5 files changed, 23 insertions, 8 deletions
diff --git a/absl/strings/cord_ring_test.cc b/absl/strings/cord_ring_test.cc
index cc8fbaf9..b1787edf 100644
--- a/absl/strings/cord_ring_test.cc
+++ b/absl/strings/cord_ring_test.cc
@@ -223,7 +223,7 @@ CordRepExternal* MakeFakeExternal(size_t length) {
     std::string s;
     explicit Rep(size_t len) {
       this->tag = EXTERNAL;
-      this->base = this->storage;
+      this->base = reinterpret_cast<const char*>(this->storage);
       this->length = len;
       this->releaser_invoker = [](CordRepExternal* self) {
         delete static_cast<Rep*>(self);
diff --git a/absl/strings/internal/cord_internal.cc b/absl/strings/internal/cord_internal.cc
index 905ffd0c..6a0b051c 100644
--- a/absl/strings/internal/cord_internal.cc
+++ b/absl/strings/internal/cord_internal.cc
@@ -25,6 +25,7 @@ namespace absl {
 ABSL_NAMESPACE_BEGIN
 namespace cord_internal {
 
+ABSL_CONST_INIT std::atomic<bool> cord_btree_enabled(kCordEnableBtreeDefault);
 ABSL_CONST_INIT std::atomic<bool> cord_ring_buffer_enabled(
     kCordEnableRingBufferDefault);
 ABSL_CONST_INIT std::atomic<bool> shallow_subcords_enabled(
diff --git a/absl/strings/internal/cord_internal.h b/absl/strings/internal/cord_internal.h
index bddc9815..371a7f9c 100644
--- a/absl/strings/internal/cord_internal.h
+++ b/absl/strings/internal/cord_internal.h
@@ -37,13 +37,19 @@ class CordzInfo;
 
 // Default feature enable states for cord ring buffers
 enum CordFeatureDefaults {
+  kCordEnableBtreeDefault = false,
   kCordEnableRingBufferDefault = false,
   kCordShallowSubcordsDefault = false
 };
 
+extern std::atomic<bool> cord_btree_enabled;
 extern std::atomic<bool> cord_ring_buffer_enabled;
 extern std::atomic<bool> shallow_subcords_enabled;
 
+inline void enable_cord_btree(bool enable) {
+  cord_btree_enabled.store(enable, std::memory_order_relaxed);
+}
+
 inline void enable_cord_ring_buffer(bool enable) {
   cord_ring_buffer_enabled.store(enable, std::memory_order_relaxed);
 }
@@ -193,7 +199,16 @@ struct CordRep {
   // If tag < FLAT, it represents CordRepKind and indicates the type of node.
   // Otherwise, the node type is CordRepFlat and the tag is the encoded size.
   uint8_t tag;
-  char storage[1];  // Starting point for flat array: MUST BE LAST FIELD
+
+  // `storage` provides two main purposes:
+  // - the starting point for FlatCordRep.Data() [flexible-array-member]
+  // - 3 bytes of additional storage for use by derived classes.
+  // The latter is used by CordrepConcat and CordRepBtree. CordRepConcat stores
+  // a 'depth' value in storage[0], and the (future) CordRepBtree class stores
+  // `height`, `begin` and `end` in the 3 entries. Otherwise we would need to
+  // allocate room for these in the derived class, as not all compilers reuse
+  // padding space from the base class (clang and gcc do, MSVC does not, etc)
+  uint8_t storage[3];
 
   inline CordRepRing* ring();
   inline const CordRepRing* ring() const;
@@ -228,8 +243,8 @@ struct CordRepConcat : public CordRep {
   CordRep* left;
   CordRep* right;
 
-  uint8_t depth() const { return static_cast<uint8_t>(storage[0]); }
-  void set_depth(uint8_t depth) { storage[0] = static_cast<char>(depth); }
+  uint8_t depth() const { return storage[0]; }
+  void set_depth(uint8_t depth) { storage[0] = depth; }
 };
 
 struct CordRepSubstring : public CordRep {
diff --git a/absl/strings/internal/cord_rep_flat.h b/absl/strings/internal/cord_rep_flat.h
index cddb282f..4d0f9886 100644
--- a/absl/strings/internal/cord_rep_flat.h
+++ b/absl/strings/internal/cord_rep_flat.h
@@ -118,8 +118,8 @@ struct CordRepFlat : public CordRep {
   }
 
   // Returns a pointer to the data inside this flat rep.
-  char* Data() { return storage; }
-  const char* Data() const { return storage; }
+  char* Data() { return reinterpret_cast<char*>(storage); }
+  const char* Data() const { return reinterpret_cast<const char*>(storage); }
 
   // Returns the maximum capacity (payload size) of this instance.
   size_t Capacity() const { return TagToLength(tag); }
diff --git a/absl/strings/str_format.h b/absl/strings/str_format.h
index 01465107..4b05c70c 100644
--- a/absl/strings/str_format.h
+++ b/absl/strings/str_format.h
@@ -536,8 +536,7 @@ using FormatArg = str_format_internal::FormatArgImpl;
 // The arguments are provided in an `absl::Span<const absl::FormatArg>`.
 // Each `absl::FormatArg` object binds to a single argument and keeps a
 // reference to it. The values used to create the `FormatArg` objects must
-// outlive this function call. (See `str_format_arg.h` for information on
-// the `FormatArg` class.)_
+// outlive this function call.
 //
 // Example:
 //