StaticAnalyzer/Checkers/BasicObjCFoundationChecks.cpp

//== BasicObjCFoundationChecks.cpp - Simple Apple-Foundation checks -*- C++ -*--
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
//  This file defines BasicObjCFoundationChecks, a class that encapsulates
//  a set of simple checks to run on Objective-C code using Apple's Foundation
//  classes.
//
//===----------------------------------------------------------------------===//

#include "ClangSACheckers.h"
#include "clang/AST/ASTContext.h"
#include "clang/AST/DeclObjC.h"
#include "clang/AST/Expr.h"
#include "clang/AST/ExprObjC.h"
#include "clang/AST/StmtObjC.h"
#include "clang/Analysis/DomainSpecific/CocoaConventions.h"
#include "clang/StaticAnalyzer/Core/BugReporter/BugType.h"
#include "clang/StaticAnalyzer/Core/Checker.h"
#include "clang/StaticAnalyzer/Core/CheckerManager.h"
#include "clang/StaticAnalyzer/Core/PathSensitive/CallEvent.h"
#include "clang/StaticAnalyzer/Core/PathSensitive/CheckerContext.h"
#include "clang/StaticAnalyzer/Core/PathSensitive/ExplodedGraph.h"
#include "clang/StaticAnalyzer/Core/PathSensitive/ExprEngine.h"
#include "clang/StaticAnalyzer/Core/PathSensitive/MemRegion.h"
#include "clang/StaticAnalyzer/Core/PathSensitive/ProgramState.h"
#include "llvm/ADT/SmallString.h"
#include "llvm/ADT/StringMap.h"
#include "llvm/Support/raw_ostream.h"

using namespace clang;
using namespace ento;

namespace {
class APIMisuse : public BugType {
public:
  APIMisuse(const char* name) : BugType(name, "API Misuse (Apple)") {}
};
} // end anonymous namespace

//===----------------------------------------------------------------------===//
// Utility functions.
//===----------------------------------------------------------------------===//

static StringRef GetReceiverInterfaceName(const ObjCMethodCall &msg) {
  if (const ObjCInterfaceDecl *ID = msg.getReceiverInterface())
    return ID->getIdentifier()->getName();
  return StringRef();
}

enum FoundationClass {
  FC_None,
  FC_NSArray,
  FC_NSDictionary,
  FC_NSEnumerator,
  FC_NSOrderedSet,
  FC_NSSet,
  FC_NSString
};

static FoundationClass findKnownClass(const ObjCInterfaceDecl *ID) {
  static llvm::StringMap<FoundationClass> Classes;
  if (Classes.empty()) {
    Classes["NSArray"] = FC_NSArray;
    Classes["NSDictionary"] = FC_NSDictionary;
    Classes["NSEnumerator"] = FC_NSEnumerator;
    Classes["NSOrderedSet"] = FC_NSOrderedSet;
    Classes["NSSet"] = FC_NSSet;
    Classes["NSString"] = FC_NSString;
  }

  // FIXME: Should we cache this at all?
  FoundationClass result = Classes.lookup(ID->getIdentifier()->getName());
  if (result == FC_None)
    if (const ObjCInterfaceDecl *Super = ID->getSuperClass())
      return findKnownClass(Super);

  return result;
}

static inline bool isNil(SVal X) {
  return X.getAs<loc::ConcreteInt>().hasValue();
}

//===----------------------------------------------------------------------===//
// NilArgChecker - Check for prohibited nil arguments to ObjC method calls.
//===----------------------------------------------------------------------===//

namespace {
  class NilArgChecker : public Checker<check::PreObjCMessage> {
    mutable OwningPtr<APIMisuse> BT;

    void WarnNilArg(CheckerContext &C,
                    const ObjCMethodCall &msg, unsigned Arg) const;

  public:
    void checkPreObjCMessage(const ObjCMethodCall &M, CheckerContext &C) const;
  };
}

void NilArgChecker::WarnNilArg(CheckerContext &C,
                               const ObjCMethodCall &msg,
                               unsigned int Arg) const
{
  if (!BT)
    BT.reset(new APIMisuse("nil argument"));

  if (ExplodedNode *N = C.generateSink()) {
    SmallString<128> sbuf;
    llvm::raw_svector_ostream os(sbuf);
    os << "Argument to '" << GetReceiverInterfaceName(msg) << "' method '"
       << msg.getSelector().getAsString() << "' cannot be nil";

