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diff --git a/third_party/abseil_cpp/absl/random/bit_gen_ref.h b/third_party/abseil_cpp/absl/random/bit_gen_ref.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
+//
+//      https://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.
+//
+// -----------------------------------------------------------------------------
+// File: bit_gen_ref.h
+// -----------------------------------------------------------------------------
+//
+// This header defines a bit generator "reference" class, for use in interfaces
+// that take both Abseil (e.g. `absl::BitGen`) and standard library (e.g.
+// `std::mt19937`) bit generators.
+
+#ifndef ABSL_RANDOM_BIT_GEN_REF_H_
+#define ABSL_RANDOM_BIT_GEN_REF_H_
+
+#include "absl/base/macros.h"
+#include "absl/meta/type_traits.h"
+#include "absl/random/internal/distribution_caller.h"
+#include "absl/random/internal/fast_uniform_bits.h"
+#include "absl/random/internal/mocking_bit_gen_base.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace random_internal {
+
+template <typename URBG, typename = void, typename = void, typename = void>
+struct is_urbg : std::false_type {};
+
+template <typename URBG>
+struct is_urbg<
+    URBG,
+    absl::enable_if_t<std::is_same<
+        typename URBG::result_type,
+        typename std::decay<decltype((URBG::min)())>::type>::value>,
+    absl::enable_if_t<std::is_same<
+        typename URBG::result_type,
+        typename std::decay<decltype((URBG::max)())>::type>::value>,
+    absl::enable_if_t<std::is_same<
+        typename URBG::result_type,
+        typename std::decay<decltype(std::declval<URBG>()())>::type>::value>>
+    : std::true_type {};
+
+}  // namespace random_internal
+
+// -----------------------------------------------------------------------------
+// absl::BitGenRef
+// -----------------------------------------------------------------------------
+//
+// `absl::BitGenRef` is a type-erasing class that provides a generator-agnostic
+// non-owning "reference" interface for use in place of any specific uniform
+// random bit generator (URBG). This class may be used for both Abseil
+// (e.g. `absl::BitGen`, `absl::InsecureBitGen`) and Standard library (e.g
+// `std::mt19937`, `std::minstd_rand`) bit generators.
+//
+// Like other reference classes, `absl::BitGenRef` does not own the
+// underlying bit generator, and the underlying instance must outlive the
+// `absl::BitGenRef`.
+//
+// `absl::BitGenRef` is particularly useful when used with an
+// `absl::MockingBitGen` to test specific paths in functions which use random
+// values.
+//
+// Example:
+//    void TakesBitGenRef(absl::BitGenRef gen) {
+//      int x = absl::Uniform<int>(gen, 0, 1000);
+//    }
+//
+class BitGenRef {
+ public:
+  using result_type = uint64_t;
+
+  BitGenRef(const absl::BitGenRef&) = default;
+  BitGenRef(absl::BitGenRef&&) = default;
+  BitGenRef& operator=(const absl::BitGenRef&) = default;
+  BitGenRef& operator=(absl::BitGenRef&&) = default;
+
+  template <typename URBG,
+            typename absl::enable_if_t<
+                (!std::is_same<URBG, BitGenRef>::value &&
+                 random_internal::is_urbg<URBG>::value)>* = nullptr>
+  BitGenRef(URBG& gen)  // NOLINT
+      : mocked_gen_ptr_(MakeMockPointer(&gen)),
+        t_erased_gen_ptr_(reinterpret_cast<uintptr_t>(&gen)),
+        generate_impl_fn_(ImplFn<URBG>) {
+  }
+
+  static constexpr result_type(min)() {
+    return (std::numeric_limits<result_type>::min)();
+  }
+
+  static constexpr result_type(max)() {
+    return (std::numeric_limits<result_type>::max)();
+  }
+
+  result_type operator()() { return generate_impl_fn_(t_erased_gen_ptr_); }
+
+ private:
+  friend struct absl::random_internal::DistributionCaller<absl::BitGenRef>;
+  using impl_fn = result_type (*)(uintptr_t);
+  using mocker_base_t = absl::random_internal::MockingBitGenBase;
+
+  // Convert an arbitrary URBG pointer into either a valid mocker_base_t
+  // pointer or a nullptr.
+  static inline mocker_base_t* MakeMockPointer(mocker_base_t* t) { return t; }
+  static inline mocker_base_t* MakeMockPointer(void*) { return nullptr; }
+
+  template <typename URBG>
+  static result_type ImplFn(uintptr_t ptr) {
+    // Ensure that the return values from operator() fill the entire
+    // range promised by result_type, min() and max().
+    absl::random_internal::FastUniformBits<result_type> fast_uniform_bits;
+    return fast_uniform_bits(*reinterpret_cast<URBG*>(ptr));
+  }
+
+  mocker_base_t* mocked_gen_ptr_;
+  uintptr_t t_erased_gen_ptr_;
+  impl_fn generate_impl_fn_;
+};
+
+namespace random_internal {
+
+template <>
+struct DistributionCaller<absl::BitGenRef> {
+  template <typename DistrT, typename... Args>
+  static typename DistrT::result_type Call(absl::BitGenRef* gen_ref,
+                                           Args&&... args) {
+    auto* mock_ptr = gen_ref->mocked_gen_ptr_;
+    if (mock_ptr == nullptr) {
+      DistrT dist(std::forward<Args>(args)...);
+      return dist(*gen_ref);
+    } else {
+      return mock_ptr->template Call<DistrT>(std::forward<Args>(args)...);
+    }
+  }
+};
+
+}  // namespace random_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_RANDOM_BIT_GEN_REF_H_