Export of internal Abseil changes

--
756156bf03da050e8b27539a8247d9af7e44c6a2 by Abseil Team <absl-team@google.com>:

Fix a typo in cord.h: "accomodate" => "accommodate"

PiperOrigin-RevId: 356168875

--
638befdb342b608ec28910ee931ee200fdbe1fef by Samuel Benzaquen <sbenza@google.com>:

Fix float conversion for PPC.
In PPC `long double` is a double-double representation which behaves weirdly
wrt numeric_limits. Don't take `long double` into account when we are not
handling `long double` natively anyway.

Fix the convert test to always run the conversion even if we are not going to
compare against libc's printf result. This allows exercising the code itself to
make sure we don't trigger assertions or UB found by sanitizers.

PiperOrigin-RevId: 355857729

--
ff5f893319fa76b273c7785b76ef6c95b1791076 by Abseil Team <absl-team@google.com>:

Example usage tweak

PiperOrigin-RevId: 355695750

--
0efc454f90023fa651b226e5e3ba7395a3b60c6d by Benjamin Barenblat <bbaren@google.com>:

Remove endian-sensitivity from Abseil’s RNG

Ensure that the Abseil random number generator produces identical output
on both big- and little-endian platforms by byte-swapping appropriately
on big-endian systems.

PiperOrigin-RevId: 355635051
GitOrigin-RevId: 756156bf03da050e8b27539a8247d9af7e44c6a2
Change-Id: Iaaa69767b8e85d626742b9ba56fefb75f07c69ee

12 files changed, 146 insertions, 66 deletions

diff --git a/absl/base/BUILD.bazel b/absl/base/BUILD.bazel
index 5d67a507..7d2ff308 100644
--- a/absl/base/BUILD.bazel
+++ b/absl/base/BUILD.bazel
@@ -479,6 +479,7 @@ cc_library(
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
+        ":base",
         ":config",
         ":core_headers",
     ],
diff --git a/absl/base/CMakeLists.txt b/absl/base/CMakeLists.txt
index 3d930b85..981b8cc0 100644
--- a/absl/base/CMakeLists.txt
+++ b/absl/base/CMakeLists.txt
@@ -418,6 +418,7 @@ absl_cc_library(
   COPTS
     ${ABSL_DEFAULT_COPTS}
   DEPS
+    absl::base
     absl::config
     absl::core_headers
   PUBLIC
diff --git a/absl/base/internal/endian.h b/absl/base/internal/endian.h
index 9677530e..dad0e9ae 100644
--- a/absl/base/internal/endian.h
+++ b/absl/base/internal/endian.h
@@ -26,6 +26,7 @@
 #endif
 
 #include <cstdint>
+#include "absl/base/casts.h"
 #include "absl/base/config.h"
 #include "absl/base/internal/unaligned_access.h"
 #include "absl/base/port.h"
@@ -173,6 +174,36 @@ inline constexpr bool IsLittleEndian() { return false; }
 
 #endif /* ENDIAN */
 
+inline uint8_t FromHost(uint8_t x) { return x; }
+inline uint16_t FromHost(uint16_t x) { return FromHost16(x); }
+inline uint32_t FromHost(uint32_t x) { return FromHost32(x); }
+inline uint64_t FromHost(uint64_t x) { return FromHost64(x); }
+inline uint8_t ToHost(uint8_t x) { return x; }
+inline uint16_t ToHost(uint16_t x) { return ToHost16(x); }
+inline uint32_t ToHost(uint32_t x) { return ToHost32(x); }
+inline uint64_t ToHost(uint64_t x) { return ToHost64(x); }
+
+inline int8_t FromHost(int8_t x) { return x; }
+inline int16_t FromHost(int16_t x) {
+  return bit_cast<int16_t>(FromHost16(bit_cast<uint16_t>(x)));
+}
+inline int32_t FromHost(int32_t x) {
+  return bit_cast<int32_t>(FromHost32(bit_cast<uint32_t>(x)));
+}
+inline int64_t FromHost(int64_t x) {
+  return bit_cast<int64_t>(FromHost64(bit_cast<uint64_t>(x)));
+}
+inline int8_t ToHost(int8_t x) { return x; }
+inline int16_t ToHost(int16_t x) {
+  return bit_cast<int16_t>(ToHost16(bit_cast<uint16_t>(x)));
+}
+inline int32_t ToHost(int32_t x) {
+  return bit_cast<int32_t>(ToHost32(bit_cast<uint32_t>(x)));
+}
+inline int64_t ToHost(int64_t x) {
+  return bit_cast<int64_t>(ToHost64(bit_cast<uint64_t>(x)));
+}
+
 // Functions to do unaligned loads and stores in little-endian order.
 inline uint16_t Load16(const void *p) {
   return ToHost16(ABSL_INTERNAL_UNALIGNED_LOAD16(p));
@@ -233,6 +264,36 @@ inline constexpr bool IsLittleEndian() { return false; }
 
