Export of internal Abseil changes

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cfd7ee2487ed9b5636d8f83d3850c02e3b4a5cb0 by Jorg Brown <jorg@google.com>:

Add union padding to AsTree, to avoid issues on 32-bit MSVC 2015 compilers

alignas() causes compiler errors as per https://godbolt.org/z/vaTKjn
This change uses explicit padding to achieve what we want: https://godbolt.org/z/Mfjhhj

PiperOrigin-RevId: 353211413

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b1ac7430ffdefe58c01b29e9acd182cda4630e1d by Jorg Brown <jorg@google.com>:

Make the casting functions flat() and ring() be static_cast, rather than reinterpret_cast.

PiperOrigin-RevId: 353149543

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c37a6761c31720317c8b0b7db62b693643a88586 by Abseil Team <absl-team@google.com>:

Integrate CordRepRing logic into cord (but do not enable it)

PiperOrigin-RevId: 353135656

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2007fd3045ed6285106795cf8f2e6d792922f5e8 by Abseil Team <absl-team@google.com>:

Fix a typo in the description of ::equal_range(). The correct return is actually a half-open range [first, last).

PiperOrigin-RevId: 353122213

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6683fa2ba7271dd1f575bd7742d97f47a034c9d2 by Abseil Team <absl-team@google.com>:

Integrate CordRepRing logic into cord (but do not enable it)

PiperOrigin-RevId: 353121763
GitOrigin-RevId: cfd7ee2487ed9b5636d8f83d3850c02e3b4a5cb0
Change-Id: I6635163cd634706f5462c4065aa278e6bf193a72

absl/container/btree_map.h

@@ -384,9 +384,8 @@ class btree_map
 
   // btree_map::equal_range()
   //
-  // Returns a closed range [first, last], defined by a `std::pair` of two
+  // Returns a half-open range [first, last), defined by a `std::pair` of two
-  // iterators, containing all elements with the passed key in the
+  // iterators, containing all elements with the passed key in the `btree_map`.
-  // `btree_map`.
   using Base::equal_range;
 
   // btree_map::find()
@@ -709,7 +708,7 @@ class btree_multimap
 
   // btree_multimap::equal_range()
   //
-  // Returns a closed range [first, last], defined by a `std::pair` of two
+  // Returns a half-open range [first, last), defined by a `std::pair` of two
   // iterators, containing all elements with the passed key in the
   // `btree_multimap`.
   using Base::equal_range;

absl/strings/internal/cord_internal.h

@@ -456,8 +456,14 @@ class InlineData {
   struct AsTree {
     explicit constexpr AsTree(absl::cord_internal::CordRep* tree)
         : rep(tree), cordz_info(kNullCordzInfo) {}
-    absl::cord_internal::CordRep* rep;
+    // This union uses up extra space so that whether rep is 32 or 64 bits,
-    alignas(sizeof(cordz_info_t)) cordz_info_t cordz_info;
+    // cordz_info will still start at the eighth byte, and the last
+    // byte of cordz_info will still be the last byte of InlineData.
+    union {
+      absl::cord_internal::CordRep* rep;
+      cordz_info_t unused_aligner;
+    };
+    cordz_info_t cordz_info;
   };
 
   char& tag() { return reinterpret_cast<char*>(this)[kMaxInline]; }
@@ -505,26 +511,6 @@ inline const CordRepExternal* CordRep::external() const {
   return static_cast<const CordRepExternal*>(this);
 }
 
-inline CordRepFlat* CordRep::flat() {
-  assert(tag >= FLAT && tag <= MAX_FLAT_TAG);
-  return reinterpret_cast<CordRepFlat*>(this);
-}
-
-inline const CordRepFlat* CordRep::flat() const {
-  assert(tag >= FLAT && tag <= MAX_FLAT_TAG);
-  return reinterpret_cast<const CordRepFlat*>(this);
-}
-
-inline CordRepRing* CordRep::ring() {
-  assert(tag == RING);
-  return reinterpret_cast<CordRepRing*>(this);
-}
-
-inline const CordRepRing* CordRep::ring() const {
-  assert(tag == RING);
-  return reinterpret_cast<const CordRepRing*>(this);
-}
-
 inline CordRep* CordRep::Ref(CordRep* rep) {
   assert(rep != nullptr);
   rep->refcount.Increment();

absl/strings/internal/cord_rep_flat.h

@@ -127,6 +127,17 @@ struct CordRepFlat : public CordRep {
   size_t AllocatedSize() const { return TagToAllocatedSize(tag); }
 };
 
+// Now that CordRepFlat is defined, we can define CordRep's helper casts:
+inline CordRepFlat* CordRep::flat() {
+  assert(tag >= FLAT && tag <= MAX_FLAT_TAG);
+  return reinterpret_cast<CordRepFlat*>(this);
+}
+
+inline const CordRepFlat* CordRep::flat() const {
+  assert(tag >= FLAT && tag <= MAX_FLAT_TAG);
+  return reinterpret_cast<const CordRepFlat*>(this);
+}
+
 }  // namespace cord_internal
 ABSL_NAMESPACE_END
 }  // namespace absl

absl/strings/internal/cord_rep_ring.h

@@ -563,6 +563,17 @@ inline CordRepRing::Position CordRepRing::FindTail(index_type head,
   return (offset == length) ? Position{tail_, 0} : FindTailSlow(head, offset);
 }
 
+// Now that CordRepRing is defined, we can define CordRep's helper casts:
+inline CordRepRing* CordRep::ring() {
+  assert(tag == RING);
+  return static_cast<CordRepRing*>(this);
+}
+
+inline const CordRepRing* CordRep::ring() const {
+  assert(tag == RING);
+  return static_cast<const CordRepRing*>(this);
+}
+
 std::ostream& operator<<(std::ostream& s, const CordRepRing& rep);
 
 #ifdef __clang__