    BugReport *R = new BugReport(*BT, os.str(), N);
    R->addRange(msg.getArgSourceRange(Arg));
    C.emitReport(R);
  }
}

void NilArgChecker::checkPreObjCMessage(const ObjCMethodCall &msg,
                                        CheckerContext &C) const {
  const ObjCInterfaceDecl *ID = msg.getReceiverInterface();
  if (!ID)
    return;

  FoundationClass Class = findKnownClass(ID);

  static const unsigned InvalidArgIndex = UINT_MAX;
  unsigned Arg = InvalidArgIndex;

  if (Class == FC_NSString) {
    Selector S = msg.getSelector();

    if (S.isUnarySelector())
      return;

    // FIXME: This is going to be really slow doing these checks with
    //  lexical comparisons.

    std::string NameStr = S.getAsString();
    StringRef Name(NameStr);
    assert(!Name.empty());

    // FIXME: Checking for initWithFormat: will not work in most cases
    //  yet because [NSString alloc] returns id, not NSString*.  We will
    //  need support for tracking expected-type information in the analyzer
    //  to find these errors.
    if (Name == "caseInsensitiveCompare:" ||
        Name == "compare:" ||
        Name == "compare:options:" ||
        Name == "compare:options:range:" ||
        Name == "compare:options:range:locale:" ||
        Name == "componentsSeparatedByCharactersInSet:" ||
        Name == "initWithFormat:") {
      Arg = 0;
    }
  } else if (Class == FC_NSArray) {
    Selector S = msg.getSelector();

    if (S.isUnarySelector())
      return;

    if (S.getNameForSlot(0).equals("addObject")) {
      Arg = 0;
    } else if (S.getNameForSlot(0).equals("insertObject") &&
               S.getNameForSlot(1).equals("atIndex")) {
      Arg = 0;
    } else if (S.getNameForSlot(0).equals("replaceObjectAtIndex") &&
               S.getNameForSlot(1).equals("withObject")) {
      Arg = 1;
    } else if (S.getNameForSlot(0).equals("setObject") &&
               S.getNameForSlot(1).equals("atIndexedSubscript")) {
      Arg = 0;
    } else if (S.getNameForSlot(0).equals("arrayByAddingObject")) {
      Arg = 0;
    }
  }

  // If argument is '0', report a warning.
  if ((Arg != InvalidArgIndex) && isNil(msg.getArgSVal(Arg)))
    WarnNilArg(C, msg, Arg);

}

//===----------------------------------------------------------------------===//
// Error reporting.
//===----------------------------------------------------------------------===//

namespace {
class CFNumberCreateChecker : public Checker< check::PreStmt<CallExpr> > {
  mutable OwningPtr<APIMisuse> BT;
  mutable IdentifierInfo* II;
public:
  CFNumberCreateChecker() : II(0) {}

  void checkPreStmt(const CallExpr *CE, CheckerContext &C) const;

private:
  void EmitError(const TypedRegion* R, const Expr *Ex,
                uint64_t SourceSize, uint64_t TargetSize, uint64_t NumberKind);
};
} // end anonymous namespace

enum CFNumberType {
  kCFNumberSInt8Type = 1,
  kCFNumberSInt16Type = 2,
  kCFNumberSInt32Type = 3,
  kCFNumberSInt64Type = 4,
  kCFNumberFloat32Type = 5,
  kCFNumberFloat64Type = 6,
  kCFNumberCharType = 7,
  kCFNumberShortType = 8,
  kCFNumberIntType = 9,
  kCFNumberLongType = 10,
  kCFNumberLongLongType = 11,
  kCFNumberFloatType = 12,
  kCFNumberDoubleType = 13,
  kCFNumberCFIndexType = 14,
  kCFNumberNSIntegerType = 15,
  kCFNumberCGFloatType = 16
};

static Optional<uint64_t> GetCFNumberSize(ASTContext &Ctx, uint64_t i) {
  static const unsigned char FixedSize[] = { 8, 16, 32, 64, 32, 64 };

  if (i < kCFNumberCharType)
    return FixedSize[i-1];

