Index: include/llvm/ADT/fallible_iterator.h
===================================================================
--- /dev/null
+++ include/llvm/ADT/fallible_iterator.h
@@ -0,0 +1,242 @@
+//===--- fallible_iterator.h - Wrapper for fallible iterators ---*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_ADT_FALLIBLE_ITERATOR_H
+#define LLVM_ADT_FALLIBLE_ITERATOR_H
+
+#include "llvm/ADT/iterator_range.h"
+#include "llvm/Support/Error.h"
+
+#include <type_traits>
+
+namespace llvm {
+
+/// A wrapper class for fallible iterators.
+///
+///   The fallible_iterator template wraps an underlying iterator-like class
+/// whose increment and decrement operations are replaced with fallible versions
+/// like:
+///
+///   @code{.cpp}
+///   Error inc();
+///   Error dec();
+///   @endcode
+///
+///   It produces an interface that is (mostly) compatible with a traditional
+/// c++ iterator, including ++ and -- operators that do not fail.
+///
+///   Instances of the wrapper are constructed with an instance of the
+/// underlying iterator and (for non-end iterators) a reference to an Error
+/// instance. If the underlying increment/decrement operations fail, the Error
+/// is returned via this reference, and the resulting iterator value set to an
+/// end-of-range sentinel value. This enables the following loop idiom:
+///
+///   @code{.cpp}
+///   class Archive { // E.g. Potentially malformed on-disk archive
+///   public:
+///     fallible_iterator<ArchiveChildItr> children_begin(Error &Err);
+///     fallible_iterator<ArchiveChildItr> children_end();
+///     iterator_range<fallible_iterator<ArchiveChildItr>>
+///     children(Error &Err) {
+///       return make_range(children_begin(Err), children_end());
+///     //...
+///   };
+///
+///   void walk(Archive &A) {
+///     Error Err = Error::success();
+///     for (auto &C : A.children()) {
+///       // Loop body only entered when increment succeeds.
+///     }
+///     if (Err) {
+///       // handle error.
+///     }
+///   }
+///   @endcode
+///
+///   The wrapper marks the referenced Error as unchecked after each increment
+/// and/or decrement operation, and clears the unchecked flag when a non-end
+/// value is compared against end (since, by the increment invariant, not being
+/// an end value proves that there was no error, and is equivalent to checking
+/// that the Error is success). This allows early exits from the loop body
+/// without requiring redundant error checks.
+template <typename Underlying> class fallible_iterator {
+private:
+  template <typename T>
+  using enable_if_struct_deref_supported = std::enable_if<
+      !std::is_void<decltype(std::declval<T>().operator->())>::value,
+      decltype(std::declval<T>().operator->())>;
+
+public:
+  using this_fallible_iterator = fallible_iterator<Underlying>;
+
+  /// Construct a fallible iterator that *can not* be used as an end-of-range
+  /// value.
+  ///
+  /// A value created by this method can be dereferenced, incremented,
+  /// decremented and compared, providing the underlying type supports it.
+  static this_fallible_iterator non_end(Underlying I, Error &Err) {
+    return this_fallible_iterator(std::move(I), &Err);
+  }
+
+  /// Construct a fallible iteratro that can be used as an end-of-range value.
+  ///
+  /// A value created by this method can be dereferenced (if the underlying
+  /// value points at a valid value) and compared, but not incremented or
+  /// decremented.
+  static this_fallible_iterator end(Underlying I) {
+    return this_fallible_iterator(std::move(I), nullptr);
+  }
+
+  /// Forward dereference to the underlying iterator.
+  auto operator*() -> decltype(*std::declval<Underlying>()) { return *I; }
+
+  /// Forward const dereference to the underlying iterator.
+  auto operator*() const -> decltype(*std::declval<const Underlying>()) {
+    return *I;
+  }
+
+  /// Forward structure dereference to the underlying iterator (if the
+  /// underlying iterator supports it).
+  template <typename T = Underlying>
+  typename enable_if_struct_deref_supported<T>::type operator->() {
+    return I.operator->();
+  }
+
+  /// Forward const structure dereference to the underlying iterator (if the
+  /// underlying iterator supports it).
+  template <typename T = Underlying>
+  typename enable_if_struct_deref_supported<const T>::type operator->() const {
+    return I.operator->();
+  }
+
+  /// Increment the fallible iterator.
+  ///
+  /// If the underlying 'inc' operation fails, this will set the Error value
+  /// and update this iterator value to point to end-of-range.
