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diff --git a/absl/hash/internal/spy_hash_state.h b/absl/hash/internal/spy_hash_state.h
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+++ b/absl/hash/internal/spy_hash_state.h
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+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_HASH_INTERNAL_SPY_HASH_STATE_H_
+#define ABSL_HASH_INTERNAL_SPY_HASH_STATE_H_
+
+#include <ostream>
+#include <string>
+#include <vector>
+
+#include "absl/hash/hash.h"
+#include "absl/strings/match.h"
+#include "absl/strings/str_format.h"
+#include "absl/strings/str_join.h"
+
+namespace absl {
+namespace hash_internal {
+
+// SpyHashState is an implementation of the HashState API that simply
+// accumulates all input bytes in an internal buffer. This makes it useful
+// for testing AbslHashValue overloads (so long as they are templated on the
+// HashState parameter), since it can report the exact hash representation
+// that the AbslHashValue overload produces.
+//
+// Sample usage:
+// EXPECT_EQ(SpyHashState::combine(SpyHashState(), foo),
+//           SpyHashState::combine(SpyHashState(), bar));
+template <typename T>
+class SpyHashStateImpl : public HashStateBase<SpyHashStateImpl<T>> {
+ public:
+  SpyHashStateImpl()
+      : error_(std::make_shared<absl::optional<std::string>>()) {
+    static_assert(std::is_void<T>::value, "");
+  }
+
+  // Move-only
+  SpyHashStateImpl(const SpyHashStateImpl&) = delete;
+  SpyHashStateImpl& operator=(const SpyHashStateImpl&) = delete;
+
+  SpyHashStateImpl(SpyHashStateImpl&& other) noexcept {
+    *this = std::move(other);
+  }
+
+  SpyHashStateImpl& operator=(SpyHashStateImpl&& other) noexcept {
+    hash_representation_ = std::move(other.hash_representation_);
+    error_ = other.error_;
+    moved_from_ = other.moved_from_;
+    other.moved_from_ = true;
+    return *this;
+  }
+
+  template <typename U>
+  SpyHashStateImpl(SpyHashStateImpl<U>&& other) {  // NOLINT
+    hash_representation_ = std::move(other.hash_representation_);
+    error_ = other.error_;
+    moved_from_ = other.moved_from_;
+    other.moved_from_ = true;
+  }
+
+  template <typename A, typename... Args>
+  static SpyHashStateImpl combine(SpyHashStateImpl s, const A& a,
+                                  const Args&... args) {
+    // Pass an instance of SpyHashStateImpl<A> when trying to combine `A`. This
+    // allows us to test that the user only uses this instance for combine calls
+    // and does not call AbslHashValue directly.
+    // See AbslHashValue implementation at the bottom.
+    s = SpyHashStateImpl<A>::HashStateBase::combine(std::move(s), a);
+    return SpyHashStateImpl::combine(std::move(s), args...);
+  }
+  static SpyHashStateImpl combine(SpyHashStateImpl s) {
+    if (direct_absl_hash_value_error_) {
+      *s.error_ = "AbslHashValue should not be invoked directly.";
+    } else if (s.moved_from_) {
+      *s.error_ = "Used moved-from instance of the hash state object.";
+    }
+    return s;
+  }
+
+  static void SetDirectAbslHashValueError() {
+    direct_absl_hash_value_error_ = true;
+  }
+
+  // Two SpyHashStateImpl objects are equal if they hold equal hash
+  // representations.
