Preliminary support for substitution with nested pack expansions

docs/ABI/Mangling.rst

@@ -675,6 +675,8 @@ Types
   type ::= assoc-type-list 'QZ'                      // shortcut for 'QYz'
   type ::= opaque-type-decl-name bound-generic-args 'Qo' INDEX // opaque type
 
+  type ::= pack-type 'Qe' INDEX              // pack element type
+
   type ::= pattern-type count-type 'Qp'      // pack expansion type
   type ::= pack-element-list 'QP'            // pack type
   type ::= pack-element-list 'QS' DIRECTNESS // SIL pack type

include/swift/AST/InFlightSubstitution.h

@@ -49,27 +49,15 @@ class InFlightSubstitution {
   InFlightSubstitution(const InFlightSubstitution &) = delete;
   InFlightSubstitution &operator=(const InFlightSubstitution &) = delete;
 
-  // TODO: when we add PackElementType, we should recognize it during
-  // substitution and either call different methods on this class or
-  // pass an extra argument for the pack-expansion depth D.  We should
-  // be able to rely on that to mark a pack-element reference instead
-  // of checking whether the original type was a pack.  Substitution
-  // should use the D'th entry from the end of ActivePackExpansions to
-  // guide the element substitution:
-  //   - project the given index of the pack substitution
-  //   - wrap it in a PackElementType if it's a subst expansion
-  //   - the depth of that PackElementType is the number of subst
-  //     expansions between the depth entry and the end of
-  //     ActivePackExpansions
-
   /// Perform primitive substitution on the given type.  Returns Type()
   /// if the type should not be substituted as a whole.
-  Type substType(SubstitutableType *origType);
+  Type substType(SubstitutableType *origType, unsigned level);
 
   /// Perform primitive conformance lookup on the given type.
   ProtocolConformanceRef lookupConformance(CanType dependentType,
                                            Type conformingReplacementType,
-                                           ProtocolDecl *conformedProtocol);
+                                           ProtocolDecl *conformedProtocol,
+                                           unsigned level);
 
   /// Given the shape type of a pack expansion, invoke the given callback
   /// for each expanded component of it.  If the substituted component

include/swift/AST/Type.h

@@ -151,7 +151,11 @@ enum class SubstFlags {
   /// Map member types to their desugared witness type.
   DesugarMemberTypes = 0x02,
   /// Substitute types involving opaque type archetypes.
-  SubstituteOpaqueArchetypes = 0x04
+  SubstituteOpaqueArchetypes = 0x04,
+  /// Don't increase pack expansion level for free pack references.
+  /// Do not introduce new usages of this flag.
+  /// FIXME: Remove this.
+  PreservePackExpansionLevel = 0x08,
 };
 
 /// Options for performing substitutions into a type.

include/swift/AST/Types.h

@@ -724,8 +724,10 @@ class alignas(1 << TypeAlignInBits) TypeBase
       SmallVectorImpl<OpenedArchetypeType *> &rootOpenedArchetypes) const;
 
   /// Retrieve the set of type parameter packs that occur within this type.
-  void getTypeParameterPacks(
-      SmallVectorImpl<Type> &rootParameterPacks);
+  void getTypeParameterPacks(SmallVectorImpl<Type> &rootParameterPacks);
+
+  /// Retrieve the set of type parameter packs that occur within this type.
+  void walkPackReferences(llvm::function_ref<bool (Type)> fn);
 
   /// Replace opened archetypes with the given root with their most
   /// specific non-dependent upper bounds throughout this type.
@@ -792,6 +794,9 @@ class alignas(1 << TypeAlignInBits) TypeBase
   /// this function will wrap into a pack containing a singleton expansion.
   PackType *getPackSubstitutionAsPackType();
 
+  /// Increase the expansion level of each parameter pack appearing in this type.
+  Type increasePackElementLevel(unsigned level);
+
   /// Determines whether this type is an lvalue. This includes both straight
   /// lvalue types as well as tuples or optionals of lvalues.
   bool hasLValueType() {

include/swift/Demangling/DemangleNodes.def

@@ -242,6 +242,7 @@ NODE(SILPackDirect)
 NODE(SILPackIndirect)
 NODE(PackExpansion)
 NODE(PackElement)
+NODE(PackElementLevel)
 NODE(Type)
 CONTEXT_NODE(TypeSymbolicReference)
 CONTEXT_NODE(TypeAlias)

