Try explicitely converting to and from utf8

src/file_dialogs.cpp

@@ -1,4 +1,5 @@
 #include "file_dialogs.hpp"
+#include "utf8_strings.hpp"
 
 #include <cstring>
 
@@ -30,8 +31,8 @@ namespace feis {
         if (utf8_path == nullptr) {
             return {};
         } else {
-            const auto u8string_path = std::u8string{utf8_path, utf8_path+std::strlen(utf8_path)};
+            const std::string utf8_string{utf8_path, utf8_path+std::strlen(utf8_path)};
-            return std::filesystem::path{u8string_path};
+            return to_path(utf8_string);
         }
     }
 }

src/main.cpp

@@ -27,6 +27,7 @@
 #include "sound_effect.hpp"
 #include "src/custom_sfml_audio/synced_sound_streams.hpp"
 #include "widgets/blank_screen.hpp"
+#include "utf8_strings.hpp"
 
 int main() {
     // TODO : Make the playfield not appear when there's no chart selected
@@ -53,7 +54,7 @@ int main() {
     if (not std::filesystem::exists(font_path)) {
         tinyfd_messageBox(
             "Error",
-            ("Could not open "+font_path.string()).c_str(),
+            ("Could not open "+to_utf8_encoded_string(font_path)).c_str(),
             "ok",
             "error",
             1
@@ -63,8 +64,9 @@ int main() {
     ImGuiIO& IO = ImGui::GetIO();
     IO.Fonts->Clear();
     IO.Fonts->AddFontFromFileTTF(
-        (assets_folder / "fonts" / "NotoSans-Medium.ttf").c_str(),
+        to_utf8_encoded_string(assets_folder / "fonts" / "NotoSans-Medium.ttf").c_str(),
-        16.f);
+        16.f
+    );
     ImGui::SFML::UpdateFontTexture();
 
     IO.ConfigWindowsMoveFromTitleBarOnly = true;

src/meson.build

@@ -31,6 +31,7 @@ sources += files(
     'sound_effect.cpp',
     'special_numeric_types.cpp',
     'toolbox.cpp',
+    'utf8_strings.cpp',
 )
 
 conf_data = configuration_data()

src/utf8_strings.cpp

@@ -0,0 +1,19 @@
+#include "utf8_strings.hpp"
+
+std::string to_utf8_encoded_string(const std::u8string& u8s) {
+    std::string result{u8s.cbegin(), u8s.cend()};
+    return result;
+}
+
+std::string to_utf8_encoded_string(const std::filesystem::path& path) {
+    return to_utf8_encoded_string(path.u8string());
+}
+
+std::u8string to_u8string(const std::string& utf8s) {
+    std::u8string result{utf8s.cbegin(), utf8s.cend()};
+    return result;
+}
+
+std::filesystem::path to_path(const std::string& utf8s) {
+    return std::filesystem::path{to_u8string(utf8s)};
+}

src/utf8_strings.hpp

@@ -0,0 +1,7 @@
+#include <filesystem>
+#include <string>
+
+std::string to_utf8_encoded_string(const std::u8string& u8s);
+std::string to_utf8_encoded_string(const std::filesystem::path& path);
+std::u8string to_u8string(const std::string& utf8s);
+std::filesystem::path to_path(const std::string& utf8s);