 #endif /* ENDIAN */
 
+inline uint8_t FromHost(uint8_t x) { return x; }
+inline uint16_t FromHost(uint16_t x) { return FromHost16(x); }
+inline uint32_t FromHost(uint32_t x) { return FromHost32(x); }
+inline uint64_t FromHost(uint64_t x) { return FromHost64(x); }
+inline uint8_t ToHost(uint8_t x) { return x; }
+inline uint16_t ToHost(uint16_t x) { return ToHost16(x); }
+inline uint32_t ToHost(uint32_t x) { return ToHost32(x); }
+inline uint64_t ToHost(uint64_t x) { return ToHost64(x); }
+
+inline int8_t FromHost(int8_t x) { return x; }
+inline int16_t FromHost(int16_t x) {
+  return bit_cast<int16_t>(FromHost16(bit_cast<uint16_t>(x)));
+}
+inline int32_t FromHost(int32_t x) {
+  return bit_cast<int32_t>(FromHost32(bit_cast<uint32_t>(x)));
+}
+inline int64_t FromHost(int64_t x) {
+  return bit_cast<int64_t>(FromHost64(bit_cast<uint64_t>(x)));
+}
+inline int8_t ToHost(int8_t x) { return x; }
+inline int16_t ToHost(int16_t x) {
+  return bit_cast<int16_t>(ToHost16(bit_cast<uint16_t>(x)));
+}
+inline int32_t ToHost(int32_t x) {
+  return bit_cast<int32_t>(ToHost32(bit_cast<uint32_t>(x)));
+}
+inline int64_t ToHost(int64_t x) {
+  return bit_cast<int64_t>(ToHost64(bit_cast<uint64_t>(x)));
+}
+
 // Functions to do unaligned loads and stores in big-endian order.
 inline uint16_t Load16(const void *p) {
   return ToHost16(ABSL_INTERNAL_UNALIGNED_LOAD16(p));
diff --git a/absl/random/CMakeLists.txt b/absl/random/CMakeLists.txt
index 7d7bec83..13093d6d 100644
--- a/absl/random/CMakeLists.txt
+++ b/absl/random/CMakeLists.txt
@@ -611,6 +611,7 @@ absl_cc_library(
     ${ABSL_DEFAULT_LINKOPTS}
   DEPS
     absl::config
+    absl::endian
   TESTONLY
 )
 
@@ -758,6 +759,7 @@ absl_cc_library(
   LINKOPTS
     ${ABSL_DEFAULT_LINKOPTS}
   DEPS
+    absl::endian
     absl::random_internal_iostream_state_saver
     absl::random_internal_randen
     absl::raw_logging_internal
@@ -1119,6 +1121,7 @@ absl_cc_test(
   LINKOPTS
     ${ABSL_DEFAULT_LINKOPTS}
   DEPS
+    absl::endian
     absl::random_internal_randen_slow
     gtest_main
 )
diff --git a/absl/random/internal/BUILD.bazel b/absl/random/internal/BUILD.bazel
index 2c1a5f4a..4e778aee 100644
--- a/absl/random/internal/BUILD.bazel
+++ b/absl/random/internal/BUILD.bazel
@@ -124,7 +124,10 @@ cc_library(
     ],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
-    deps = ["//absl/base:config"],
+    deps = [
+        "//absl/base:config",
+        "//absl/base:endian",
+    ],
 )
 
 cc_library(
@@ -242,6 +245,7 @@ cc_library(
     deps = [
         ":iostream_state_saver",
         ":randen",
+        "//absl/base:endian",
         "//absl/meta:type_traits",
     ],
 )
@@ -606,6 +610,7 @@ cc_test(
     deps = [
         ":platform",
         ":randen_slow",
+        "//absl/base:endian",
         "@com_google_googletest//:gtest_main",
     ],
 )
diff --git a/absl/random/internal/explicit_seed_seq.h b/absl/random/internal/explicit_seed_seq.h
index 6a743eaf..e3aa31a1 100644
--- a/absl/random/internal/explicit_seed_seq.h
+++ b/absl/random/internal/explicit_seed_seq.h
@@ -23,6 +23,7 @@
 #include <vector>
 