  QualType T;

  switch (i) {
    case kCFNumberCharType:     T = Ctx.CharTy;     break;
    case kCFNumberShortType:    T = Ctx.ShortTy;    break;
    case kCFNumberIntType:      T = Ctx.IntTy;      break;
    case kCFNumberLongType:     T = Ctx.LongTy;     break;
    case kCFNumberLongLongType: T = Ctx.LongLongTy; break;
    case kCFNumberFloatType:    T = Ctx.FloatTy;    break;
    case kCFNumberDoubleType:   T = Ctx.DoubleTy;   break;
    case kCFNumberCFIndexType:
    case kCFNumberNSIntegerType:
    case kCFNumberCGFloatType:
      // FIXME: We need a way to map from names to Type*.
    default:
      return None;
  }

  return Ctx.getTypeSize(T);
}

#if 0
static const char* GetCFNumberTypeStr(uint64_t i) {
  static const char* Names[] = {
    "kCFNumberSInt8Type",
    "kCFNumberSInt16Type",
    "kCFNumberSInt32Type",
    "kCFNumberSInt64Type",
    "kCFNumberFloat32Type",
    "kCFNumberFloat64Type",
    "kCFNumberCharType",
    "kCFNumberShortType",
    "kCFNumberIntType",
    "kCFNumberLongType",
    "kCFNumberLongLongType",
    "kCFNumberFloatType",
    "kCFNumberDoubleType",
    "kCFNumberCFIndexType",
    "kCFNumberNSIntegerType",
    "kCFNumberCGFloatType"
  };

  return i <= kCFNumberCGFloatType ? Names[i-1] : "Invalid CFNumberType";
}
#endif

void CFNumberCreateChecker::checkPreStmt(const CallExpr *CE,
                                         CheckerContext &C) const {
  ProgramStateRef state = C.getState();
  const FunctionDecl *FD = C.getCalleeDecl(CE);
  if (!FD)
    return;

  ASTContext &Ctx = C.getASTContext();
  if (!II)
    II = &Ctx.Idents.get("CFNumberCreate");

  if (FD->getIdentifier() != II || CE->getNumArgs() != 3)
    return;

  // Get the value of the "theType" argument.
  const LocationContext *LCtx = C.getLocationContext();
  SVal TheTypeVal = state->getSVal(CE->getArg(1), LCtx);

  // FIXME: We really should allow ranges of valid theType values, and
  //   bifurcate the state appropriately.
  Optional<nonloc::ConcreteInt> V = TheTypeVal.getAs<nonloc::ConcreteInt>();
  if (!V)
    return;

  uint64_t NumberKind = V->getValue().getLimitedValue();
  Optional<uint64_t> OptTargetSize = GetCFNumberSize(Ctx, NumberKind);

  // FIXME: In some cases we can emit an error.
  if (!OptTargetSize)
    return;

  uint64_t TargetSize = *OptTargetSize;

  // Look at the value of the integer being passed by reference.  Essentially
  // we want to catch cases where the value passed in is not equal to the
  // size of the type being created.
  SVal TheValueExpr = state->getSVal(CE->getArg(2), LCtx);

  // FIXME: Eventually we should handle arbitrary locations.  We can do this
  //  by having an enhanced memory model that does low-level typing.
  Optional<loc::MemRegionVal> LV = TheValueExpr.getAs<loc::MemRegionVal>();
  if (!LV)
    return;

  const TypedValueRegion* R = dyn_cast<TypedValueRegion>(LV->stripCasts());
  if (!R)
    return;

  QualType T = Ctx.getCanonicalType(R->getValueType());

  // FIXME: If the pointee isn't an integer type, should we flag a warning?
  //  People can do weird stuff with pointers.

  if (!T->isIntegerType())
    return;

  uint64_t SourceSize = Ctx.getTypeSize(T);

  // CHECK: is SourceSize == TargetSize
  if (SourceSize == TargetSize)
    return;

  // Generate an error.  Only generate a sink if 'SourceSize < TargetSize';
  // otherwise generate a regular node.
  //
  // FIXME: We can actually create an abstract "CFNumber" object that has
  //  the bits initialized to the provided values.
  //
  if (ExplodedNode *N = SourceSize < TargetSize ? C.generateSink()
                                                : C.addTransition()) {
    SmallString<128> sbuf;
    llvm::raw_svector_ostream os(sbuf);

    os << (SourceSize == 8 ? "An " : "A ")
       << SourceSize << " bit integer is used to initialize a CFNumber "
                        "object that represents "
       << (TargetSize == 8 ? "an " : "a ")
       << TargetSize << " bit integer. ";

    if (SourceSize < TargetSize)
      os << (TargetSize - SourceSize)
      << " bits of the CFNumber value will be garbage." ;
    else
      os << (SourceSize - TargetSize)
      << " bits of the input integer will be lost.";

    if (!BT)
      BT.reset(new APIMisuse("Bad use of CFNumberCreate"));

