allows clients to handle fewer
cases.  It also removes an ambiguity in the representation which could
lead us to have two canonical types for the same type.

This is definitely not working yet, but I'm not making progress on
it quickly enough to unblock what we need to unblock; it'll have to
be fixed in parallel.

Author: rjmccall

include/swift/AST/Types.h

@@ -6699,6 +6699,10 @@ class PackType final : public TypeBase, public llvm::FoldingSetNode,
                        TypeArrayView<GenericTypeParamType> params,
                        ArrayRef<Type> args);
 
+  /// Given a pack parameter or pack archetype `T`, produce the
+  /// pack type `Pack{repeat each T}`.
+  static PackType *getSingletonPackExpansion(Type packParam);
+
 public:
   /// Retrieves the number of elements in this pack.
   unsigned getNumElements() const { return Bits.PackType.Count; }

lib/AST/ASTContext.cpp

@@ -3302,6 +3302,13 @@ PackType *PackType::getEmpty(const ASTContext &C) {
   return cast<PackType>(CanType(C.TheEmptyPackType));
 }
 
+PackType *PackType::getSingletonPackExpansion(Type param) {
+  assert((param->is<GenericTypeParamType>() &&
+          param->castTo<GenericTypeParamType>()->isParameterPack()) ||
+         (param->is<PackArchetypeType>()));
+  return get(param->getASTContext(), {PackExpansionType::get(param, param)});
+}
+
 CanPackType CanPackType::get(const ASTContext &C, ArrayRef<CanType> elements) {
   SmallVector<Type, 8> ncElements(elements.begin(), elements.end());
   return CanPackType(PackType::get(C, ncElements));

lib/AST/Decl.cpp

@@ -4657,8 +4657,12 @@ static Type computeNominalType(NominalTypeDecl *decl, DeclTypeKind kind) {
       // the generic parameter list directly instead of looking
       // at the signature.
       SmallVector<Type, 4> args;
-      for (auto param : decl->getGenericParams()->getParams())
-        args.push_back(param->getDeclaredInterfaceType());
+      for (auto param : decl->getGenericParams()->getParams()) {
+        auto argTy = param->getDeclaredInterfaceType();
+        if (param->isParameterPack())
+          argTy = PackType::getSingletonPackExpansion(argTy);
+        args.push_back(argTy);
+      }
 
       return BoundGenericType::get(decl, ParentTy, args);
     }

lib/AST/GenericEnvironment.cpp

@@ -681,10 +681,37 @@ GenericEnvironment::mapElementTypeIntoPackContext(Type type) const {
   }, LookUpConformanceInSignature(sig.getPointer()));
 }
 
+namespace {
+/// A function suitable for use as a \c TypeSubstitutionFn that produces
+/// correct forwarding substitutions for a generic environment.
+///
+/// This differs from QueryInterfaceTypeSubstitutions only in that it
+/// always produces PackTypes for pack parameters.
+class BuildForwardingSubstitutions {
+  QueryInterfaceTypeSubstitutions Query;
+
+public:
+  BuildForwardingSubstitutions(const GenericEnvironment *self)
+    : Query(self) { }
+
+  Type operator()(SubstitutableType *type) const;
+};
+} // end anonymous namespace
+
+Type BuildForwardingSubstitutions::operator()(SubstitutableType *type) const {
+  if (auto resultType = Query(type)) {
+    auto param = type->castTo<GenericTypeParamType>();
+    if (!param->isParameterPack())
+      return resultType;
+    return PackType::getSingletonPackExpansion(resultType);
+  }
+  return Type();
+}
+
 SubstitutionMap GenericEnvironment::getForwardingSubstitutionMap() const {
   auto genericSig = getGenericSignature();
   return SubstitutionMap::get(genericSig,
-                              QueryInterfaceTypeSubstitutions(this),
+                              BuildForwardingSubstitutions(this),
                               MakeAbstractConformanceForGenericType());
 }
 

