[PrintAsObjC] Handle circularities introduced by ObjC generics.

Like Swift generics, Objective-C generics may have constraints; unlike
Swift generics, Objective-C doesn't do separate parsing and
type-checking passes. This means that any generic arguments for
constrained generic parameters must be fully-defined, in order to
check that they satisfy the constraints.

This commit addresses this problem with three different mechanisms,
one for each kind of declaration that might run into this issue:

- For classes, if a member references a type with constrained generic
  parameter, and the corresponding argument type hasn't been printed
  yet, that member is "delayed", which means it is put into a category
  at the end of the file.

- Protocols cannot have categories, so for protocols we instead see if
  we can print the definition of the other type first. To break
  circular dependencies, the printer will not attempt this if both the
  type and the protocol are already being depended on. This isn't
  perfect (see below).

- Rather than delaying members of extensions, we just delay them
  wholesale. This keeps related members together, but also has
  problems (see below).

These approaches solve the most common cases while still not crashing
in the uncommon ones. However, there are still a number of problems:

- The protocol heuristic is overly negative, which means we may generate
  an invalid header even when there's a reasonable ordering. For example,
  a single class might inherit from a class A and conform to protocol P,
  and protocol P depends on class A as a generic argument. In this case,
  defining class A first is the right thing to do, but it's possible for
  the printer to decide that there's circularity here and just forward-
  declare A instead.

- Protocols really can be circular. This can be fixed by printing a
  forward-declared protocol alongside the generic constraints, i.e.
  'id <MoreThanNSCopying, NSCopying>' instead of just
  'id <MoreThanNSCopying>'.

- Extensions can introduce protocols as well. This is not modeled at
  all; if a member depends on a protocol conformance, it's assumed
  that simply printing the class would be sufficient. This could be
  fixed by checking how a generic argument satisfies its constraints,
  possibly delaying individual members from extensions in order to
  print them sooner.

- More cases I haven't thought about.

Test cases for some of these problems are in the new
circularity-errors.swift file, mostly to make sure the ObjC printer
doesn't crash when it encounters them.

rdar://problem/27109377
(cherry picked from commit 8282160)

Author: jrose-apple

lib/PrintAsObjC/PrintAsObjC.cpp

@@ -118,6 +118,8 @@ static bool isMistakableForInit(ObjCSelector selector) {
 
 
 namespace {
+using DelayedMemberSet = llvm::SmallSetVector<const ValueDecl *, 32>;
+
 class ObjCPrinter : private DeclVisitor<ObjCPrinter>,
                     private TypeVisitor<ObjCPrinter, void, 
                                         Optional<OptionalTypeKind>>
@@ -134,6 +136,7 @@ class ObjCPrinter : private DeclVisitor<ObjCPrinter>,
   raw_ostream &os;
 
   SmallVector<const FunctionType *, 4> openFunctionTypes;
+  const DelayedMemberSet &delayedMembers;
 
   Accessibility minRequiredAccess;
   bool protocolMembersOptional = false;
@@ -144,14 +147,31 @@ class ObjCPrinter : private DeclVisitor<ObjCPrinter>,
   friend TypeVisitor<ObjCPrinter>;
 
 public:
-  explicit ObjCPrinter(Module &mod, raw_ostream &out, Accessibility access)
-    : M(mod), os(out), minRequiredAccess(access) {}
+  explicit ObjCPrinter(Module &mod, raw_ostream &out,
+                       DelayedMemberSet &delayed, Accessibility access)
+    : M(mod), os(out), delayedMembers(delayed), minRequiredAccess(access) {}
 
   void print(const Decl *D) {
     PrettyStackTraceDecl trace("printing", D);
     visit(const_cast<Decl *>(D));
   }
 