    BugReport *report = new BugReport(*BT, os.str(), N);
    report->addRange(CE->getArg(2)->getSourceRange());
    C.emitReport(report);
  }
}

//===----------------------------------------------------------------------===//
// CFRetain/CFRelease/CFMakeCollectable checking for null arguments.
//===----------------------------------------------------------------------===//

namespace {
class CFRetainReleaseChecker : public Checker< check::PreStmt<CallExpr> > {
  mutable OwningPtr<APIMisuse> BT;
  mutable IdentifierInfo *Retain, *Release, *MakeCollectable;
public:
  CFRetainReleaseChecker(): Retain(0), Release(0), MakeCollectable(0) {}
  void checkPreStmt(const CallExpr *CE, CheckerContext &C) const;
};
} // end anonymous namespace


void CFRetainReleaseChecker::checkPreStmt(const CallExpr *CE,
                                          CheckerContext &C) const {
  // If the CallExpr doesn't have exactly 1 argument just give up checking.
  if (CE->getNumArgs() != 1)
    return;

  ProgramStateRef state = C.getState();
  const FunctionDecl *FD = C.getCalleeDecl(CE);
  if (!FD)
    return;

  if (!BT) {
    ASTContext &Ctx = C.getASTContext();
    Retain = &Ctx.Idents.get("CFRetain");
    Release = &Ctx.Idents.get("CFRelease");
    MakeCollectable = &Ctx.Idents.get("CFMakeCollectable");
    BT.reset(
      new APIMisuse("null passed to CFRetain/CFRelease/CFMakeCollectable"));
  }

  // Check if we called CFRetain/CFRelease/CFMakeCollectable.
  const IdentifierInfo *FuncII = FD->getIdentifier();
  if (!(FuncII == Retain || FuncII == Release || FuncII == MakeCollectable))
    return;

  // FIXME: The rest of this just checks that the argument is non-null.
  // It should probably be refactored and combined with NonNullParamChecker.

  // Get the argument's value.
  const Expr *Arg = CE->getArg(0);
  SVal ArgVal = state->getSVal(Arg, C.getLocationContext());
  Optional<DefinedSVal> DefArgVal = ArgVal.getAs<DefinedSVal>();
  if (!DefArgVal)
    return;

  // Get a NULL value.
  SValBuilder &svalBuilder = C.getSValBuilder();
  DefinedSVal zero =
      svalBuilder.makeZeroVal(Arg->getType()).castAs<DefinedSVal>();

  // Make an expression asserting that they're equal.
  DefinedOrUnknownSVal ArgIsNull = svalBuilder.evalEQ(state, zero, *DefArgVal);

  // Are they equal?
  ProgramStateRef stateTrue, stateFalse;
  llvm::tie(stateTrue, stateFalse) = state->assume(ArgIsNull);

  if (stateTrue && !stateFalse) {
    ExplodedNode *N = C.generateSink(stateTrue);
    if (!N)
      return;

    const char *description;
    if (FuncII == Retain)
      description = "Null pointer argument in call to CFRetain";
    else if (FuncII == Release)
      description = "Null pointer argument in call to CFRelease";
    else if (FuncII == MakeCollectable)
      description = "Null pointer argument in call to CFMakeCollectable";
    else
      llvm_unreachable("impossible case");

    BugReport *report = new BugReport(*BT, description, N);
    report->addRange(Arg->getSourceRange());
    bugreporter::trackNullOrUndefValue(N, Arg, *report);
    C.emitReport(report);
    return;
  }

  // From here on, we know the argument is non-null.
  C.addTransition(stateFalse);
}

//===----------------------------------------------------------------------===//
// Check for sending 'retain', 'release', or 'autorelease' directly to a Class.
//===----------------------------------------------------------------------===//

namespace {
class ClassReleaseChecker : public Checker<check::PreObjCMessage> {
  mutable Selector releaseS;
  mutable Selector retainS;
  mutable Selector autoreleaseS;
  mutable Selector drainS;
  mutable OwningPtr<BugType> BT;

public:
  void checkPreObjCMessage(const ObjCMethodCall &msg, CheckerContext &C) const;
};
}

void ClassReleaseChecker::checkPreObjCMessage(const ObjCMethodCall &msg,
                                              CheckerContext &C) const {

  if (!BT) {
    BT.reset(new APIMisuse("message incorrectly sent to class instead of class "
                           "instance"));