lib/AST/GenericSignature.cpp

@@ -566,10 +566,13 @@ unsigned GenericParamKey::findIndexIn(
 
 SubstitutionMap GenericSignatureImpl::getIdentitySubstitutionMap() const {
   return SubstitutionMap::get(const_cast<GenericSignatureImpl *>(this),
-                              [](SubstitutableType *t) -> Type {
-                                return Type(cast<GenericTypeParamType>(t));
-                              },
-                              MakeAbstractConformanceForGenericType());
+    [](SubstitutableType *t) -> Type {
+      auto param = cast<GenericTypeParamType>(t);
+      if (!param->isParameterPack())
+        return param;
+      return PackType::getSingletonPackExpansion(param);
+    },
+    MakeAbstractConformanceForGenericType());
 }
 
 GenericTypeParamType *GenericSignatureImpl::getSugaredType(

lib/AST/SubstitutionMap.cpp

@@ -226,6 +226,11 @@ SubstitutionMap SubstitutionMap::get(GenericSignature genericSig,
 
     // Record the replacement.
     Type replacement = Type(gp).subst(subs, lookupConformance);
+
+    assert((!replacement || replacement->hasError() ||
+            gp->isParameterPack() == replacement->is<PackType>()) &&
+           "replacement for pack parameter must be a pack type");
+
     replacementTypes.push_back(replacement);
   });
 

lib/AST/Type.cpp

@@ -4489,6 +4489,24 @@ operator()(CanType dependentType, Type conformingReplacementType,
 ProtocolConformanceRef MakeAbstractConformanceForGenericType::
 operator()(CanType dependentType, Type conformingReplacementType,
            ProtocolDecl *conformedProtocol) const {
+  // The places that use this can also produce conformance packs, generally
+  // just for singleton pack expansions.
+  if (auto conformingPack = conformingReplacementType->getAs<PackType>()) {
+    SmallVector<ProtocolConformanceRef, 4> conformances;
+    for (auto conformingPackElt : conformingPack->getElementTypes()) {
+      // Look through pack expansions; there's no equivalent conformance
+      // expansion right now.
+      auto expansion = conformingPackElt->getAs<PackExpansionType>();
+      if (expansion) conformingPackElt = expansion->getPatternType();
+
+      auto conformance =
+        (*this)(dependentType, conformingPackElt, conformedProtocol);
+      conformances.push_back(conformance);
+    }
+    return ProtocolConformanceRef(
+        PackConformance::get(conformingPack, conformedProtocol, conformances));
+  }
+
   assert((conformingReplacementType->is<ErrorType>() ||
           conformingReplacementType->is<SubstitutableType>() ||
           conformingReplacementType->is<DependentMemberType>() ||
@@ -4954,6 +4972,9 @@ TypeBase::getContextSubstitutions(const DeclContext *dc,
     else if (genericEnv)
       substTy = genericEnv->mapTypeIntoContext(gp);
 
+    if (gp->isParameterPack() && !substTy->hasError())
+      substTy = PackType::getSingletonPackExpansion(substTy);
+
     auto result = substitutions.insert(
       {gp->getCanonicalType()->castTo<GenericTypeParamType>(),
        substTy});

lib/IRGen/GenPack.cpp

@@ -263,6 +263,18 @@ static llvm::Value *bindWitnessTableAtIndex(IRGenFunction &IGF,
   return wtable;
 }
 
