Fix strict-aliasing violation in typeinfo::hash_code()

[EricWF]

Summary

The current implementation of hash_code() for uniqued RTTI strings violates strict aliasing by dereferencing a type-punned pointer. Specifically it generates a const char** pointer from the address of the __name member before casting it to const size_t* and dereferencing it to get the hash. This is really just a complex and incorrect way of writing reinterpret_cast<size_t>(__name).

This patch changes the conversion sequence so that it no longer contains UB.

[rjmccall]

LGTM, but I'm not a code owner here.

[mclow.lists]

Adding a reference here: https://llvm.org/bugs/show_bug.cgi?id=30613

[mclow.lists]

LGTM.

include/__config

 #endif
 
 #ifdef __GNUC__
 #define _NOALIAS __attribute__((__malloc__))
 #else
 #define _NOALIAS
 #endif
 
-#ifdef __GNUC__
-#define _LIBCPP_MAY_ALIAS __attribute__((__may_alias__))
-#else
-#define _LIBCPP_MAY_ALIAS
-#endif
-
 #if __has_feature(cxx_explicit_conversions) || defined(__IBMCPP__)
 #   define _LIBCPP_EXPLICIT explicit
 #else
 #   define _LIBCPP_EXPLICIT
 #endif
 
 #if !__has_builtin(__builtin_operator_new) || !__has_builtin(__builtin_operator_delete)
 #   define _LIBCPP_HAS_NO_BUILTIN_OPERATOR_NEW_DELETE

include/typeinfo

 namespace std  // purposefully not using versioning namespace
 {
 
 class _LIBCPP_EXCEPTION_ABI type_info
 {
     type_info& operator=(const type_info&);
     type_info(const type_info&);
 
-    typedef size_t _LIBCPP_MAY_ALIAS _ASizeT; // Avoid strict-aliasing issues.
-
 protected:
 #ifndef _LIBCPP_NONUNIQUE_RTTI_BIT
     const char* __type_name;
 #else
     // A const char* with the non-unique RTTI bit possibly set.
     uintptr_t __type_name;
 #endif
 
#else
         {if (!((__type_name & __arg.__type_name) & _LIBCPP_NONUNIQUE_RTTI_BIT))
            return __type_name < __arg.__type_name;
          return __compare_nonunique_names(__arg) < 0;}
 #endif
 
     _LIBCPP_INLINE_VISIBILITY
     size_t hash_code() const _NOEXCEPT
 #ifndef _LIBCPP_NONUNIQUE_RTTI_BIT
-        {return *reinterpret_cast<const _ASizeT *>(&__type_name);}
+        {return reinterpret_cast<size_t>(__type_name);}
 #else
         {if (!(__type_name & _LIBCPP_NONUNIQUE_RTTI_BIT)) return __type_name;
          const char *__ptr = name();
          size_t __hash = 5381;
          while (unsigned char __c = static_cast<unsigned char>(*__ptr++))
            __hash = (__hash * 33) ^ __c;
          return __hash;}
 #endif