    ASTContext &Ctx = C.getASTContext();
    releaseS = GetNullarySelector("release", Ctx);
    retainS = GetNullarySelector("retain", Ctx);
    autoreleaseS = GetNullarySelector("autorelease", Ctx);
    drainS = GetNullarySelector("drain", Ctx);
  }

  if (msg.isInstanceMessage())
    return;
  const ObjCInterfaceDecl *Class = msg.getReceiverInterface();
  assert(Class);

  Selector S = msg.getSelector();
  if (!(S == releaseS || S == retainS || S == autoreleaseS || S == drainS))
    return;

  if (ExplodedNode *N = C.addTransition()) {
    SmallString<200> buf;
    llvm::raw_svector_ostream os(buf);

    os << "The '" << S.getAsString() << "' message should be sent to instances "
          "of class '" << Class->getName()
       << "' and not the class directly";

    BugReport *report = new BugReport(*BT, os.str(), N);
    report->addRange(msg.getSourceRange());
    C.emitReport(report);
  }
}

//===----------------------------------------------------------------------===//
// Check for passing non-Objective-C types to variadic methods that expect
// only Objective-C types.
//===----------------------------------------------------------------------===//

namespace {
class VariadicMethodTypeChecker : public Checker<check::PreObjCMessage> {
  mutable Selector arrayWithObjectsS;
  mutable Selector dictionaryWithObjectsAndKeysS;
  mutable Selector setWithObjectsS;
  mutable Selector orderedSetWithObjectsS;
  mutable Selector initWithObjectsS;
  mutable Selector initWithObjectsAndKeysS;
  mutable OwningPtr<BugType> BT;

  bool isVariadicMessage(const ObjCMethodCall &msg) const;

public:
  void checkPreObjCMessage(const ObjCMethodCall &msg, CheckerContext &C) const;
};
}

/// isVariadicMessage - Returns whether the given message is a variadic message,
/// where all arguments must be Objective-C types.
bool
VariadicMethodTypeChecker::isVariadicMessage(const ObjCMethodCall &msg) const {
  const ObjCMethodDecl *MD = msg.getDecl();

  if (!MD || !MD->isVariadic() || isa<ObjCProtocolDecl>(MD->getDeclContext()))
    return false;

  Selector S = msg.getSelector();

  if (msg.isInstanceMessage()) {
    // FIXME: Ideally we'd look at the receiver interface here, but that's not
    // useful for init, because alloc returns 'id'. In theory, this could lead
    // to false positives, for example if there existed a class that had an
    // initWithObjects: implementation that does accept non-Objective-C pointer
    // types, but the chance of that happening is pretty small compared to the
    // gains that this analysis gives.
    const ObjCInterfaceDecl *Class = MD->getClassInterface();

    switch (findKnownClass(Class)) {
    case FC_NSArray:
    case FC_NSOrderedSet:
    case FC_NSSet:
      return S == initWithObjectsS;
    case FC_NSDictionary:
      return S == initWithObjectsAndKeysS;
    default:
      return false;
    }
  } else {
    const ObjCInterfaceDecl *Class = msg.getReceiverInterface();

    switch (findKnownClass(Class)) {
      case FC_NSArray:
        return S == arrayWithObjectsS;
      case FC_NSOrderedSet:
        return S == orderedSetWithObjectsS;
      case FC_NSSet:
        return S == setWithObjectsS;
      case FC_NSDictionary:
        return S == dictionaryWithObjectsAndKeysS;
      default:
        return false;
    }
  }
}

void VariadicMethodTypeChecker::checkPreObjCMessage(const ObjCMethodCall &msg,
                                                    CheckerContext &C) const {
  if (!BT) {
    BT.reset(new APIMisuse("Arguments passed to variadic method aren't all "
                           "Objective-C pointer types"));

    ASTContext &Ctx = C.getASTContext();
    arrayWithObjectsS = GetUnarySelector("arrayWithObjects", Ctx);
    dictionaryWithObjectsAndKeysS =
      GetUnarySelector("dictionaryWithObjectsAndKeys", Ctx);
    setWithObjectsS = GetUnarySelector("setWithObjects", Ctx);
    orderedSetWithObjectsS = GetUnarySelector("orderedSetWithObjects", Ctx);

    initWithObjectsS = GetUnarySelector("initWithObjects", Ctx);
    initWithObjectsAndKeysS = GetUnarySelector("initWithObjectsAndKeys", Ctx);
  }

  if (!isVariadicMessage(msg))
      return;

