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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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+++ 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/internal/fast_type_id.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"
+
+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 {};
+
+template <typename>
+struct DistributionCaller;
+class MockHelpers;
+
+}  // 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 {
+  // SFINAE to detect whether the URBG type includes a member matching
+  // bool InvokeMock(base_internal::FastTypeIdType, void*, void*).
+  //
+  // These live inside BitGenRef so that they have friend access
+  // to MockingBitGen. (see similar methods in DistributionCaller).
+  template <template <class...> class Trait, class AlwaysVoid, class... Args>
+  struct detector : std::false_type {};
+  template <template <class...> class Trait, class... Args>
+  struct detector<Trait, absl::void_t<Trait<Args...>>, Args...>
+      : std::true_type {};
+
+  template <class T>
+  using invoke_mock_t = decltype(std::declval<T*>()->InvokeMock(
+      std::declval<base_internal::FastTypeIdType>(), std::declval<void*>(),
+      std::declval<void*>()));
+
+  template <typename T>
+  using HasInvokeMock = typename detector<invoke_mock_t, void, T>::type;
+
+ public:
+  BitGenRef(const BitGenRef&) = default;
+  BitGenRef(BitGenRef&&) = default;
+  BitGenRef& operator=(const BitGenRef&) = default;
+  BitGenRef& operator=(BitGenRef&&) = default;
+
+  template <typename URBG, typename absl::enable_if_t<
+                               (!std::is_same<URBG, BitGenRef>::value &&
+                                random_internal::is_urbg<URBG>::value &&
+                                !HasInvokeMock<URBG>::value)>* = nullptr>
+  BitGenRef(URBG& gen)  // NOLINT
+      : t_erased_gen_ptr_(reinterpret_cast<uintptr_t>(&gen)),
+        mock_call_(NotAMock),
+        generate_impl_fn_(ImplFn<URBG>) {}
+
+  template <typename URBG,
+            typename absl::enable_if_t<(!std::is_same<URBG, BitGenRef>::value &&
+                                        random_internal::is_urbg<URBG>::value &&
+                                        HasInvokeMock<URBG>::value)>* = nullptr>
+  BitGenRef(URBG& gen)  // NOLINT
+      : t_erased_gen_ptr_(reinterpret_cast<uintptr_t>(&gen)),
+        mock_call_(&MockCall<URBG>),
+        generate_impl_fn_(ImplFn<URBG>) {}
+
+  using result_type = uint64_t;
+
+  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:
+  using impl_fn = result_type (*)(uintptr_t);
+  using mock_call_fn = bool (*)(uintptr_t, base_internal::FastTypeIdType, void*,
+                                void*);
+
+  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));
+  }
+
+  // Get a type-erased InvokeMock pointer.
+  template <typename URBG>
+  static bool MockCall(uintptr_t gen_ptr, base_internal::FastTypeIdType type,
+                       void* result, void* arg_tuple) {
+    return reinterpret_cast<URBG*>(gen_ptr)->InvokeMock(type, result,
+                                                        arg_tuple);
+  }
+  static bool NotAMock(uintptr_t, base_internal::FastTypeIdType, void*, void*) {
+    return false;
+  }
+
+  inline bool InvokeMock(base_internal::FastTypeIdType type, void* args_tuple,
+                         void* result) {
+    if (mock_call_ == NotAMock) return false;  // avoids an indirect call.
+    return mock_call_(t_erased_gen_ptr_, type, args_tuple, result);
+  }
+
+  uintptr_t t_erased_gen_ptr_;
+  mock_call_fn mock_call_;
+  impl_fn generate_impl_fn_;
+
+  template <typename>
+  friend struct ::absl::random_internal::DistributionCaller;  // for InvokeMock
+  friend class ::absl::random_internal::MockHelpers;          // for InvokeMock
+};
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_RANDOM_BIT_GEN_REF_H_