+/// Find the pack archetype for the given interface type in the given
+/// opened element context, which is known to be a forwarding context.
+static CanPackArchetypeType
+getMappedPackArchetypeType(const OpenedElementContext &context, CanType ty) {
+  auto packType = cast<PackType>(
+    context.environment->maybeApplyOuterContextSubstitutions(ty)
+        ->getCanonicalType());
+  auto archetype = getForwardedPackArchetypeType(packType);
+  assert(archetype);
+  return archetype;
+}
+
 static void bindElementSignatureRequirementsAtIndex(
     IRGenFunction &IGF, OpenedElementContext const &context, llvm::Value *index,
     DynamicMetadataRequest request) {
@@ -275,9 +287,7 @@ static void bindElementSignatureRequirementsAtIndex(
           break;
         case GenericRequirement::Kind::MetadataPack: {
           auto ty = requirement.getTypeParameter();
-          auto patternPackArchetype = cast<PackArchetypeType>(
-              context.environment->maybeApplyOuterContextSubstitutions(ty)
-                  ->getCanonicalType());
+          auto patternPackArchetype = getMappedPackArchetypeType(context, ty);
           auto response =
               IGF.emitTypeMetadataRef(patternPackArchetype, request);
           auto elementArchetype =
@@ -295,9 +305,7 @@ static void bindElementSignatureRequirementsAtIndex(
         case GenericRequirement::Kind::WitnessTablePack: {
           auto ty = requirement.getTypeParameter();
           auto proto = requirement.getProtocol();
-          auto patternPackArchetype = cast<PackArchetypeType>(
-              context.environment->maybeApplyOuterContextSubstitutions(ty)
-                  ->getCanonicalType());
+          auto patternPackArchetype = getMappedPackArchetypeType(context, ty);
           auto elementArchetype =
               context.environment
                   ->mapPackTypeIntoElementContext(

lib/Sema/CSApply.cpp

@@ -95,8 +95,12 @@ Solution::computeSubstitutions(GenericSignature sig,
     return SubstitutionMap();
 
   TypeSubstitutionMap subs;
-  for (const auto &opened : openedTypes->second)
-    subs[opened.first] = getFixedType(opened.second);
+  for (const auto &opened : openedTypes->second) {
+    auto type = getFixedType(opened.second);
+    if (opened.first->isParameterPack() && !type->is<PackType>())
+      type = PackType::getSingletonPackExpansion(type);
+    subs[opened.first] = type;
+  }
 
   auto lookupConformanceFn =
       [&](CanType original, Type replacement,

lib/Sema/CSBindings.cpp

@@ -1483,6 +1483,7 @@ void PotentialBindings::infer(Constraint *constraint) {
       auto *elementEnv = CS.getPackElementEnvironment(constraint->getLocator(),
                                                       shapeClass);
       auto elementType = elementEnv->mapPackTypeIntoElementContext(packType);
+      assert(!elementType->is<PackType>());
       addPotentialBinding({elementType, AllowedBindingKind::Exact, constraint});
 
       break;

lib/Sema/CSSimplify.cpp

@@ -8782,6 +8782,19 @@ ConstraintSystem::simplifyBindTupleOfFunctionParamsConstraint(
   return SolutionKind::Solved;
 }
 
+static Type lookThroughSingletonPackExpansion(Type ty) {
+  if (auto pack = ty->getAs<PackType>()) {
+    if (pack->getNumElements() == 1) {
+      if (auto expansion = pack->getElementType(0)->getAs<PackExpansionType>()) {
+        auto countType = expansion->getCountType();
+        if (countType->isEqual(expansion->getPatternType()))
+          return countType;
+      }
+    }
+  }
+  return ty;
+}
+
 ConstraintSystem::SolutionKind
 ConstraintSystem::simplifyPackElementOfConstraint(Type first, Type second,
                                                   TypeMatchOptions flags,
@@ -8801,6 +8814,11 @@ ConstraintSystem::simplifyPackElementOfConstraint(Type first, Type second,
     return SolutionKind::Solved;
   }
 
+  // FIXME: I'm not sure this is actually necessary; I may only be seeing
+  // this because of something I've screwed up in element generic
+  // environments.
+  elementType = lookThroughSingletonPackExpansion(elementType);
+
   // This constraint only exists to vend bindings.
   auto *packEnv = DC->getGenericEnvironmentOfContext();
   if ((!elementType->hasElementArchetype() && packType->isEqual(elementType)) ||