std::unordered_map<int64_t, int64_t> tensor_usage_;

  // If false, all activations are deleted in the first call to ComputeGradient.
  // Else, only when this is destructed.
  bool persistent_;
};

// Describes a callback for special-cased and more efficient jvp computation.
//
// Could just be a simple typedef in ForwardAccumulator, but MSVC chokes on
// that.
template <typename Gradient>
class ForwardFunction

diff --git a/misc/sys/cdefs.h b/misc/sys/cdefs.h
index 30482a1..551d5fd 100644
--- a/misc/sys/cdefs.h
+++ b/misc/sys/cdefs.h
@@ -391,6 +391,15 @@
 
 #include <bits/wordsize.h>
 
+#if __GNUC_PREREQ (8, 0)
+/* Describes a char array whose address can safely be passed as the first
+   argument to strncpy and strncat, as the char array is not necessarily
+   a NUL-terminated string.  */