pick util/stringformat.h from mixxxdj#13236

`always_false_v<T>` already exists in midimessage.h, move to new /util/always_false_v.h

src/controllers/midi/midimessage.h

@@ -11,6 +11,7 @@
 #include <variant>
 
 #include "preferences/usersettings.h"
+#include "util/always_false_v.h"
 #include "util/compatibility/qhash.h"
 
 // The second value of each OpCode will be the channel number the message
@@ -179,9 +180,6 @@ struct MidiKey {
     };
 };
 
-template<class>
-inline constexpr bool always_false_v = false;
-
 struct MidiInputMapping {
     MidiInputMapping() {
     }

src/util/always_false_v.h

@@ -0,0 +1,6 @@
+#pragma once
+
+// always false, used for static_assert and workaround for compilers without
+// https://cplusplus.github.io/CWG/issues/2518.html
+template<typename T>
+static constexpr bool always_false_v = false;

src/util/stringformat.h

@@ -0,0 +1,39 @@
+#pragma once
+
+#include <QString>
+#include <type_traits>
+#include <utility>
+
+#include "util/always_false_v.h"
+
+namespace {
+
+// taken from Qt
+template<typename T>
+static constexpr bool is_convertible_to_view_or_qstring_v =
+        std::is_convertible_v<T, QString> ||
+        std::is_convertible_v<T, QStringView> ||
+        std::is_convertible_v<T, QLatin1String>;
+
+// check if we can call QString::number(T) with T
+template<typename T>
+static constexpr bool is_number_compatible_v =
+        std::is_invocable_v<decltype(QString::number(std::declval<T>()))(T), T>;
+} // namespace
+
+// Try to convert T to a type that would be accepted by QString::args(Args&&...)
+template<typename T>
+auto convertToQStringConvertible(T&& arg) {
+    if constexpr (is_convertible_to_view_or_qstring_v<T>) {
+        // no need to do anything, just return verbatim
+        return std::forward<T>(arg);
+    } else if constexpr (is_number_compatible_v<T>) {
+        return QString::number(std::forward<T>(arg));
+    } else {
+        static_assert(always_false_v<T>, "Unsupported type for QString::arg");
+        // unreachable, but returning a QString results in a better error message
+        // because the log won't be spammed with all the QString::arg overloads
+        // it couldn't match with `void`.
+        return QString();
+    }
+}