 #include "absl/base/config.h"
+#include "absl/base/internal/endian.h"
 
 namespace absl {
 ABSL_NAMESPACE_BEGIN
@@ -73,7 +74,7 @@ class ExplicitSeedSeq {
   template <typename OutIterator>
   void generate(OutIterator begin, OutIterator end) {
     for (size_t index = 0; begin != end; begin++) {
-      *begin = state_.empty() ? 0 : state_[index++];
+      *begin = state_.empty() ? 0 : little_endian::FromHost32(state_[index++]);
       if (index >= state_.size()) {
         index = 0;
       }
diff --git a/absl/random/internal/randen_engine.h b/absl/random/internal/randen_engine.h
index 6b337313..92bb8905 100644
--- a/absl/random/internal/randen_engine.h
+++ b/absl/random/internal/randen_engine.h
@@ -23,6 +23,7 @@
 #include <limits>
 #include <type_traits>
 
+#include "absl/base/internal/endian.h"
 #include "absl/meta/type_traits.h"
 #include "absl/random/internal/iostream_state_saver.h"
 #include "absl/random/internal/randen.h"
@@ -76,7 +77,7 @@ class alignas(16) randen_engine {
       impl_.Generate(state_);
     }
 
-    return state_[next_++];
+    return little_endian::ToHost(state_[next_++]);
   }
 
   template <class SeedSequence>
@@ -181,7 +182,8 @@ class alignas(16) randen_engine {
       // In the case that `elem` is `uint8_t`, it must be cast to something
       // larger so that it prints as an integer rather than a character. For
       // simplicity, apply the cast all circumstances.
-      os << static_cast<numeric_type>(elem) << os.fill();
+      os << static_cast<numeric_type>(little_endian::FromHost(elem))
+         << os.fill();
     }
     os << engine.next_;
     return os;
@@ -200,7 +202,7 @@ class alignas(16) randen_engine {
       // necessary to read a wider type and then cast it to uint8_t.
       numeric_type value;
       is >> value;
-      elem = static_cast<result_type>(value);
+      elem = little_endian::ToHost(static_cast<result_type>(value));
     }
     is >> next;
     if (is.fail()) {
diff --git a/absl/random/internal/randen_slow_test.cc b/absl/random/internal/randen_slow_test.cc
index 4a535837..4861ffa4 100644
--- a/absl/random/internal/randen_slow_test.cc
+++ b/absl/random/internal/randen_slow_test.cc
@@ -17,6 +17,7 @@
 #include <cstring>
 
 #include "gtest/gtest.h"
+#include "absl/base/internal/endian.h"
 #include "absl/random/internal/randen_traits.h"
 
 namespace {
@@ -56,7 +57,7 @@ TEST(RandenSlowTest, Default) {
 
   uint64_t* id = d.state;
   for (const auto& elem : kGolden) {
-    EXPECT_EQ(elem, *id++);
+    EXPECT_EQ(absl::little_endian::FromHost64(elem), *id++);
   }
 }
 
diff --git a/absl/strings/cord.h b/absl/strings/cord.h
index 320226d2..fa9cb913 100644
--- a/absl/strings/cord.h
+++ b/absl/strings/cord.h
@@ -25,7 +25,7 @@
 //
 // Because a Cord consists of these chunks, data can be added to or removed from
 // a Cord during its lifetime. Chunks may also be shared between Cords. Unlike a
-// `std::string`, a Cord can therefore accomodate data that changes over its
+// `std::string`, a Cord can therefore accommodate data that changes over its
 // lifetime, though it's not quite "mutable"; it can change only in the
 // attachment, detachment, or rearrangement of chunks of its constituent data.
 //
diff --git a/absl/strings/internal/str_format/convert_test.cc b/absl/strings/internal/str_format/convert_test.cc
index 375db0a0..926283cf 100644
--- a/absl/strings/internal/str_format/convert_test.cc
+++ b/absl/strings/internal/str_format/convert_test.cc
@@ -554,7 +554,8 @@ TEST_F(FormatConvertTest, Uint128) {
 }
 