+  friend bool operator==(const SpyHashStateImpl& lhs,
+                         const SpyHashStateImpl& rhs) {
+    return lhs.hash_representation_ == rhs.hash_representation_;
+  }
+
+  friend bool operator!=(const SpyHashStateImpl& lhs,
+                         const SpyHashStateImpl& rhs) {
+    return !(lhs == rhs);
+  }
+
+  enum class CompareResult {
+    kEqual,
+    kASuffixB,
+    kBSuffixA,
+    kUnequal,
+  };
+
+  static CompareResult Compare(const SpyHashStateImpl& a,
+                               const SpyHashStateImpl& b) {
+    const std::string a_flat = absl::StrJoin(a.hash_representation_, "");
+    const std::string b_flat = absl::StrJoin(b.hash_representation_, "");
+    if (a_flat == b_flat) return CompareResult::kEqual;
+    if (absl::EndsWith(a_flat, b_flat)) return CompareResult::kBSuffixA;
+    if (absl::EndsWith(b_flat, a_flat)) return CompareResult::kASuffixB;
+    return CompareResult::kUnequal;
+  }
+
+  // operator<< prints the hash representation as a hex and ASCII dump, to
+  // facilitate debugging.
+  friend std::ostream& operator<<(std::ostream& out,
+                                  const SpyHashStateImpl& hash_state) {
+    out << "[\n";
+    for (auto& s : hash_state.hash_representation_) {
+      size_t offset = 0;
+      for (char c : s) {
+        if (offset % 16 == 0) {
+          out << absl::StreamFormat("\n0x%04x: ", offset);
+        }
+        if (offset % 2 == 0) {
+          out << " ";
+        }
+        out << absl::StreamFormat("%02x", c);
+        ++offset;
+      }
+      out << "\n";
+    }
+    return out << "]";
+  }
+
+  // The base case of the combine recursion, which writes raw bytes into the
+  // internal buffer.
+  static SpyHashStateImpl combine_contiguous(SpyHashStateImpl hash_state,
+                                             const unsigned char* begin,
+                                             size_t size) {
+    hash_state.hash_representation_.emplace_back(
+        reinterpret_cast<const char*>(begin), size);
+    return hash_state;
+  }
+
+  using SpyHashStateImpl::HashStateBase::combine_contiguous;
+
+  absl::optional<std::string> error() const {
+    if (moved_from_) {
+      return "Returned a moved-from instance of the hash state object.";
+    }
+    return *error_;
+  }
+
+ private:
+  template <typename U>
+  friend class SpyHashStateImpl;
+
+  // This is true if SpyHashStateImpl<T> has been passed to a call of
+  // AbslHashValue with the wrong type. This detects that the user called
+  // AbslHashValue directly (because the hash state type does not match).
+  static bool direct_absl_hash_value_error_;
+
+
+  std::vector<std::string> hash_representation_;
+  // This is a shared_ptr because we want all instances of the particular
+  // SpyHashState run to share the field. This way we can set the error for
+  // use-after-move and all the copies will see it.
+  std::shared_ptr<absl::optional<std::string>> error_;
+  bool moved_from_ = false;
+};
+
+template <typename T>
+bool SpyHashStateImpl<T>::direct_absl_hash_value_error_;
+
+template <bool& B>
+struct OdrUse {
+  constexpr OdrUse() {}
+  bool& b = B;
+};
+
+template <void (*)()>
+struct RunOnStartup {
+  static bool run;
+  static constexpr OdrUse<run> kOdrUse{};
+};
+
+template <void (*f)()>
+bool RunOnStartup<f>::run = (f(), true);
+
+template <
+    typename T, typename U,
+    // Only trigger for when (T != U),
+    absl::enable_if_t<!std::is_same<T, U>::value, int> = 0,
+    // This statement works in two ways:
+    //  - First, it instantiates RunOnStartup and forces the initialization of
+    //    `run`, which set the global variable.
+    //  - Second, it triggers a SFINAE error disabling the overload to prevent
+    //    compile time errors. If we didn't disable the overload we would get
+    //    ambiguous overload errors, which we don't want.
+    int = RunOnStartup<SpyHashStateImpl<T>::SetDirectAbslHashValueError>::run>
+void AbslHashValue(SpyHashStateImpl<T>, const U&);
+
+using SpyHashState = SpyHashStateImpl<void>;
+
+}  // namespace hash_internal
+}  // namespace absl
+
+#endif  // ABSL_HASH_INTERNAL_SPY_HASH_STATE_H_