+  void printAdHocCategory(iterator_range<const ValueDecl * const *> members) {
+    assert(members.begin() != members.end());
+
+    const DeclContext *origDC = (*members.begin())->getDeclContext();
+    auto *baseClass = dyn_cast<ClassDecl>(origDC);
+    if (!baseClass) {
+      Type extendedTy = cast<ExtensionDecl>(origDC)->getExtendedType();
+      baseClass = extendedTy->getClassOrBoundGenericClass();
+    }
+
+    os << "@interface " << getNameForObjC(baseClass)
+    << " (SWIFT_EXTENSION(" << origDC->getParentModule()->getName() << "))\n";
+    printMembers</*allowDelayed*/true>(members);
+    os << "@end\n\n";
+  }
+  
   bool shouldInclude(const ValueDecl *VD, bool checkParent = true) {
     if (!(VD->isObjC() || VD->getAttrs().hasAttribute<CDeclAttr>()))
       return false;
@@ -198,19 +218,24 @@ class ObjCPrinter : private DeclVisitor<ObjCPrinter>,
   }
 
   /// Prints the members of a class, extension, or protocol.
-  void printMembers(DeclRange members) {
-    for (auto member : members) {
+  template <bool AllowDelayed = false, typename R>
+  void printMembers(R &&members) {
+    for (const Decl *member : members) {
       auto VD = dyn_cast<ValueDecl>(member);
       if (!VD || !shouldInclude(VD) || isa<TypeDecl>(VD))
         continue;
       if (auto FD = dyn_cast<FuncDecl>(VD))
         if (FD->isAccessor())
           continue;
+      if (!AllowDelayed && delayedMembers.count(VD)) {
+        os << "// '" << VD->getFullName() << "' below\n";
+        continue;
+      }
       if (VD->getAttrs().hasAttribute<OptionalAttr>() != protocolMembersOptional) {
         protocolMembersOptional = VD->getAttrs().hasAttribute<OptionalAttr>();
         os << (protocolMembersOptional ? "@optional\n" : "@required\n");
       }
-      visit(VD);
+      visit(const_cast<ValueDecl*>(VD));
     }
   }
 
@@ -1509,6 +1534,7 @@ class ReferencedTypeFinder : private TypeVisitor<ReferencedTypeFinder> {
   friend TypeVisitor;
 
   llvm::function_ref<void(ReferencedTypeFinder &, const TypeDecl *)> Callback;
+  bool IsWithinConstrainedObjCGeneric = false;
 
   ReferencedTypeFinder(decltype(Callback) callback) : Callback(callback) {}
 
@@ -1568,18 +1594,48 @@ class ReferencedTypeFinder : private TypeVisitor<ReferencedTypeFinder> {
     visit(inout->getObjectType());
   }
 
+  /// Returns true if \p archetype has any constraints other than being
+  /// class-bound ("conforms to" AnyObject).
+  static bool isConstrainedArchetype(const ArchetypeType *archetype) {
+    if (archetype->getSuperclass())
+      return true;
+
+    if (archetype->getConformsTo().size() > 1)
+      return true;
+    if (archetype->getConformsTo().size() == 0)
+      return false;
+
+    const ProtocolDecl *proto = archetype->getConformsTo().front();
+    if (auto knownKind = proto->getKnownProtocolKind())
+      return knownKind.getValue() != KnownProtocolKind::AnyObject;
+    return true;
+  }
+
   void visitBoundGenericType(BoundGenericType *boundGeneric) {
-    for (auto argTy : boundGeneric->getGenericArgs())
+    bool isObjCGeneric = boundGeneric->getDecl()->hasClangNode();
+
+    for_each(boundGeneric->getGenericArgs(),
+             boundGeneric->getDecl()->getGenericParams()->getPrimaryArchetypes(),
+             [&](Type argTy, const ArchetypeType *archetype) {
+      if (isObjCGeneric && isConstrainedArchetype(archetype))
+        IsWithinConstrainedObjCGeneric = true;
       visit(argTy);
-    // Ignore the base type; that can't be exposed to Objective-C. Every
-    // bound generic type we care about gets mapped to a particular construct
-    // in Objective-C we care about. (For example, Optional<NSFoo> is mapped to
-    // NSFoo *.)
+      IsWithinConstrainedObjCGeneric = false;
+    });
+
+    // Ignore the base type; that either can't be exposed to Objective-C or
+    // was an Objective-C type to begin with. Every bound generic Swift type we
+    // care about gets mapped to a particular construct in Objective-C.
+    // (For example, Optional<NSFoo> is mapped to NSFoo *.)
   }
 
 public:
   using TypeVisitor::visit;
 
