diff --git a/include/pybind11/detail/descr.h b/include/pybind11/detail/descr.h
index 14f9223a5..0f93e06b2 100644
--- a/include/pybind11/detail/descr.h
+++ b/include/pybind11/detail/descr.h
@@ -59,6 +59,7 @@ constexpr descr<0> const_name(char const(&)[1]) { return {}; }
 
 template <size_t Rem, size_t... Digits> struct int_to_str : int_to_str<Rem/10, Rem%10, Digits...> { };
 template <size_t...Digits> struct int_to_str<0, Digits...> {
+    // WARNING: This only works with C++17 or higher.
     static constexpr auto digits = descr<sizeof...(Digits)>(('0' + Digits)...);
 };
 
@@ -84,9 +85,12 @@ auto constexpr const_name() -> remove_cv_t<decltype(int_to_str<Size / 10, Size %
 
 template <typename Type> constexpr descr<1, Type> const_name() { return {'%'}; }
 
-// The "_" might be defined as a macro - don't define it if so.
-// Repeating the const_name code to avoid introducing a #define.
+// If "_" is defined as a macro, py::detail::_ cannot be provided.
+// It is therefore best to use py::detail::const_name universally.
+// This block is for backward compatibility only.
+// (The const_name code is repeated to avoid introducing a "_" #define ourselves.)
 #ifndef _
+#define PYBIND11_DETAIL_UNDERSCORE_BACKWARD_COMPATIBILITY
 template <size_t N>
 constexpr descr<N-1> _(char const(&text)[N]) { return const_name<N>(text); }
 template <bool B, size_t N1, size_t N2>
@@ -107,7 +111,7 @@ auto constexpr _() -> remove_cv_t<decltype(int_to_str<Size / 10, Size % 10>::dig
     return const_name<Size>();
 }
 template <typename Type> constexpr descr<1, Type> _() { return const_name<Type>(); }
-#endif
+#endif  // #ifndef _
 
 constexpr descr<0> concat() { return {}; }
 
diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index 8b6d0a9ac..c2d7dc5ac 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -101,6 +101,7 @@ set(PYBIND11_TEST_FILES
     test_callbacks.cpp
     test_chrono.cpp
     test_class.cpp
+    test_const_name.cpp
     test_constants_and_functions.cpp
     test_copy_move.cpp
     test_custom_type_casters.cpp
diff --git a/tests/test_const_name.cpp b/tests/test_const_name.cpp
new file mode 100644
index 000000000..5cb3d16c1
--- /dev/null
+++ b/tests/test_const_name.cpp
@@ -0,0 +1,70 @@
+// Copyright (c) 2021 The Pybind Development Team.
+// All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+#include "pybind11_tests.h"
+
+#if defined(_MSC_VER) && _MSC_VER < 1910
+
+// MSVC 2015 fails in bizarre ways.
+#    define PYBIND11_SKIP_TEST_CONST_NAME
+
+#else // Only test with MSVC 2017 or newer.
+
+// IUT = Implementation Under Test
+#    define CONST_NAME_TESTS(TEST_FUNC, IUT)                                                      \
+        std::string TEST_FUNC(int selector) {                                                     \
+            switch (selector) {                                                                   \
+                case 0:                                                                           \
+                    return IUT("").text;                                                          \
+                case 1:                                                                           \
+                    return IUT("A").text;                                                         \
+                case 2:                                                                           \
+                    return IUT("Bd").text;                                                        \
+                case 3:                                                                           \
+                    return IUT("Cef").text;                                                       \
+                case 4:                                                                           \
+                    return IUT<int>().text; /*NOLINT(bugprone-macro-parentheses)*/                \
+                case 5:                                                                           \
+                    return IUT<std::string>().text; /*NOLINT(bugprone-macro-parentheses)*/        \
+                case 6:                                                                           \
+                    return IUT<true>("T1", "T2").text; /*NOLINT(bugprone-macro-parentheses)*/     \
+                case 7:                                                                           \
+                    return IUT<false>("U1", "U2").text; /*NOLINT(bugprone-macro-parentheses)*/    \
+                case 8:                                                                           \
+                    /*NOLINTNEXTLINE(bugprone-macro-parentheses)*/                                \
+                    return IUT<true>(IUT("D1"), IUT("D2")).text;                                  \
+                case 9:                                                                           \
+                    /*NOLINTNEXTLINE(bugprone-macro-parentheses)*/                                \
+                    return IUT<false>(IUT("E1"), IUT("E2")).text;                                 \
+                case 10:                                                                          \
+                    return IUT("KeepAtEnd").text;                                                 \
+                default:                                                                          \
+                    break;                                                                        \
+            }                                                                                     \
+            throw std::runtime_error("Invalid selector value.");                                  \
+        }
+
+CONST_NAME_TESTS(const_name_tests, py::detail::const_name)
+
+#    ifdef PYBIND11_DETAIL_UNDERSCORE_BACKWARD_COMPATIBILITY
+CONST_NAME_TESTS(underscore_tests, py::detail::_)
+#    endif
+
+#endif // MSVC >= 2017
+
+TEST_SUBMODULE(const_name, m) {
+#ifdef PYBIND11_SKIP_TEST_CONST_NAME
+    m.attr("const_name_tests") = "PYBIND11_SKIP_TEST_CONST_NAME";
+#else
+    m.def("const_name_tests", const_name_tests);
+#endif
+
+#ifdef PYBIND11_SKIP_TEST_CONST_NAME
+    m.attr("underscore_tests") = "PYBIND11_SKIP_TEST_CONST_NAME";
+#elif defined(PYBIND11_DETAIL_UNDERSCORE_BACKWARD_COMPATIBILITY)
+    m.def("underscore_tests", underscore_tests);
+#else
+    m.attr("underscore_tests") = "PYBIND11_DETAIL_UNDERSCORE_BACKWARD_COMPATIBILITY not defined.";
+#endif
+}
diff --git a/tests/test_const_name.py b/tests/test_const_name.py
new file mode 100644
index 000000000..d4e45e5e9
--- /dev/null
+++ b/tests/test_const_name.py
@@ -0,0 +1,31 @@
+# -*- coding: utf-8 -*-
+import pytest
+
+import env
+from pybind11_tests import const_name as m
+
+
+@pytest.mark.parametrize("func", (m.const_name_tests, m.underscore_tests))
+@pytest.mark.parametrize(
+    "selector, expected",
+    enumerate(
+        (
+            "",
+            "A",
+            "Bd",
+            "Cef",
+            "%",
+            "%",
+            "T1",
+            "U2",
+            "D1",
+            "E2",
+            "KeepAtEnd",
+        )
+    ),
+)
+def test_const_name(func, selector, expected):
+    if isinstance(func, type(u"") if env.PY2 else str):
+        pytest.skip(func)
+    text = func(selector)
+    assert text == expected
diff --git a/tests/test_custom_type_casters.cpp b/tests/test_custom_type_casters.cpp
index 8ad584de6..48613ee5a 100644
--- a/tests/test_custom_type_casters.cpp
+++ b/tests/test_custom_type_casters.cpp
@@ -19,7 +19,7 @@ namespace pybind11 { namespace detail {
 template <> struct type_caster<ArgInspector1> {
 public:
     // Classic
-#ifndef _
+#ifdef PYBIND11_DETAIL_UNDERSCORE_BACKWARD_COMPATIBILITY
     PYBIND11_TYPE_CASTER(ArgInspector1, _("ArgInspector1"));
 #else
     PYBIND11_TYPE_CASTER(ArgInspector1, const_name("ArgInspector1"));