Add C++20 char8_t/u8string support (#2026)

* Fix test build in C++20

* Add C++20 char8_t/u8string support

include/pybind11/cast.h

@@ -32,6 +32,10 @@
 #include <string_view>
 #endif
 
+#if defined(__cpp_lib_char8_t) && __cpp_lib_char8_t >= 201811L
+#  define PYBIND11_HAS_U8STRING
+#endif
+
 NAMESPACE_BEGIN(PYBIND11_NAMESPACE)
 NAMESPACE_BEGIN(detail)
 
@@ -988,6 +992,9 @@ public:
 
 template <typename CharT> using is_std_char_type = any_of<
     std::is_same<CharT, char>, /* std::string */
+#if defined(PYBIND11_HAS_U8STRING)
+    std::is_same<CharT, char8_t>, /* std::u8string */
+#endif
     std::is_same<CharT, char16_t>, /* std::u16string */
     std::is_same<CharT, char32_t>, /* std::u32string */
     std::is_same<CharT, wchar_t> /* std::wstring */
@@ -1191,6 +1198,9 @@ template <typename StringType, bool IsView = false> struct string_caster {
     // Simplify life by being able to assume standard char sizes (the standard only guarantees
     // minimums, but Python requires exact sizes)
     static_assert(!std::is_same<CharT, char>::value || sizeof(CharT) == 1, "Unsupported char size != 1");
+#if defined(PYBIND11_HAS_U8STRING)
+    static_assert(!std::is_same<CharT, char8_t>::value || sizeof(CharT) == 1, "Unsupported char8_t size != 1");
+#endif
     static_assert(!std::is_same<CharT, char16_t>::value || sizeof(CharT) == 2, "Unsupported char16_t size != 2");
     static_assert(!std::is_same<CharT, char32_t>::value || sizeof(CharT) == 4, "Unsupported char32_t size != 4");
     // wchar_t can be either 16 bits (Windows) or 32 (everywhere else)
@@ -1209,7 +1219,7 @@ template <typename StringType, bool IsView = false> struct string_caster {
 #if PY_MAJOR_VERSION >= 3
             return load_bytes(load_src);
 #else
-            if (sizeof(CharT) == 1) {
+            if (std::is_same<CharT, char>::value) {
                 return load_bytes(load_src);
             }
 
@@ -1269,7 +1279,7 @@ private:
     // without any encoding/decoding attempt).  For other C++ char sizes this is a no-op.
     // which supports loading a unicode from a str, doesn't take this path.
     template <typename C = CharT>
-    bool load_bytes(enable_if_t<sizeof(C) == 1, handle> src) {
+    bool load_bytes(enable_if_t<std::is_same<C, char>::value, handle> src) {
         if (PYBIND11_BYTES_CHECK(src.ptr())) {
             // We were passed a Python 3 raw bytes; accept it into a std::string or char*
             // without any encoding attempt.
@@ -1284,7 +1294,7 @@ private:
     }
 
     template <typename C = CharT>
-    bool load_bytes(enable_if_t<sizeof(C) != 1, handle>) { return false; }
+    bool load_bytes(enable_if_t<!std::is_same<C, char>::value, handle>) { return false; }
 };
 
 template <typename CharT, class Traits, class Allocator>

tests/test_builtin_casters.cpp

@@ -30,7 +30,7 @@ TEST_SUBMODULE(builtin_casters, m) {
     else { wstr.push_back((wchar_t) mathbfA32); } // 𝐀, utf32
     wstr.push_back(0x7a); // z
 
-    m.def("good_utf8_string", []() { return std::string(u8"Say utf8\u203d \U0001f382 \U0001d400"); }); // Say utf8‽ 🎂 𝐀
+    m.def("good_utf8_string", []() { return std::string((const char*)u8"Say utf8\u203d \U0001f382 \U0001d400"); }); // Say utf8‽ 🎂 𝐀
     m.def("good_utf16_string", [=]() { return std::u16string({ b16, ib16, cake16_1, cake16_2, mathbfA16_1, mathbfA16_2, z16 }); }); // b‽🎂𝐀z
     m.def("good_utf32_string", [=]() { return std::u32string({ a32, mathbfA32, cake32, ib32, z32 }); }); // a𝐀🎂‽z
     m.def("good_wchar_string", [=]() { return wstr; }); // a‽𝐀z
@@ -60,6 +60,18 @@ TEST_SUBMODULE(builtin_casters, m) {
     m.def("strlen", [](char *s) { return strlen(s); });
     m.def("string_length", [](std::string s) { return s.length(); });
 