include/swift/SIL/AbstractionPattern.h

@@ -938,10 +938,9 @@ class AbstractionPattern {
     case Kind::Discard: {
       auto type = getType();
       if (isa<DependentMemberType>(type) ||
-          isa<GenericTypeParamType>(type)) {
-        return true;
-      }
-      if (isa<ArchetypeType>(type)) {
+          isa<GenericTypeParamType>(type) ||
+          isa<PackElementType>(type) ||
+          isa<ArchetypeType>(type)) {
         return true;
       }
       return false;
@@ -960,7 +959,8 @@ class AbstractionPattern {
     case Kind::Discard: {
       auto type = getType();
       if (isa<DependentMemberType>(type) ||
-          isa<GenericTypeParamType>(type)) {
+          isa<GenericTypeParamType>(type) ||
+          isa<PackElementType>(type)) {
         return true;
       }
       if (auto archetype = dyn_cast<ArchetypeType>(type)) {
@@ -1448,6 +1448,10 @@ class AbstractionPattern {
   /// the abstraction pattern for an element type.
   AbstractionPattern getTupleElementType(unsigned index) const;
 
+  /// Given that the value being abstracted is a pack element type, return
+  /// the abstraction pattern for its pack type.
+  AbstractionPattern getPackElementPackType() const;
+
   /// Given that the value being abstracted is a pack type, return
   /// the abstraction pattern for an element type.
   AbstractionPattern getPackElementType(unsigned index) const;

lib/AST/ASTMangler.cpp

@@ -659,7 +659,8 @@ static Type getTypeForDWARFMangling(Type t) {
       return t;
     },
     MakeAbstractConformanceForGenericType(),
-    SubstFlags::AllowLoweredTypes);
+    SubstFlags::AllowLoweredTypes |
+    SubstFlags::PreservePackExpansionLevel);
 }
 
 std::string ASTMangler::mangleTypeForDebugger(Type Ty, GenericSignature sig) {
@@ -1288,8 +1289,9 @@ void ASTMangler::appendType(Type type, GenericSignature sig,
     case TypeKind::PackElement: {
       auto elementType = cast<PackElementType>(tybase);
       appendType(elementType->getPackType(), sig, forDecl);
-
-      // FIXME: append expansion depth
+      // If this ever changes, just mangle level 0 as a plain type parameter.
+      assert(elementType->getLevel() > 0);
+      appendOperator("Qe", Index(elementType->getLevel() - 1));
 
       return;
     }

lib/AST/ASTPrinter.cpp

@@ -6075,12 +6075,24 @@ class TypePrinter : public TypeVisitor<TypePrinter> {
   }
 
   void visitPackExpansionType(PackExpansionType *T) {
+    SmallVector<Type, 2> rootParameterPacks;
+    T->getPatternType()->getTypeParameterPacks(rootParameterPacks);
+
+    if (rootParameterPacks.empty() &&
+        (T->getCountType()->isParameterPack() ||
+         T->getCountType()->is<PackArchetypeType>())) {
+      Printer << "/* shape: ";
+      visit(T->getCountType());
+      Printer << " */ ";
+    }
+
     Printer << "repeat ";
+
     visit(T->getPatternType());
   }
 
   void visitPackElementType(PackElementType *T) {
-    Printer << "@level(" << T->getLevel() << ") ";
+    Printer << "/* level: " << T->getLevel() << " */ ";
     visit(T->getPackType());
   }
 

lib/AST/GenericEnvironment.cpp

@@ -432,7 +432,8 @@ Type TypeBase::mapTypeOutOfContext() {
   assert(!hasTypeParameter() && "already have an interface type");
   return Type(this).subst(MapTypeOutOfContext(),
     MakeAbstractConformanceForGenericType(),
-    SubstFlags::AllowLoweredTypes);
+    SubstFlags::AllowLoweredTypes |
+    SubstFlags::PreservePackExpansionLevel);
 }
 
