Update charconv.cc
diff --git a/absl/strings/charconv.cc b/absl/strings/charconv.cc
index c08623c..25ac449 100644
--- a/absl/strings/charconv.cc
+++ b/absl/strings/charconv.cc
@@ -347,7 +347,8 @@
     // https://bugs.llvm.org/show_bug.cgi?id=37778
     // https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86113
     constexpr ptrdiff_t kNanBufferSize = 128;
-#if defined(__GNUC__) || (defined(__clang__) && __clang_major__ < 7)
+#if (defined(__GNUC__) && !defined(__clang__)) || \
+    (defined(__clang__) && __clang_major__ < 7)
     volatile char n_char_sequence[kNanBufferSize];
 #else
     char n_char_sequence[kNanBufferSize];