  // We are not interested in the selector arguments since they have
  // well-defined types, so the compiler will issue a warning for them.
  unsigned variadicArgsBegin = msg.getSelector().getNumArgs();

  // We're not interested in the last argument since it has to be nil or the
  // compiler would have issued a warning for it elsewhere.
  unsigned variadicArgsEnd = msg.getNumArgs() - 1;

  if (variadicArgsEnd <= variadicArgsBegin)
    return;

  // Verify that all arguments have Objective-C types.
  Optional<ExplodedNode*> errorNode;
  ProgramStateRef state = C.getState();

  for (unsigned I = variadicArgsBegin; I != variadicArgsEnd; ++I) {
    QualType ArgTy = msg.getArgExpr(I)->getType();
    if (ArgTy->isObjCObjectPointerType())
      continue;

    // Block pointers are treaded as Objective-C pointers.
    if (ArgTy->isBlockPointerType())
      continue;

    // Ignore pointer constants.
    if (msg.getArgSVal(I).getAs<loc::ConcreteInt>())
      continue;

    // Ignore pointer types annotated with 'NSObject' attribute.
    if (C.getASTContext().isObjCNSObjectType(ArgTy))
      continue;

    // Ignore CF references, which can be toll-free bridged.
    if (coreFoundation::isCFObjectRef(ArgTy))
      continue;

    // Generate only one error node to use for all bug reports.
    if (!errorNode.hasValue())
      errorNode = C.addTransition();

    if (!errorNode.getValue())
      continue;

    SmallString<128> sbuf;
    llvm::raw_svector_ostream os(sbuf);

    StringRef TypeName = GetReceiverInterfaceName(msg);
    if (!TypeName.empty())
      os << "Argument to '" << TypeName << "' method '";
    else
      os << "Argument to method '";

    os << msg.getSelector().getAsString()
       << "' should be an Objective-C pointer type, not '";
    ArgTy.print(os, C.getLangOpts());
    os << "'";

    BugReport *R = new BugReport(*BT, os.str(), errorNode.getValue());
    R->addRange(msg.getArgSourceRange(I));
    C.emitReport(R);
  }
}

//===----------------------------------------------------------------------===//
// Improves the modeling of loops over Cocoa collections.
//===----------------------------------------------------------------------===//

namespace {
class ObjCLoopChecker
  : public Checker<check::PostStmt<ObjCForCollectionStmt> > {

public:
  void checkPostStmt(const ObjCForCollectionStmt *FCS, CheckerContext &C) const;
};
}

static bool isKnownNonNilCollectionType(QualType T) {
  const ObjCObjectPointerType *PT = T->getAs<ObjCObjectPointerType>();
  if (!PT)
    return false;

  const ObjCInterfaceDecl *ID = PT->getInterfaceDecl();
  if (!ID)
    return false;

  switch (findKnownClass(ID)) {
  case FC_NSArray:
  case FC_NSDictionary:
  case FC_NSEnumerator:
  case FC_NSOrderedSet:
  case FC_NSSet:
    return true;
  default:
    return false;
  }
}

void ObjCLoopChecker::checkPostStmt(const ObjCForCollectionStmt *FCS,
                                    CheckerContext &C) const {
  ProgramStateRef State = C.getState();

  // Check if this is the branch for the end of the loop.
  SVal CollectionSentinel = State->getSVal(FCS, C.getLocationContext());
  if (CollectionSentinel.isZeroConstant())
    return;

  // See if the collection is one where we /know/ the elements are non-nil.
  const Expr *Collection = FCS->getCollection();
  if (!isKnownNonNilCollectionType(Collection->getType()))
    return;

  // FIXME: Copied from ExprEngineObjC.
  const Stmt *Element = FCS->getElement();
  SVal ElementVar;
  if (const DeclStmt *DS = dyn_cast<DeclStmt>(Element)) {
    const VarDecl *ElemDecl = cast<VarDecl>(DS->getSingleDecl());
    assert(ElemDecl->getInit() == 0);
    ElementVar = State->getLValue(ElemDecl, C.getLocationContext());
  } else {
    ElementVar = State->getSVal(Element, C.getLocationContext());
  }

  if (!ElementVar.getAs<Loc>())
    return;