 class GenericEnvironment::NestedTypeStorage
@@ -637,7 +638,8 @@ Type GenericEnvironment::mapTypeIntoContext(
   type = maybeApplyOuterContextSubstitutions(type);
   Type result = type.subst(QueryInterfaceTypeSubstitutions(this),
                            lookupConformance,
-                           SubstFlags::AllowLoweredTypes);
+                           SubstFlags::AllowLoweredTypes |
+                           SubstFlags::PreservePackExpansionLevel);
   assert((!result->hasTypeParameter() || result->hasError() ||
           getKind() == Kind::Opaque) &&
          "not fully substituted");
@@ -787,16 +789,19 @@ GenericEnvironment::mapElementTypeIntoPackContext(Type type) const {
   // Map element archetypes to the pack archetypes by converting
   // element types to interface types and adding the isParameterPack
   // bit. Then, map type parameters to archetypes.
-  return type.subst([&](SubstitutableType *type) {
-    auto *genericParam = type->getAs<GenericTypeParamType>();
-    if (!genericParam)
-      return Type();
-
-    if (auto *packParam = packParamForElement[{genericParam}])
-      return substitutions(packParam);
-
-    return substitutions(genericParam);
-  }, LookUpConformanceInSignature(sig.getPointer()));
+  return type.subst(
+      [&](SubstitutableType *type) {
+        auto *genericParam = type->getAs<GenericTypeParamType>();
+        if (!genericParam)
+          return Type();
+
+        if (auto *packParam = packParamForElement[{genericParam}])
+          return substitutions(packParam);
+
+        return substitutions(genericParam);
+      },
+      LookUpConformanceInSignature(sig.getPointer()),
+      SubstFlags::PreservePackExpansionLevel);
 }
 
 namespace {

lib/AST/ParameterPack.cpp

@@ -29,12 +29,13 @@ namespace {
 
 /// Collects all unique pack type parameters referenced from the pattern type,
 /// skipping those captured by nested pack expansion types.
-struct PackTypeParameterCollector: TypeWalker {
-  llvm::SetVector<Type> typeParams;
+struct PackReferenceCollector: TypeWalker {
+  llvm::function_ref<bool (Type)> fn;
   unsigned expansionLevel;
   SmallVector<unsigned, 2> elementLevel;
 
-  PackTypeParameterCollector() : expansionLevel(0) {
+  PackReferenceCollector(llvm::function_ref<bool (Type)> fn)
+    : fn(fn), expansionLevel(0) {
     elementLevel.push_back(0);
   }
 
@@ -72,12 +73,8 @@ struct PackTypeParameterCollector: TypeWalker {
     }
 
     if (elementLevel.back() == expansionLevel) {
-      if (auto *paramTy = t->getAs<GenericTypeParamType>()) {
-        if (paramTy->isParameterPack())
-          typeParams.insert(paramTy);
-      } else if (auto *archetypeTy = t->getAs<PackArchetypeType>()) {
-        typeParams.insert(archetypeTy->getRoot());
-      }
+      if (fn(t))
+        return Action::Stop;
     }
 
     return Action::Continue;
@@ -96,13 +93,28 @@ struct PackTypeParameterCollector: TypeWalker {
 
 }
 
+void TypeBase::walkPackReferences(
+    llvm::function_ref<bool (Type)> fn) {
+  Type(this).walk(PackReferenceCollector(fn));
+}
+
 void TypeBase::getTypeParameterPacks(
     SmallVectorImpl<Type> &rootParameterPacks) {
-  PackTypeParameterCollector collector;
-  Type(this).walk(collector);
+  llvm::SmallSetVector<Type, 2> rootParameterPackSet;
+
+  walkPackReferences([&](Type t) {
+    if (auto *paramTy = t->getAs<GenericTypeParamType>()) {
+      if (paramTy->isParameterPack())
+        rootParameterPackSet.insert(paramTy);
+    } else if (auto *archetypeTy = t->getAs<PackArchetypeType>()) {
+      rootParameterPackSet.insert(archetypeTy->getRoot());
+    }
+
+    return false;
+  });
 
-  rootParameterPacks.append(collector.typeParams.begin(),
-                            collector.typeParams.end());
+  rootParameterPacks.append(rootParameterPackSet.begin(),
+                            rootParameterPackSet.end());
 }
 
 bool GenericTypeParamType::isParameterPack() const {
@@ -133,6 +145,45 @@ PackType *TypeBase::getPackSubstitutionAsPackType() {
   }
 }
 