 template <typename Floating>
-void TestWithMultipleFormatsHelper(const std::vector<Floating> &floats) {
+void TestWithMultipleFormatsHelper(const std::vector<Floating> &floats,
+                                   const std::set<Floating> &skip_verify) {
   const NativePrintfTraits &native_traits = VerifyNativeImplementation();
   // Reserve the space to ensure we don't allocate memory in the output itself.
   std::string str_format_result;
@@ -602,7 +603,16 @@ void TestWithMultipleFormatsHelper(const std::vector<Floating> &floats) {
           AppendPack(&str_format_result, format, absl::MakeSpan(args));
         }
 
-        if (string_printf_result != str_format_result) {
+#ifdef _MSC_VER
+        // MSVC has a different rounding policy than us so we can't test our
+        // implementation against the native one there.
+        continue;
+#elif defined(__APPLE__)
+        // Apple formats NaN differently (+nan) vs. (nan)
+        if (std::isnan(d)) continue;
+#endif
+        if (string_printf_result != str_format_result &&
+            skip_verify.find(d) == skip_verify.end()) {
           // We use ASSERT_EQ here because failures are usually correlated and a
           // bug would print way too many failed expectations causing the test
           // to time out.
@@ -616,12 +626,6 @@ void TestWithMultipleFormatsHelper(const std::vector<Floating> &floats) {
 }
 
 TEST_F(FormatConvertTest, Float) {
-#ifdef _MSC_VER
-  // MSVC has a different rounding policy than us so we can't test our
-  // implementation against the native one there.
-  return;
-#endif  // _MSC_VER
-
   std::vector<float> floats = {0.0f,
                                -0.0f,
                                .9999999f,
@@ -635,7 +639,8 @@ TEST_F(FormatConvertTest, Float) {
                                std::numeric_limits<float>::epsilon(),
                                std::numeric_limits<float>::epsilon() + 1.0f,
                                std::numeric_limits<float>::infinity(),
-                               -std::numeric_limits<float>::infinity()};
+                               -std::numeric_limits<float>::infinity(),
+                               std::nanf("")};
 
   // Some regression tests.
   floats.push_back(0.999999989f);
@@ -664,21 +669,14 @@ TEST_F(FormatConvertTest, Float) {
   std::sort(floats.begin(), floats.end());
   floats.erase(std::unique(floats.begin(), floats.end()), floats.end());
 
-#ifndef __APPLE__
-  // Apple formats NaN differently (+nan) vs. (nan)
-  floats.push_back(std::nan(""));
-#endif
-
-  TestWithMultipleFormatsHelper(floats);
+  TestWithMultipleFormatsHelper(floats, {});
 }
 
 TEST_F(FormatConvertTest, Double) {
-#ifdef _MSC_VER
-  // MSVC has a different rounding policy than us so we can't test our
-  // implementation against the native one there.
-  return;
-#endif  // _MSC_VER
-
+  // For values that we know won't match the standard library implementation we
+  // skip verification, but still run the algorithm to catch asserts/sanitizer
+  // bugs.
+  std::set<double> skip_verify;
   std::vector<double> doubles = {0.0,
                                  -0.0,
                                  .99999999999999,
@@ -692,7 +690,8 @@ TEST_F(FormatConvertTest, Double) {
                                  std::numeric_limits<double>::epsilon(),
                                  std::numeric_limits<double>::epsilon() + 1,
                                  std::numeric_limits<double>::infinity(),
-                                 -std::numeric_limits<double>::infinity()};
+                                 -std::numeric_limits<double>::infinity(),
+                                 std::nan("")};
 
   // Some regression tests.
   doubles.push_back(0.99999999999999989);
@@ -722,33 +721,29 @@ TEST_F(FormatConvertTest, Double) {
       "5084551339423045832369032229481658085593321233482747978262041447231"
       "68738177180919299881250404026184124858368.000000";
 
