diff --git a/aten/src/ATen/core/UndefinedTensorImpl.h b/aten/src/ATen/core/UndefinedTensorImpl.h
index fcfdffa75dd..885f6e195f0 100644
--- a/aten/src/ATen/core/UndefinedTensorImpl.h
+++ b/aten/src/ATen/core/UndefinedTensorImpl.h
@@ -1,34 +1 @@
-#pragma once
-
-#include <c10/core/TensorImpl.h>
-
-namespace at {
-
-struct CAFFE2_API UndefinedTensorImpl final : public TensorImpl {
- public:
-  // Without this, we get:
-  //  error: identifier "at::UndefinedTensorImpl::_singleton" is undefined in device code
-  // (ostensibly because the constexpr tricks MSVC into trying to compile this
-  // function for device as well).
-#ifdef _WIN32
-  static inline TensorImpl * singleton() {
-#else
-  static constexpr inline TensorImpl * singleton() {
-#endif
-    return &_singleton;
-  }
-  IntList sizes() const override;
-  IntList strides() const override;
-  int64_t size(int64_t d) const override;
-  int64_t stride(int64_t d) const override;
-  int64_t dim() const override;
-  const Storage& storage() const override;
-  int64_t storage_offset() const override;
-private:
-  UndefinedTensorImpl();
-  static UndefinedTensorImpl _singleton;
-public:
-  friend struct UndefinedType;
-};
-
-} // namespace at
+#include <c10/core/UndefinedTensorImpl.h>
diff --git a/aten/src/ATen/core/UndefinedTensorImpl.cpp b/c10/core/UndefinedTensorImpl.cpp
similarity index 94%
rename from aten/src/ATen/core/UndefinedTensorImpl.cpp
rename to c10/core/UndefinedTensorImpl.cpp
index 704f975bd95..2e2265397ea 100644
--- a/aten/src/ATen/core/UndefinedTensorImpl.cpp
+++ b/c10/core/UndefinedTensorImpl.cpp
@@ -1,7 +1,7 @@
-#include <ATen/core/UndefinedTensorImpl.h>
+#include <c10/core/UndefinedTensorImpl.h>
 #include <c10/util/Exception.h>
 
-namespace at {
+namespace c10 {
 
 // should this use the globalContext?  Can it get a context passed in somehow?
 UndefinedTensorImpl::UndefinedTensorImpl()
diff --git a/c10/core/UndefinedTensorImpl.h b/c10/core/UndefinedTensorImpl.h
new file mode 100644
index 00000000000..5d268b7b821
--- /dev/null
+++ b/c10/core/UndefinedTensorImpl.h
@@ -0,0 +1,34 @@
+#pragma once
+
+#include <c10/core/TensorImpl.h>
+
+namespace c10 {
+
+struct C10_API UndefinedTensorImpl final : public TensorImpl {
+ public:
+  // Without this, we get:
+  //  error: identifier "at::UndefinedTensorImpl::_singleton" is undefined in device code
+  // (ostensibly because the constexpr tricks MSVC into trying to compile this
+  // function for device as well).
+#ifdef _WIN32
+  static inline TensorImpl * singleton() {
+#else
+  static constexpr inline TensorImpl * singleton() {
+#endif
+    return &_singleton;
+  }
+  IntList sizes() const override;
+  IntList strides() const override;
+  int64_t size(int64_t d) const override;
+  int64_t stride(int64_t d) const override;
+  int64_t dim() const override;
+  const Storage& storage() const override;
+  int64_t storage_offset() const override;
+private:
+  UndefinedTensorImpl();
+  static UndefinedTensorImpl _singleton;
+public:
+  friend struct UndefinedType;
+};
+
+} // namespace c10