+  ///
+  /// The Error value is marked as needing checking, regardless of whether the
+  /// 'inc' operation succeeds or fails.
+  this_fallible_iterator &operator++() {
+    assert(getErrPtr() && "Can not increment end iterator");
+    if (auto Err = I.inc())
+      handleError(std::move(Err));
+    else
+      resetCheckedFlag();
+    return *this;
+  }
+
+  /// Decrement the fallible iterator.
+  ///
+  /// If the underlying 'dec' operation fails, this will set the Error value
+  /// and update this iterator value to point to end-of-range.
+  ///
+  /// The Error value is marked as needing checking, regardless of whether the
+  /// 'dec' operation succeeds or fails.
+  this_fallible_iterator &operator--() {
+    assert(getErrPtr() && "Can not decrement end iterator");
+    if (auto Err = I.dec())
+      handleError(std::move(Err));
+    else
+      resetCheckedFlag();
+    return *this;
+  }
+
+  /// Compare fallible iterators for equality.
+  ///
+  /// Returns true if both LHS and RHS are end-of-range values, or if both are
+  /// non-end-of-range values whose underlying iterator values compare equal.
+  ///
+  /// If this is a comparison between an end-of-range iterator and a
+  /// non-end-of-range iterator, then the Error (referenced by the
+  /// non-end-of-range value) is marked as checked: Since all
+  /// increment/decrement operations result in an end-of-range value, comparing
+  /// false against end-of-range is equivalent to checking that the Error value
+  /// is success. This flag management enables early returns from loop bodies
+  /// without redundant Error checks.
+  friend bool operator==(const this_fallible_iterator &LHS,
+                         const this_fallible_iterator &RHS) {
+    // If both iterators are in the end state they compare
+    // equal, regardless of whether either is valid.
+    if (LHS.isEnd() && RHS.isEnd())
+      return true;
+
+    assert(LHS.isValid() && RHS.isValid() &&
+           "Invalid iterators can only be compared against end");
+
+    bool Equal = LHS.I == RHS.I;
+
+    // If the iterators differ and this is a comparison against end then mark
+    // the Error as checked.
+    if (!Equal) {
+      if (LHS.isEnd())
+        (void)!!*RHS.getErrPtr();
+      else
+        (void)!!*LHS.getErrPtr();
+    }
+
+    return Equal;
+  }
+
+  /// Compare fallible iterators for inequality.
+  ///
+  /// See notes for operator==.
+  friend bool operator!=(const this_fallible_iterator &LHS,
+                         const this_fallible_iterator &RHS) {
+    return !(LHS == RHS);
+  }
+
+private:
+  fallible_iterator(Underlying I, Error *Err)
+      : I(std::move(I)), ErrState(Err, false) {}
+
+  Error *getErrPtr() const { return ErrState.getPointer(); }
+
+  bool isEnd() const { return getErrPtr() == nullptr; }
+
+  bool isValid() const { return !ErrState.getInt(); }
+
+  void handleError(Error Err) {
+    *getErrPtr() = std::move(Err);
+    ErrState.setPointer(nullptr);
+    ErrState.setInt(true);
+  }
+
+  void resetCheckedFlag() {
+    (void)!!*getErrPtr();
+    assert(!*getErrPtr() && "Incrementing failure value");
+    *getErrPtr() = Error::success();
+  }
+
+  Underlying I;
+  mutable PointerIntPair<Error *, 1> ErrState;
+};
+
+/// Convenience wrapper to make a fallible_iterator value from an instance
+/// of an underlying iterator and an Error reference.
+template <typename Underlying>
+fallible_iterator<Underlying> make_fallible_iter(Underlying I, Error &Err) {
+  return fallible_iterator<Underlying>::non_end(std::move(I), Err);
+}
+
+/// Convenience wrapper to make a fallible_iterator end value from an instance
+/// of an underlying iterator.
