From d01f15152cdf9a4b693d5c768cef31a0b2a5b012 Mon Sep 17 00:00:00 2001
From: Pearu Peterson <pearu.peterson@gmail.com>
Date: Sun, 19 Oct 2025 12:54:52 +0300
Subject: [PATCH] Move toUnderlying to headeronly (#165694)

As in the title. Required in upper PRs of this ghstack.

Pull Request resolved: https://github.com/pytorch/pytorch/pull/165694
Approved by: https://github.com/janeyx99
---
 c10/core/ScalarType.h                       | 16 ----------------
 test/cpp/aoti_abi_check/test_scalartype.cpp | 16 ++++++++++++++++
 torch/header_only_apis.txt                  |  1 +
 torch/headeronly/core/ScalarType.h          | 17 +++++++++++++++++
 4 files changed, 34 insertions(+), 16 deletions(-)

diff --git a/c10/core/ScalarType.h b/c10/core/ScalarType.h
index 24396630417..e0c84370e87 100644
--- a/c10/core/ScalarType.h
+++ b/c10/core/ScalarType.h
@@ -137,22 +137,6 @@ inline ScalarType toQIntType(ScalarType t) {
   }
 }
 
-inline ScalarType toUnderlying(ScalarType t) {
-  switch (t) {
-    case ScalarType::QUInt8:
-    case ScalarType::QUInt4x2:
-      [[fallthrough]];
-    case ScalarType::QUInt2x4:
-      return ScalarType::Byte;
-    case ScalarType::QInt8:
-      return ScalarType::Char;
-    case ScalarType::QInt32:
-      return ScalarType::Int;
-    default:
-      return t;
-  }
-}
-
 inline bool isSignedType(ScalarType t) {
 #define CASE_ISSIGNED(name)     \
   case ScalarType::name:        \
diff --git a/test/cpp/aoti_abi_check/test_scalartype.cpp b/test/cpp/aoti_abi_check/test_scalartype.cpp
index 13d1b98a770..e0952a48e5a 100644
--- a/test/cpp/aoti_abi_check/test_scalartype.cpp
+++ b/test/cpp/aoti_abi_check/test_scalartype.cpp
@@ -74,3 +74,19 @@ TEST(TestScalarType, operator_left_shift) {
   AT_FORALL_SCALAR_TYPES_WITH_COMPLEX_AND_QINTS(DEFINE_CHECK);
 #undef DEFINE_CHECK
 }
+
+TEST(TestScalarType, toUnderlying) {
+  using torch::headeronly::ScalarType;
+  using torch::headeronly::toUnderlying;
+
+  EXPECT_EQ(toUnderlying(ScalarType::QUInt8), ScalarType::Byte);
+  EXPECT_EQ(toUnderlying(ScalarType::QUInt4x2), ScalarType::Byte);
+  EXPECT_EQ(toUnderlying(ScalarType::QUInt2x4), ScalarType::Byte);
+  EXPECT_EQ(toUnderlying(ScalarType::QInt8), ScalarType::Char);
+  EXPECT_EQ(toUnderlying(ScalarType::QInt32), ScalarType::Int);
+#define DEFINE_CHECK(_, name) \
+  EXPECT_EQ(toUnderlying(ScalarType::name), ScalarType::name);
+  AT_FORALL_SCALAR_TYPES_WITH_COMPLEX(DEFINE_CHECK);
+  AT_FORALL_FLOAT8_TYPES(DEFINE_CHECK);
+#undef DEFINE_CHECK
+}
diff --git a/torch/header_only_apis.txt b/torch/header_only_apis.txt
index 8fe36f78063..70165a7493e 100644
--- a/torch/header_only_apis.txt
+++ b/torch/header_only_apis.txt
@@ -135,3 +135,4 @@ AT_FORALL_FLOAT8_TYPES
 AT_FORALL_COMPLEX_TYPES
 toString
 <<
+toUnderlying
diff --git a/torch/headeronly/core/ScalarType.h b/torch/headeronly/core/ScalarType.h
index e1451e9cbb2..624792c88d4 100644
--- a/torch/headeronly/core/ScalarType.h
+++ b/torch/headeronly/core/ScalarType.h
@@ -318,6 +318,22 @@ inline std::ostream& operator<<(
   return stream << toString(scalar_type);
 }
 
+inline ScalarType toUnderlying(ScalarType t) {
+  switch (t) {
+    case ScalarType::QUInt8:
+    case ScalarType::QUInt4x2:
+      [[fallthrough]];
+    case ScalarType::QUInt2x4:
+      return ScalarType::Byte;
+    case ScalarType::QInt8:
+      return ScalarType::Char;
+    case ScalarType::QInt32:
+      return ScalarType::Int;
+    default:
+      return t;
+  }
+}
+
 } // namespace c10
 
 namespace torch::headeronly {
@@ -330,4 +346,5 @@ using c10::impl::ScalarTypeToCPPTypeT;
 } // namespace impl
 using c10::toString;
 using c10::operator<<;
+using c10::toUnderlying;
 } // namespace torch::headeronly