+  bool isWithinConstrainedObjCGeneric() const {
+    return IsWithinConstrainedObjCGeneric;
+  }
+
   static void walk(Type ty, decltype(Callback) callback) {
     ReferencedTypeFinder(callback).visit(ty);
   }
@@ -1603,13 +1659,14 @@ struct PointerLikeComparator {
 
 class ModuleWriter {
   enum class EmissionState {
-    DefinitionRequested = 0,
-    DefinitionInProgress,
+    NotYetDefined = 0,
+    DefinitionRequested,
     Defined
   };
 
   llvm::DenseMap<const TypeDecl *, std::pair<EmissionState, bool>> seenTypes;
   std::vector<const Decl *> declsToWrite;
+  DelayedMemberSet delayedMembers;
 
   using ImportModuleTy = PointerUnion<Module*, const clang::Module*>;
   SmallSetVector<ImportModuleTy, 8,
@@ -1623,7 +1680,7 @@ class ModuleWriter {
   ObjCPrinter printer;
 public:
   ModuleWriter(Module &mod, StringRef header, Accessibility access)
-    : M(mod), bridgingHeader(header), printer(M, os, access) {}
+    : M(mod), bridgingHeader(header), printer(M, os, delayedMembers, access) {}
 
   /// Returns true if we added the decl's module to the import set, false if
   /// the decl is a local decl.
@@ -1660,6 +1717,19 @@ class ModuleWriter {
     return true;
   }
 
+  bool hasBeenRequested(const TypeDecl *D) const {
+    return seenTypes.lookup(D).first >= EmissionState::DefinitionRequested;
+  }
+
+  bool tryRequire(const TypeDecl *D) {
+    if (addImport(D)) {
+      seenTypes[D] = { EmissionState::Defined, true };
+      return true;
+    }
+    auto &state = seenTypes[D];
+    return state.first == EmissionState::Defined;
+  }
+
   bool require(const TypeDecl *D) {
     if (addImport(D)) {
       seenTypes[D] = { EmissionState::Defined, true };
@@ -1668,11 +1738,11 @@ class ModuleWriter {
 
     auto &state = seenTypes[D];
     switch (state.first) {
+    case EmissionState::NotYetDefined:
     case EmissionState::DefinitionRequested:
+      state.first = EmissionState::DefinitionRequested;
       declsToWrite.push_back(D);
       return false;
-    case EmissionState::DefinitionInProgress:
-      llvm_unreachable("circular requirements");
     case EmissionState::Defined:
       return true;
     }
@@ -1717,7 +1787,17 @@ class ModuleWriter {
     });
   }
 
-  void forwardDeclareMemberTypes(DeclRange members) {
+  bool forwardDeclareMemberTypes(DeclRange members, const Decl *container) {
+    switch (container->getKind()) {
+    case DeclKind::Class:
+    case DeclKind::Protocol:
+    case DeclKind::Extension:
+      break;
+    default:
+      llvm_unreachable("unexpected container kind");
+    }
+
+    bool hadAnyDelayedMembers = false;
     SmallVector<ValueDecl *, 4> nestedTypes;
     for (auto member : members) {
       auto VD = dyn_cast<ValueDecl>(member);
@@ -1738,9 +1818,44 @@ class ModuleWriter {
         continue;
       }
 