+#ifdef PYBIND11_HAS_U8STRING
+    m.attr("has_u8string") = true;
+    m.def("good_utf8_u8string", []() { return std::u8string(u8"Say utf8\u203d \U0001f382 \U0001d400"); }); // Say utf8‽ 🎂 𝐀
+    m.def("bad_utf8_u8string", []()  { return std::u8string((const char8_t*)"abc\xd0" "def"); });
+
+    m.def("u8_char8_Z", []() -> char8_t { return u8'Z'; });
+
+    // test_single_char_arguments
+    m.def("ord_char8", [](char8_t c) -> int { return static_cast<unsigned char>(c); });
+    m.def("ord_char8_lv", [](char8_t &c) -> int { return static_cast<unsigned char>(c); });
+#endif
+
     // test_string_view
 #ifdef PYBIND11_HAS_STRING_VIEW
     m.attr("has_string_view") = true;
@@ -69,9 +81,15 @@ TEST_SUBMODULE(builtin_casters, m) {
     m.def("string_view_chars",   [](std::string_view s)    { py::list l; for (auto c : s) l.append((std::uint8_t) c); return l; });
     m.def("string_view16_chars", [](std::u16string_view s) { py::list l; for (auto c : s) l.append((int) c); return l; });
     m.def("string_view32_chars", [](std::u32string_view s) { py::list l; for (auto c : s) l.append((int) c); return l; });
-    m.def("string_view_return",   []() { return std::string_view(u8"utf8 secret \U0001f382"); });
+    m.def("string_view_return",   []() { return std::string_view((const char*)u8"utf8 secret \U0001f382"); });
     m.def("string_view16_return", []() { return std::u16string_view(u"utf16 secret \U0001f382"); });
     m.def("string_view32_return", []() { return std::u32string_view(U"utf32 secret \U0001f382"); });
+
+#   ifdef PYBIND11_HAS_U8STRING
+    m.def("string_view8_print",  [](std::u8string_view s) { py::print(s, s.size()); });
+    m.def("string_view8_chars",  [](std::u8string_view s) { py::list l; for (auto c : s) l.append((std::uint8_t) c); return l; });
+    m.def("string_view8_return", []() { return std::u8string_view(u8"utf8 secret \U0001f382"); });
+#   endif
 #endif
 
     // test_integer_casting

tests/test_builtin_casters.py

@@ -15,6 +15,8 @@ def test_unicode_conversion():
     assert m.good_utf16_string() == u"b‽🎂𝐀z"
     assert m.good_utf32_string() == u"a𝐀🎂‽z"
     assert m.good_wchar_string() == u"a⸘𝐀z"
+    if hasattr(m, "has_u8string"):
+        assert m.good_utf8_u8string() == u"Say utf8‽ 🎂 𝐀"
 
     with pytest.raises(UnicodeDecodeError):
         m.bad_utf8_string()
@@ -29,12 +31,17 @@ def test_unicode_conversion():
     if hasattr(m, "bad_wchar_string"):
         with pytest.raises(UnicodeDecodeError):
             m.bad_wchar_string()
+    if hasattr(m, "has_u8string"):
+        with pytest.raises(UnicodeDecodeError):
+            m.bad_utf8_u8string()
 
     assert m.u8_Z() == 'Z'
     assert m.u8_eacute() == u'é'
     assert m.u16_ibang() == u'‽'
     assert m.u32_mathbfA() == u'𝐀'
     assert m.wchar_heart() == u'♥'
+    if hasattr(m, "has_u8string"):
+        assert m.u8_char8_Z() == 'Z'
 
 
 def test_single_char_arguments():
@@ -92,6 +99,17 @@ def test_single_char_arguments():
         assert m.ord_wchar(u'aa')
     assert str(excinfo.value) == toolong_message
 
+    if hasattr(m, "has_u8string"):
+        assert m.ord_char8(u'a') == 0x61  # simple ASCII
+        assert m.ord_char8_lv(u'b') == 0x62
+        assert m.ord_char8(u'é') == 0xE9  # requires 2 bytes in utf-8, but can be stuffed in a char
+        with pytest.raises(ValueError) as excinfo:
+            assert m.ord_char8(u'Ā') == 0x100  # requires 2 bytes, doesn't fit in a char
+        assert str(excinfo.value) == toobig_message(0x100)
+        with pytest.raises(ValueError) as excinfo:
+            assert m.ord_char8(u'ab')
+        assert str(excinfo.value) == toolong_message
+
 
 def test_bytes_to_string():
     """Tests the ability to pass bytes to C++ string-accepting functions.  Note that this is
@@ -116,10 +134,15 @@ def test_string_view(capture):
     assert m.string_view_chars("Hi 🎂") == [72, 105, 32, 0xf0, 0x9f, 0x8e, 0x82]
     assert m.string_view16_chars("Hi 🎂") == [72, 105, 32, 0xd83c, 0xdf82]
     assert m.string_view32_chars("Hi 🎂") == [72, 105, 32, 127874]
+    if hasattr(m, "has_u8string"):
+        assert m.string_view8_chars("Hi") == [72, 105]
+        assert m.string_view8_chars("Hi 🎂") == [72, 105, 32, 0xf0, 0x9f, 0x8e, 0x82]
 
     assert m.string_view_return() == "utf8 secret 🎂"
     assert m.string_view16_return() == "utf16 secret 🎂"
     assert m.string_view32_return() == "utf32 secret 🎂"
+    if hasattr(m, "has_u8string"):
+        assert m.string_view8_return() == "utf8 secret 🎂"
 
     with capture:
         m.string_view_print("Hi")
@@ -132,6 +155,14 @@ def test_string_view(capture):
         utf16 🎂 8
         utf32 🎂 7
     """
+    if hasattr(m, "has_u8string"):
+        with capture:
+            m.string_view8_print("Hi")
+            m.string_view8_print("utf8 🎂")
+        assert capture == """
+            Hi 2
+            utf8 🎂 9
+        """
 
     with capture:
         m.string_view_print("Hi, ascii")
@@ -144,6 +175,14 @@ def test_string_view(capture):
         Hi, utf16 🎂 12
         Hi, utf32 🎂 11
     """
+    if hasattr(m, "has_u8string"):
+        with capture:
+            m.string_view8_print("Hi, ascii")
+            m.string_view8_print("Hi, utf8 🎂")
+        assert capture == """
+            Hi, ascii 9
+            Hi, utf8 🎂 13
+        """
 
 
 def test_integer_casting():