Fixes StrCat() performance regression when not using libc++

65d7b6d4 changed StrCat() to not use an intermediate buffer when the
result fits in the SSO buffer, but only libc++ has an SSO buffer large
enough for this optimization to work.

PiperOrigin-RevId: 564447163
Change-Id: I0c7fa4afed3369b36e13e7d1691eb7f933ea0091

absl/strings/str_cat.h

@@ -498,11 +498,28 @@ inline std::string SingleArgStrCat(unsigned long long x) {
 inline std::string SingleArgStrCat(float x) { return FloatToString(x); }
 inline std::string SingleArgStrCat(double x) { return FloatToString(x); }
 
-
-template <typename T, typename = std::enable_if_t<std::is_arithmetic<T>{} &&
-                                                  !std::is_same<T, char>{}>>
+// As of September 2023, the SingleArgStrCat() optimization is only enabled for
+// libc++. The reasons for this are:
+// 1) The SSO size for libc++ is 23, while libstdc++ and MSSTL have an SSO size
+// of 15. Since IntegerToString unconditionally resizes the string to 22 bytes,
+// this causes both libstdc++ and MSSTL to allocate.
+// 2) strings_internal::STLStringResizeUninitialized() only has an
+// implementation that avoids initialization when using libc++. This isn't as
+// relevant as (1), and the cost should be benchmarked if (1) ever changes on
+// libstc++ or MSSTL.
+#ifdef _LIBCPP_VERSION
+#define ABSL_INTERNAL_STRCAT_ENABLE_FAST_CASE true
+#else
+#define ABSL_INTERNAL_STRCAT_ENABLE_FAST_CASE false
+#endif
+
+template <typename T, typename = std::enable_if_t<
+                          ABSL_INTERNAL_STRCAT_ENABLE_FAST_CASE &&
+                          std::is_arithmetic<T>{} && !std::is_same<T, char>{}>>
 using EnableIfFastCase = T;
 
+#undef ABSL_INTERNAL_STRCAT_ENABLE_FAST_CASE
+
 }  // namespace strings_internal
 
 ABSL_MUST_USE_RESULT inline std::string StrCat() { return std::string(); }