+template <typename Underlying>
+fallible_iterator<Underlying> make_fallible_end(Underlying E) {
+  return fallible_iterator<Underlying>::end(std::move(E));
+}
+
+template <typename Underlying>
+iterator_range<fallible_iterator<Underlying>>
+make_fallible_range(Underlying I, Underlying E, Error &Err) {
+  return make_range(make_fallible_iter(std::move(I), Err),
+                    make_fallible_end(std::move(E)));
+}
+
+} // end namespace llvm
+
+#endif // LLVM_ADT_FALLIBLE_ITERATOR_H
Index: unittests/ADT/CMakeLists.txt
===================================================================
--- unittests/ADT/CMakeLists.txt
+++ unittests/ADT/CMakeLists.txt
@@ -18,6 +18,7 @@
   DenseSetTest.cpp
   DepthFirstIteratorTest.cpp
   EquivalenceClassesTest.cpp
+  FallibleIteratorTest.cpp
   FoldingSet.cpp
   FunctionExtrasTest.cpp
   FunctionRefTest.cpp
@@ -71,4 +72,6 @@
   VariadicFunctionTest.cpp
   )
 
+target_link_libraries(ADTTests PRIVATE LLVMTestingSupport)
+
 add_dependencies(ADTTests intrinsics_gen)
Index: unittests/ADT/FallibleIteratorTest.cpp
===================================================================
--- /dev/null
+++ unittests/ADT/FallibleIteratorTest.cpp
@@ -0,0 +1,239 @@
+//===- unittests/ADT/FallibleIteratorTest.cpp - fallible_iterator.h tests -===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/ADT/fallible_iterator.h"
+#include "llvm/Testing/Support/Error.h"
+
+#include "gtest/gtest-spi.h"
+#include "gtest/gtest.h"
+
+#include <utility>
+#include <vector>
+
+using namespace llvm;
+
+class FallibleIteratorTest : public testing::Test {
+public:
+  using ObjectValid = enum { ValidObject, InvalidObject };
+  using LinkValid = enum { ValidLink, InvalidLink };
+
+  class Object {
+  public:
+    Object(ObjectValid V) : V(V) {}
+    bool isValid() const { return V == ValidObject; }
+
+  private:
+    ObjectValid V;
+  };
+
+  using Archive = std::vector<std::pair<Object, LinkValid>>;
+
+  class ArchiveWalker {
+  public:
+    ArchiveWalker(Archive &A, unsigned Idx) : A(A), Idx(Idx) {}
+
+    Object &operator*() { return A[Idx].first; }
+
+    const Object &operator*() const { return A[Idx].first; }
+
+    Error inc() {
+      assert(Idx != A.size() && "Walking off end of (fake) archive");
+      if (A[Idx].second == ValidLink) {
+        ++Idx;
+        return Error::success();
+      }
+      return make_error<StringError>("cant get next object in (fake) archive",
+                                     inconvertibleErrorCode());
+    }
+
+    Error dec() {
+      assert(Idx != 0 && "Walking off start of (fake) archive");
+      --Idx;
+      if (A[Idx].second == ValidLink)
+        return Error::success();
+      return make_error<StringError>("cant get prev object in (fake) archiev",
+                                     inconvertibleErrorCode());
+    }
+
+    friend bool operator==(const ArchiveWalker &LHS, const ArchiveWalker &RHS) {
+      assert(&LHS.A == &RHS.A && "Comparing iterators across archives.");
+      return LHS.Idx == RHS.Idx;
+    }
+
+    friend bool operator!=(const ArchiveWalker &LHS, const ArchiveWalker &RHS) {
+      return !(LHS == RHS);
+    }
+
+  private:
+    Archive &A;
+    unsigned Idx;
+  };
+
+  class ArchiveWalkerWithDeref : public ArchiveWalker {
+  public:
+    using ArchiveWalker::ArchiveWalker;
+
+    Object *operator->() { return &this->operator*(); }
+
+    const Object *operator->() const { return &this->operator*(); }
+  };
+};
+
+namespace {
+
+TEST_F(FallibleIteratorTest, BasicSuccess) {
+
+  // Check that a basic use-case involing successful iteration over a dummy
+  // "Archive" works.
+
+  Archive A({{ValidObject, ValidLink},
+             {ValidObject, ValidLink},
+             {InvalidObject, InvalidLink}});
+
+  ArchiveWalker begin(A, 0);
+  ArchiveWalker end(A, 2);
+
+  Error Err = Error::success();
+  for (auto &Elem : make_fallible_range<ArchiveWalker>(begin, end, Err))
+    EXPECT_TRUE(Elem.isValid());
+  cantFail(std::move(Err));
+}
+
+TEST_F(FallibleIteratorTest, BasicFailure) {
+
+  // Check that a iteration failure (due to the InvalidLink state on element one
+  // of the dummy archive) breaks out of the loop and raises an Error.
