Export of internal Abseil changes

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803abc2dcad8b2354c988e9bf58dac4a17683832 by Gennadiy Rozental <rogeeff@google.com>:

Avoid warning when RTTI is not enabled.

PiperOrigin-RevId: 294247546

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5a7b0b4d07d1d6e56fbb0b0ffbf4f8fcab772dbf by Derek Mauro <dmauro@google.com>:

Add a public Abseil FAQ

PiperOrigin-RevId: 294226960

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6945c4a6df7d7679711fea31aacf4fba6ac7baa1 by Gennadiy Rozental <rogeeff@google.com>:

Re-enable type mismatch check, which works in all the cases including shared libraries.

We will use RTTI in case when our hand written approximation of it reports a type mismatch. This way we can ensure that if a flag is defined in one shared object and referenced in another we do not report spurious errors.

PiperOrigin-RevId: 293905563
GitOrigin-RevId: 803abc2dcad8b2354c988e9bf58dac4a17683832
Change-Id: I1a23776d227ed2734c2e7183323786b7a95c3cc7

absl/flags/internal/commandlineflag.h

@@ -21,9 +21,11 @@
 
 #include <memory>
 #include <string>
+#include <typeinfo>
 
 #include "absl/base/config.h"
 #include "absl/base/macros.h"
+#include "absl/flags/config.h"
 #include "absl/flags/marshalling.h"
 #include "absl/strings/string_view.h"
 #include "absl/types/optional.h"
@@ -41,7 +43,10 @@ enum FlagOp {
   kCopyConstruct,
   kSizeof,
   kParse,
-  kUnparse
+  kUnparse,
+#if defined(ABSL_FLAGS_INTERNAL_HAS_RTTI)
+  kRuntimeTypeId
+#endif
 };
 using FlagOpFn = void* (*)(FlagOp, const void*, void*);
 using FlagMarshallingOpFn = void* (*)(FlagOp, const void*, void*, void*);
@@ -84,6 +89,11 @@ void* FlagOps(FlagOp op, const void* v1, void* v2) {
       return nullptr;
     case kSizeof:
       return reinterpret_cast<void*>(sizeof(T));
+#if defined(ABSL_FLAGS_INTERNAL_HAS_RTTI)
+    case kRuntimeTypeId:
+      return const_cast<std::type_info*>(&typeid(T));
+      break;
+#endif
     default:
       return nullptr;
   }
@@ -146,6 +156,13 @@ inline size_t Sizeof(FlagOpFn op) {
       op(flags_internal::kSizeof, nullptr, nullptr)));
 }
 
+#if defined(ABSL_FLAGS_INTERNAL_HAS_RTTI)
+inline const std::type_info& RuntimeTypeId(FlagOpFn op) {
+  return *static_cast<const std::type_info*>(
+      op(flags_internal::kRuntimeTypeId, nullptr, nullptr));
+}
+#endif
+
 // Handle to FlagState objects. Specific flag state objects will restore state
 // of a flag produced this flag state from method CommandLineFlag::SaveState().
 class FlagStateInterface {