Fix test build in C++20

Author: VemundH

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
@@ -69,7 +69,7 @@ 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"); });
 #endif