  // Go ahead and assume the value is non-nil.
  SVal Val = State->getSVal(ElementVar.castAs<Loc>());
  State = State->assume(Val.castAs<DefinedOrUnknownSVal>(), true);
  C.addTransition(State);
}

namespace {
/// \class ObjCNonNilReturnValueChecker
/// \brief The checker restricts the return values of APIs known to
/// never (or almost never) return 'nil'.
class ObjCNonNilReturnValueChecker
  : public Checker<check::PostObjCMessage> {
    mutable bool Initialized;
    mutable Selector ObjectAtIndex;
    mutable Selector ObjectAtIndexedSubscript;

public:
  ObjCNonNilReturnValueChecker() : Initialized(false) {}
  void checkPostObjCMessage(const ObjCMethodCall &M, CheckerContext &C) const;
};
}

static ProgramStateRef assumeExprIsNonNull(const Expr *NonNullExpr,
                                           ProgramStateRef State,
                                           CheckerContext &C) {
  SVal Val = State->getSVal(NonNullExpr, C.getLocationContext());
  if (Optional<DefinedOrUnknownSVal> DV = Val.getAs<DefinedOrUnknownSVal>())
    return State->assume(*DV, true);
  return State;
}

void ObjCNonNilReturnValueChecker::checkPostObjCMessage(const ObjCMethodCall &M,
                                                        CheckerContext &C)
                                                        const {
  ProgramStateRef State = C.getState();

  if (!Initialized) {
    ASTContext &Ctx = C.getASTContext();
    ObjectAtIndex = GetUnarySelector("objectAtIndex", Ctx);
    ObjectAtIndexedSubscript = GetUnarySelector("objectAtIndexedSubscript", Ctx);
  }

  // Check the receiver type.
  if (const ObjCInterfaceDecl *Interface = M.getReceiverInterface()) {

    // Assume that object returned from '[self init]' or '[super init]' is not
    // 'nil' if we are processing an inlined function/method.
    //
    // A defensive callee will (and should) check if the object returned by
    // '[super init]' is 'nil' before doing it's own initialization. However,
    // since 'nil' is rarely returned in practice, we should not warn when the
    // caller to the defensive constructor uses the object in contexts where
    // 'nil' is not accepted.
    if (!C.inTopFrame() && M.getDecl() &&
        M.getDecl()->getMethodFamily() == OMF_init &&
        M.isReceiverSelfOrSuper()) {
      State = assumeExprIsNonNull(M.getOriginExpr(), State, C);
    }

    // Objects returned from
    // [NSArray|NSOrderedSet]::[ObjectAtIndex|ObjectAtIndexedSubscript]
    // are never 'nil'.
    FoundationClass Cl = findKnownClass(Interface);
    if (Cl == FC_NSArray || Cl == FC_NSOrderedSet) {
      Selector Sel = M.getSelector();
      if (Sel == ObjectAtIndex || Sel == ObjectAtIndexedSubscript) {
        // Go ahead and assume the value is non-nil.
        State = assumeExprIsNonNull(M.getOriginExpr(), State, C);
      }
    }
  }
  C.addTransition(State);
}

//===----------------------------------------------------------------------===//
// Check registration.
//===----------------------------------------------------------------------===//

void ento::registerNilArgChecker(CheckerManager &mgr) {
  mgr.registerChecker<NilArgChecker>();
}

void ento::registerCFNumberCreateChecker(CheckerManager &mgr) {
  mgr.registerChecker<CFNumberCreateChecker>();
}

void ento::registerCFRetainReleaseChecker(CheckerManager &mgr) {
  mgr.registerChecker<CFRetainReleaseChecker>();
}

void ento::registerClassReleaseChecker(CheckerManager &mgr) {
  mgr.registerChecker<ClassReleaseChecker>();
}

void ento::registerVariadicMethodTypeChecker(CheckerManager &mgr) {
  mgr.registerChecker<VariadicMethodTypeChecker>();
}

void ento::registerObjCLoopChecker(CheckerManager &mgr) {
  mgr.registerChecker<ObjCLoopChecker>();
}

void ento::registerObjCNonNilReturnValueChecker(CheckerManager &mgr) {
  mgr.registerChecker<ObjCNonNilReturnValueChecker>();
}