-  if (!gcc_bug_22142) {
-    for (int exp = -300; exp <= 300; ++exp) {
-      const double all_ones_mantissa = 0x1fffffffffffff;
-      doubles.push_back(std::ldexp(all_ones_mantissa, exp));
+  for (int exp = -300; exp <= 300; ++exp) {
+    const double all_ones_mantissa = 0x1fffffffffffff;
+    doubles.push_back(std::ldexp(all_ones_mantissa, exp));
+    if (gcc_bug_22142) {
+      skip_verify.insert(doubles.back());
     }
   }
 
   if (gcc_bug_22142) {
-    for (auto &d : doubles) {
-      using L = std::numeric_limits<double>;
-      double d2 = std::abs(d);
-      if (d2 == L::max() || d2 == L::min() || d2 == L::denorm_min()) {
-        d = 0;
-      }
-    }
+    using L = std::numeric_limits<double>;
+    skip_verify.insert(L::max());
+    skip_verify.insert(L::min());  // NOLINT
+    skip_verify.insert(L::denorm_min());
+    skip_verify.insert(-L::max());
+    skip_verify.insert(-L::min());  // NOLINT
+    skip_verify.insert(-L::denorm_min());
   }
 
   // Remove duplicates to speed up the logic below.
   std::sort(doubles.begin(), doubles.end());
   doubles.erase(std::unique(doubles.begin(), doubles.end()), doubles.end());
 
-#ifndef __APPLE__
-  // Apple formats NaN differently (+nan) vs. (nan)
-  doubles.push_back(std::nan(""));
-#endif
-
-  TestWithMultipleFormatsHelper(doubles);
+  TestWithMultipleFormatsHelper(doubles, skip_verify);
 }
 
 TEST_F(FormatConvertTest, DoubleRound) {
@@ -1069,11 +1064,6 @@ TEST_F(FormatConvertTest, ExtremeWidthPrecision) {
 }
 
 TEST_F(FormatConvertTest, LongDouble) {
-#ifdef _MSC_VER
-  // MSVC has a different rounding policy than us so we can't test our
-  // implementation against the native one there.
-  return;
-#endif  // _MSC_VER
   const NativePrintfTraits &native_traits = VerifyNativeImplementation();
   const char *const kFormats[] = {"%",    "%.3", "%8.5", "%9",  "%.5000",
                                   "%.60", "%+",  "% ",   "%-10"};
@@ -1134,10 +1124,18 @@ TEST_F(FormatConvertTest, LongDouble) {
       for (auto d : doubles) {
         FormatArgImpl arg(d);
         UntypedFormatSpecImpl format(fmt_str);
+        std::string result = FormatPack(format, {&arg, 1});
+
+#ifdef _MSC_VER
+        // MSVC has a different rounding policy than us so we can't test our
+        // implementation against the native one there.
+        continue;
+#endif  // _MSC_VER
+
         // We use ASSERT_EQ here because failures are usually correlated and a
         // bug would print way too many failed expectations causing the test to
         // time out.
-        ASSERT_EQ(StrPrint(fmt_str.c_str(), d), FormatPack(format, {&arg, 1}))
+        ASSERT_EQ(StrPrint(fmt_str.c_str(), d), result)
             << fmt_str << " " << StrPrint("%.18Lg", d) << " "
             << StrPrint("%La", d) << " " << StrPrint("%.1080Lf", d);
       }
diff --git a/absl/strings/internal/str_format/float_conversion.cc b/absl/strings/internal/str_format/float_conversion.cc
index 2aa41aa7..2b1fd8cb 100644
--- a/absl/strings/internal/str_format/float_conversion.cc
+++ b/absl/strings/internal/str_format/float_conversion.cc
@@ -112,12 +112,22 @@ inline uint64_t DivideBy10WithCarry(uint64_t *v, uint64_t carry) {
   return next_carry % divisor;
 }
 
