PR #1860: Add unsigned to character buffers to ensure they can provide storage (https://eel.is/c++draft/intro.object#3)

Imported from GitHub PR https://github.com/abseil/abseil-cpp/pull/1860

I just learned today that `char` cannot "provide storage" in the way I previously thought. Only `std::byte` and `unsigned char` are allowed. So I decided to fix my past mistakes in `absl::InlinedVector` and `absl::Cleanup` as well as any others I could find in Abseil. See: https://eel.is/c++draft/intro.object#3

Merge ae9c1ef57a into c4ff4d561c

Merging this change closes #1860

COPYBARA_INTEGRATE_REVIEW=https://github.com/abseil/abseil-cpp/pull/1860 from CJ-Johnson:master ae9c1ef57a
PiperOrigin-RevId: 739220589
Change-Id: I533b040e4cf4ee2f48008affb6603c25f34d5b4b

absl/cleanup/internal/cleanup.h

@@ -88,7 +88,7 @@ class Storage {
 
  private:
   bool is_callback_engaged_;
-  alignas(Callback) char callback_buffer_[sizeof(Callback)];
+  alignas(Callback) unsigned char callback_buffer_[sizeof(Callback)];
 };
 
 }  // namespace cleanup_internal

absl/container/fixed_array.h

@@ -447,7 +447,8 @@ class ABSL_ATTRIBUTE_WARN_UNUSED FixedArray {
 
    private:
     ABSL_ADDRESS_SANITIZER_REDZONE(redzone_begin_);
-    alignas(StorageElement) char buff_[sizeof(StorageElement[inline_elements])];
+    alignas(StorageElement) unsigned char buff_[sizeof(
+        StorageElement[inline_elements])];
     ABSL_ADDRESS_SANITIZER_REDZONE(redzone_end_);
   };
 

absl/container/internal/inlined_vector.h

@@ -543,7 +543,7 @@ class Storage {
       (std::max)(N, sizeof(Allocated) / sizeof(ValueType<A>));
 
   struct Inlined {
-    alignas(ValueType<A>) char inlined_data[sizeof(
+    alignas(ValueType<A>) unsigned char inlined_data[sizeof(
         ValueType<A>[kOptimalInlinedSize])];
   };
 

absl/flags/internal/flag.h

@@ -783,7 +783,7 @@ class FlagImpl final : public CommandLineFlag {
   // heap allocation during initialization, which is both slows program startup
   // and can fail. Using reserved space + placement new allows us to avoid both
   // problems.
-  alignas(absl::Mutex) mutable char data_guard_[sizeof(absl::Mutex)];
+  alignas(absl::Mutex) mutable unsigned char data_guard_[sizeof(absl::Mutex)];
 };
 #if defined(__GNUC__) && !defined(__clang__)
 #pragma GCC diagnostic pop
@@ -876,7 +876,8 @@ class FlagImplPeer {
 template <typename T>
 void* FlagOps(FlagOp op, const void* v1, void* v2, void* v3) {
   struct AlignedSpace {
-    alignas(MaskedPointer::RequiredAlignment()) alignas(T) char buf[sizeof(T)];
+    alignas(MaskedPointer::RequiredAlignment()) alignas(
+        T) unsigned char buf[sizeof(T)];
   };
   using Allocator = std::allocator<AlignedSpace>;
   switch (op) {

absl/flags/internal/registry.h

@@ -73,7 +73,7 @@ void FinalizeRegistry();
 //
 
 // Retire flag with name "name" and type indicated by ops.
-void Retire(const char* name, FlagFastTypeId type_id, char* buf);
+void Retire(const char* name, FlagFastTypeId type_id, unsigned char* buf);
 
 constexpr size_t kRetiredFlagObjSize = 3 * sizeof(void*);
 constexpr size_t kRetiredFlagObjAlignment = alignof(void*);
@@ -87,7 +87,7 @@ class RetiredFlag {
   }
 
  private:
-  alignas(kRetiredFlagObjAlignment) char buf_[kRetiredFlagObjSize];
+  alignas(kRetiredFlagObjAlignment) unsigned char buf_[kRetiredFlagObjSize];
 };
 
 }  // namespace flags_internal

absl/flags/reflection.cc

@@ -289,11 +289,10 @@ class RetiredFlagObj final : public CommandLineFlag {
 
 }  // namespace
 
-void Retire(const char* name, FlagFastTypeId type_id, char* buf) {
+void Retire(const char* name, FlagFastTypeId type_id, unsigned char* buf) {
   static_assert(sizeof(RetiredFlagObj) == kRetiredFlagObjSize, "");
   static_assert(alignof(RetiredFlagObj) == kRetiredFlagObjAlignment, "");
-  auto* flag = ::new (static_cast<void*>(buf))
-      flags_internal::RetiredFlagObj(name, type_id);
+  auto* flag = ::new (buf) flags_internal::RetiredFlagObj(name, type_id);
   FlagRegistry::GlobalRegistry().RegisterFlag(*flag, nullptr);
 }
 

absl/functional/internal/any_invocable.h

@@ -174,7 +174,7 @@ union TypeErasedState {
   } remote;
 
   // Local-storage for the type-erased object when small and trivial enough
-  alignas(kAlignment) char storage[kStorageSize];
+  alignas(kAlignment) unsigned char storage[kStorageSize];
 };
 
 // A typed accessor for the object in `TypeErasedState` storage

absl/synchronization/mutex_test.cc

@@ -1725,7 +1725,7 @@ TEST(Mutex, Logging) {
 TEST(Mutex, LoggingAddressReuse) {
   // Repeatedly re-create a Mutex with debug logging at the same address.
   ScopedInvariantDebugging scoped_debugging;
-  alignas(absl::Mutex) char storage[sizeof(absl::Mutex)];
+  alignas(absl::Mutex) unsigned char storage[sizeof(absl::Mutex)];
   auto invariant =
       +[](void *alive) { EXPECT_TRUE(*static_cast<bool *>(alive)); };
   constexpr size_t kIters = 10;