+static Type increasePackElementLevelImpl(
+    Type type, unsigned level, unsigned outerLevel) {
+  assert(level > 0);
+
+  return type.transformRec([&](TypeBase *t) -> Optional<Type> {
+    if (auto *elementType = dyn_cast<PackElementType>(t)) {
+      if (elementType->getLevel() >= outerLevel) {
+        elementType = PackElementType::get(elementType->getPackType(),
+                                           elementType->getLevel() + level);
+      }
+
+      return Type(elementType);
+    }
+
+    if (auto *expansionType = dyn_cast<PackExpansionType>(t)) {
+      return Type(PackExpansionType::get(
+          increasePackElementLevelImpl(expansionType->getPatternType(),
+                                       level, outerLevel + 1),
+          expansionType->getCountType()));
+    }
+
+    if (t->isParameterPack() || isa<PackArchetypeType>(t)) {
+      if (outerLevel == 0)
+        return Type(PackElementType::get(t, level));
+
+      return Type(t);
+    }
+
+    return None;
+  });
+}
+
+Type TypeBase::increasePackElementLevel(unsigned level) {
+  if (level == 0)
+    return Type(this);
+
+  return increasePackElementLevelImpl(Type(this), level, 0);
+}
+
 CanType PackExpansionType::getReducedShape() {
   auto reducedShape = countType->getReducedShape();
   if (reducedShape == getASTContext().TheEmptyTupleType)

lib/AST/ProtocolConformanceRef.cpp

@@ -119,7 +119,9 @@ ProtocolConformanceRef::subst(Type origType, InFlightSubstitution &IFS) const {
   }
 
   // Check the conformance map.
-  return IFS.lookupConformance(origType->getCanonicalType(), substType, proto);
+  // FIXME: Pack element level?
+  return IFS.lookupConformance(origType->getCanonicalType(), substType, proto,
+                               /*level=*/0);
 }
 
 ProtocolConformanceRef ProtocolConformanceRef::mapConformanceOutOfContext() const {
@@ -130,7 +132,8 @@ ProtocolConformanceRef ProtocolConformanceRef::mapConformanceOutOfContext() cons
             return archetypeType->getInterfaceType();
           return type;
         },
-        MakeAbstractConformanceForGenericType());
+        MakeAbstractConformanceForGenericType(),
+        SubstFlags::PreservePackExpansionLevel);
     return ProtocolConformanceRef(concrete);
   } else if (isPack()) {
     return getPack()->subst(
@@ -139,7 +142,8 @@ ProtocolConformanceRef ProtocolConformanceRef::mapConformanceOutOfContext() cons
             return archetypeType->getInterfaceType();
           return type;
         },
-        MakeAbstractConformanceForGenericType());
+        MakeAbstractConformanceForGenericType(),
+        SubstFlags::PreservePackExpansionLevel);
   }
 
   return *this;

lib/AST/RequirementEnvironment.cpp

@@ -69,7 +69,9 @@ RequirementEnvironment::RequirementEnvironment(
       MakeAbstractConformanceForGenericType();
 
   auto substConcreteType = concreteType.subst(
-      conformanceToWitnessThunkTypeFn, conformanceToWitnessThunkConformanceFn);
+      conformanceToWitnessThunkTypeFn,
+      conformanceToWitnessThunkConformanceFn,
+      SubstFlags::PreservePackExpansionLevel);
 
   // Calculate the depth at which the requirement's generic parameters
   // appear in the witness thunk signature.

lib/AST/SubstitutionMap.cpp

@@ -247,7 +247,8 @@ SubstitutionMap SubstitutionMap::get(GenericSignature genericSig,
     CanType depTy = req.getFirstType()->getCanonicalType();
     auto replacement = depTy.subst(IFS);
     auto *proto = req.getProtocolDecl();
-    auto conformance = IFS.lookupConformance(depTy, replacement, proto);
+    auto conformance = IFS.lookupConformance(depTy, replacement, proto,
+                                             /*level=*/0);
     conformances.push_back(conformance);
   }
 
@@ -440,7 +441,9 @@ SubstitutionMap::lookupConformance(CanType type, ProtocolDecl *proto) const {
 }
 
 SubstitutionMap SubstitutionMap::mapReplacementTypesOutOfContext() const {
-  return subst(MapTypeOutOfContext(), MakeAbstractConformanceForGenericType());
+  return subst(MapTypeOutOfContext(),
+               MakeAbstractConformanceForGenericType(),
+               SubstFlags::PreservePackExpansionLevel);
 }
 
 SubstitutionMap SubstitutionMap::subst(SubstitutionMap subMap,
@@ -829,5 +832,7 @@ SubstitutionMap SubstitutionMap::mapIntoTypeExpansionContext(
   ReplaceOpaqueTypesWithUnderlyingTypes replacer(
       context.getContext(), context.getResilienceExpansion(),
       context.isWholeModuleContext());
-  return this->subst(replacer, replacer, SubstFlags::SubstituteOpaqueArchetypes);
+  return this->subst(replacer, replacer,
+                     SubstFlags::SubstituteOpaqueArchetypes |
+                     SubstFlags::PreservePackExpansionLevel);
 }