+constexpr bool IsDoubleDouble() {
+  // This is the `double-double` representation of `long double`.
+  // We do not handle it natively. Fallback to snprintf.
+  return std::numeric_limits<long double>::digits ==
+         2 * std::numeric_limits<double>::digits;
+}
+
+using MaxFloatType =
+    typename std::conditional<IsDoubleDouble(), double, long double>::type;
+
 // Generates the decimal representation for an integer of the form `v * 2^exp`,
 // where `v` and `exp` are both positive integers.
 // It generates the digits from the left (ie the most significant digit first)
 // to allow for direct printing into the sink.
 //
-// Requires `0 <= exp` and `exp <= numeric_limits<long double>::max_exponent`.
+// Requires `0 <= exp` and `exp <= numeric_limits<MaxFloatType>::max_exponent`.
 class BinaryToDecimal {
   static constexpr int ChunksNeeded(int exp) {
     // We will left shift a uint128 by `exp` bits, so we need `128+exp` total
@@ -132,10 +142,10 @@ class BinaryToDecimal {
   static void RunConversion(uint128 v, int exp,
                             absl::FunctionRef<void(BinaryToDecimal)> f) {
     assert(exp > 0);
-    assert(exp <= std::numeric_limits<long double>::max_exponent);
+    assert(exp <= std::numeric_limits<MaxFloatType>::max_exponent);
     static_assert(
         static_cast<int>(StackArray::kMaxCapacity) >=
-            ChunksNeeded(std::numeric_limits<long double>::max_exponent),
+            ChunksNeeded(std::numeric_limits<MaxFloatType>::max_exponent),
         "");
 
     StackArray::RunWithCapacity(
@@ -232,14 +242,14 @@ class BinaryToDecimal {
 
 // Converts a value of the form `x * 2^-exp` into a sequence of decimal digits.
 // Requires `-exp < 0` and
-// `-exp >= limits<long double>::min_exponent - limits<long double>::digits`.
+// `-exp >= limits<MaxFloatType>::min_exponent - limits<MaxFloatType>::digits`.
 class FractionalDigitGenerator {
  public:
   // Run the conversion for `v * 2^exp` and call `f(generator)`.
   // This function will allocate enough stack space to perform the conversion.
   static void RunConversion(
       uint128 v, int exp, absl::FunctionRef<void(FractionalDigitGenerator)> f) {
-    using Limits = std::numeric_limits<long double>;
+    using Limits = std::numeric_limits<MaxFloatType>;
     assert(-exp < 0);
     assert(-exp >= Limits::min_exponent - 128);
     static_assert(StackArray::kMaxCapacity >=
@@ -871,10 +881,10 @@ void FormatA(const HexFloatTypeParams float_traits, Int mantissa, int exp,
   // This buffer holds the "0x1.ab1de3" portion of "0x1.ab1de3pe+2". Compute the
   // size with long double which is the largest of the floats.
   constexpr size_t kBufSizeForHexFloatRepr =
-      2                                               // 0x
-      + std::numeric_limits<long double>::digits / 4  // number of hex digits
-      + 1                                             // round up
-      + 1;                                            // "." (dot)
+      2                                                // 0x
+      + std::numeric_limits<MaxFloatType>::digits / 4  // number of hex digits
+      + 1                                              // round up
+      + 1;                                             // "." (dot)
   char digits_buffer[kBufSizeForHexFloatRepr];
   char *digits_iter = digits_buffer;
   const char *const digits =
@@ -1393,10 +1403,7 @@ bool FloatToSink(const Float v, const FormatConversionSpecImpl &conv,
 
 bool ConvertFloatImpl(long double v, const FormatConversionSpecImpl &conv,
                       FormatSinkImpl *sink) {
-  if (std::numeric_limits<long double>::digits ==
-      2 * std::numeric_limits<double>::digits) {
-    // This is the `double-double` representation of `long double`.
-    // We do not handle it natively. Fallback to snprintf.
+  if (IsDoubleDouble()) {
     return FallbackToSnprintf(v, conv, sink);
   }
 
diff --git a/absl/synchronization/mutex.h b/absl/synchronization/mutex.h
index 4dd51fec..8c6d573d 100644
--- a/absl/synchronization/mutex.h
+++ b/absl/synchronization/mutex.h
@@ -147,7 +147,7 @@ class ABSL_LOCKABLE Mutex {
   //
   // Example usage:
   //   namespace foo {
-  //   ABSL_CONST_INIT Mutex mu(absl::kConstInit);
+  //   ABSL_CONST_INIT absl::Mutex mu(absl::kConstInit);
   //   }
   explicit constexpr Mutex(absl::ConstInitType);