Work around an implicit conversion signedness compiler warning

Addition and subtraction operators std::array::iterator are defined only for
ptrdiff_t, which is signed, instead of size_t, which is unsigned. Therefore,
adding the index variable to ar.begin() will trigger -Wsign-conversion if
std::array::iterator is not a raw pointer because the index variable will be
implicitly converted from size_t (an unsigned type) to ptrdiff_t (a signed
type). To fix this, we explicitly static_cast index to a ptrdiff_t.
PiperOrigin-RevId: 613662928
Change-Id: I5e06c2261d7b8f167fae7bb6acece076257f8579

absl/strings/internal/str_split_internal.h

@@ -30,6 +30,7 @@
 #define ABSL_STRINGS_INTERNAL_STR_SPLIT_INTERNAL_H_
 
 #include <array>
+#include <cstddef>
 #include <initializer_list>
 #include <iterator>
 #include <tuple>
@@ -402,7 +403,10 @@ class Splitter {
           ar[index].size = it->size();
           ++it;
         } while (++index != ar.size() && !it.at_end());
-        v.insert(v.end(), ar.begin(), ar.begin() + index);
+        // We static_cast index to a signed type to work around overzealous
+        // compiler warnings about signedness.
+        v.insert(v.end(), ar.begin(),
+                 ar.begin() + static_cast<ptrdiff_t>(index));
       }
       return v;
     }