+
+  Archive A({{ValidObject, ValidLink},
+             {ValidObject, InvalidLink},
+             {InvalidObject, InvalidLink}});
+
+  ArchiveWalker begin(A, 0);
+  ArchiveWalker end(A, 2);
+
+  Error Err = Error::success();
+  for (auto &Elem : make_fallible_range<ArchiveWalker>(begin, end, Err))
+    EXPECT_TRUE(Elem.isValid());
+
+  EXPECT_THAT_ERROR(std::move(Err), Failed()) << "Expected failure value";
+}
+
+TEST_F(FallibleIteratorTest, NoRedundantErrorCheckOnEarlyExit) {
+
+  // Check that an early return from the loop body does not require a redundant
+  // check of Err.
+
+  Archive A({{ValidObject, ValidLink},
+             {ValidObject, ValidLink},
+             {InvalidObject, InvalidLink}});
+
+  ArchiveWalker begin(A, 0);
+  ArchiveWalker end(A, 2);
+
+  Error Err = Error::success();
+  for (auto &Elem : make_fallible_range<ArchiveWalker>(begin, end, Err)) {
+    (void)Elem;
+    return;
+  }
+  // Err not checked, but should be ok because we exit from the loop
+  // body.
+}
+
+TEST_F(FallibleIteratorTest, RegularLoopExitRequiresErrorCheck) {
+
+  // Check that Err must be checked after a normal (i.e. not early) loop exit
+  // by failing to check and expecting program death (due to the unchecked
+  // error).
+
+  EXPECT_DEATH(
+      {
+        Archive A({{ValidObject, ValidLink},
+                   {ValidObject, ValidLink},
+                   {InvalidObject, InvalidLink}});
+
+        ArchiveWalker begin(A, 0);
+        ArchiveWalker end(A, 2);
+
+        Error Err = Error::success();
+        for (auto &Elem : make_fallible_range<ArchiveWalker>(begin, end, Err))
+          (void)Elem;
+      },
+      "Program aborted due to an unhandled Error:")
+      << "Normal (i.e. not early) loop exit should require an error check";
+}
+
+TEST_F(FallibleIteratorTest, RawIncrementAndDecrementBehavior) {
+
+  // Check the exact behavior of increment / decrement.
+
+  Archive A({{ValidObject, ValidLink},
+             {ValidObject, InvalidLink},
+             {ValidObject, ValidLink},
+             {ValidObject, InvalidLink}});
+
+  {
+    // One increment from begin succeeds.
+    Error Err = Error::success();
+    auto I = make_fallible_iter(ArchiveWalker(A, 0), Err);
+    ++I;
+    EXPECT_THAT_ERROR(std::move(Err), Succeeded());
+  }
+
+  {
+    // Two increments from begin fail.
+    Error Err = Error::success();
+    auto I = make_fallible_iter(ArchiveWalker(A, 0), Err);
+    ++I;
+    (void)!!Err; // Manualy reset the checked flag.
+    ++I;
+    EXPECT_THAT_ERROR(std::move(Err), Failed()) << "Expected failure value";
+    cantFail(std::move(Err));
+  }
+
+  {
+    // One decement from element three succeeds.
+    Error Err = Error::success();
+    auto I = make_fallible_iter(ArchiveWalker(A, 3), Err);
+    --I;
+    EXPECT_THAT_ERROR(std::move(Err), Succeeded());
+  }
+
+  {
+    // One decement from element three succeeds.
+    Error Err = Error::success();
+    auto I = make_fallible_iter(ArchiveWalker(A, 3), Err);
+    --I;
+    (void)!!Err; // Manually reset the checked flag.
+    --I;
+    EXPECT_THAT_ERROR(std::move(Err), Failed());
+  }
+}
+
+TEST_F(FallibleIteratorTest, CheckDerefOperatorSupport) {
+  Archive A({{ValidObject, ValidLink},
+             {ValidObject, ValidLink},
+             {InvalidObject, InvalidLink}});
+
+  ArchiveWalkerWithDeref begin(A, 0);
+
+  {
+    Error Err = Error::success();
+    auto I = make_fallible_iter(begin, Err);
+    EXPECT_TRUE(I->isValid());
+    cantFail(std::move(Err));
+  }
+
+  {
+    Error Err = Error::success();
+    const auto I = make_fallible_iter(begin, Err);
+    EXPECT_TRUE(I->isValid());
+    cantFail(std::move(Err));
+  }
+}
+
+} // namespace