+      bool needsToBeIndividuallyDelayed = false;
       ReferencedTypeFinder::walk(VD->getType(),
-                                 [this](ReferencedTypeFinder &finder,
-                                        const TypeDecl *TD) {
+                                 [&](ReferencedTypeFinder &finder,
+                                     const TypeDecl *TD) {
+        if (TD == container)
+          return;
+
+        if (finder.isWithinConstrainedObjCGeneric()) {
+          // We can delay individual members of classes; do so if necessary.
+          if (isa<ClassDecl>(container)) {
+            if (!tryRequire(TD)) {
+              needsToBeIndividuallyDelayed = true;
+              hadAnyDelayedMembers = true;
+            }
+            return;
+          }
+
+          // Extensions can always be delayed wholesale.
+          if (isa<ExtensionDecl>(container)) {
+            if (!require(TD))
+              hadAnyDelayedMembers = true;
+            return;
+          }
+
+          // Protocols should be delayed wholesale unless we might have a cycle.
+          auto *proto = cast<ProtocolDecl>(container);
+          if (!hasBeenRequested(proto) || !hasBeenRequested(TD)) {
+            if (!require(TD))
+              hadAnyDelayedMembers = true;
+            return;
+          }
+
+          // Otherwise, we have a cyclic dependency. Give up and continue with
+          // regular forward-declarations even though this will lead to an
+          // error; there's nothing we can do here.
+          // FIXME: It would be nice to diagnose this.
+        }
+
         if (auto CD = dyn_cast<ClassDecl>(TD)) {
           if (!forwardDeclare(CD)) {
             (void)addImport(CD);
@@ -1758,13 +1873,18 @@ class ModuleWriter {
         else
           assert(false && "unknown local type decl");
       });
+
+      if (needsToBeIndividuallyDelayed) {
+        assert(isa<ClassDecl>(container));
+        delayedMembers.insert(VD);
+      }
     }
 
     declsToWrite.insert(declsToWrite.end()-1, nestedTypes.rbegin(),
                         nestedTypes.rend());
 
     // Separate forward declarations from the class itself.
-    os << '\n';
+    return !hadAnyDelayedMembers;
   }
 
   bool writeClass(const ClassDecl *CD) {
@@ -1789,8 +1909,9 @@ class ModuleWriter {
     if (!allRequirementsSatisfied)
       return false;
 
+    (void)forwardDeclareMemberTypes(CD->getMembers(), CD);
     seenTypes[CD] = { EmissionState::Defined, true };
-    forwardDeclareMemberTypes(CD->getMembers());
+    os << '\n';
     printer.print(CD);
     return true;
   }
@@ -1824,8 +1945,11 @@ class ModuleWriter {
     if (!allRequirementsSatisfied)
       return false;
 
+    if (!forwardDeclareMemberTypes(PD->getMembers(), PD))
+      return false;
+
     seenTypes[PD] = { EmissionState::Defined, true };
-    forwardDeclareMemberTypes(PD->getMembers());
+    os << '\n';
     printer.print(PD);
     return true;
   }
@@ -1842,7 +1966,13 @@ class ModuleWriter {
     if (!allRequirementsSatisfied)
       return false;
 
-    forwardDeclareMemberTypes(ED->getMembers());
+    // This isn't rolled up into the previous set of requirements because
+    // it /also/ prints forward declarations, and the header is a little
+    // prettier if those are as close as possible to the necessary extension.
+    if (!forwardDeclareMemberTypes(ED->getMembers(), ED))
+      return false;
+
+    os << '\n';
     printer.print(ED);
     return true;
   }
@@ -2186,6 +2316,17 @@ class ModuleWriter {
       }
     }
 
+    if (!delayedMembers.empty()) {
+      auto groupBegin = delayedMembers.begin();
+      for (auto i = groupBegin, e = delayedMembers.end(); i != e; ++i) {
+        if ((*i)->getDeclContext() != (*groupBegin)->getDeclContext()) {
+          printer.printAdHocCategory(make_range(groupBegin, i));
+          groupBegin = i;
+        }
+      }
+      printer.printAdHocCategory(make_range(groupBegin, delayedMembers.end()));
+    }
+
     writePrologue(out);
     writeImports(out);
     out <<

test/PrintAsObjC/Inputs/circularity.h

@@ -0,0 +1,22 @@
+// This file is meant to be used with the mock SDK, not the real one.
+#import <Foundation.h>
+
+#define SWIFT_NAME(x) __attribute__((swift_name(#x)))
+
+@protocol Proto
+@end
+
+@interface ProtoImpl : NSObject <Proto>
+@end
+
+@interface Parent : NSObject
+@end
+
+@interface Unconstrained<T> : NSObject
+@end
+
+@interface NeedsProto<T: id <Proto>> : NSObject
+@end
+
+@interface NeedsParent<T: Parent *> : NSObject
+@end