Strings: get rid of the last std::char_traits use

LLVM's libc++ became pedantic and insists on only accepting
instantiations for one of the standard Char types, so remove all uses of
it with a template parameter.

Complements dc2ae08e02730ab795445bc047221aa56914f723.

Pick-to: 6.7
Fixes: QTBUG-126214
Task-number: QTBUG-122753
Change-Id: I8c5c4dba62924541bfb0fffd17d7d8ddc78338bb
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
Reviewed-by: Marc Mutz <marc.mutz@qt.io>
(cherry picked from commit ab05e05f60253b9be615c09aa340ee75f2e5bcaf)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/text/qanystringview.h

@@ -137,7 +137,7 @@ private:
     static constexpr qsizetype lengthHelperPointer(const Char *str) noexcept
     {
         if (q20::is_constant_evaluated())
-            return qsizetype(std::char_traits<Char>::length(str));
+            return QtPrivate::lengthHelperPointer(str);
         if constexpr (sizeof(Char) == sizeof(char16_t))
             return QtPrivate::qustrlen(reinterpret_cast<const char16_t*>(str));
         else

src/corelib/text/qbytearrayview.h

@@ -68,7 +68,15 @@ struct IsContainerCompatibleWithQByteArrayView<T, std::enable_if_t<
 template <typename Char>
 static constexpr qsizetype lengthHelperPointer(const Char *data) noexcept
 {
-    return qsizetype(std::char_traits<Char>::length(data));
+    // std::char_traits can only be used with one of the regular char types
+    // (char, char16_t, wchar_t, but not uchar or QChar), so we roll the loop
+    // out by ourselves.
+    qsizetype i = 0;
+    if (!data)
+        return i;
+    while (data[i] != Char(0))
+        ++i;
+    return i;
 }
 
 } // namespace QtPrivate

src/corelib/text/qstringview.h

@@ -108,7 +108,7 @@ private:
     static constexpr qsizetype lengthHelperPointer(const Char *str) noexcept
     {
         if (q20::is_constant_evaluated())
-            return std::char_traits<Char>::length(str);
+            return QtPrivate::lengthHelperPointer(str);
         return QtPrivate::qustrlen(reinterpret_cast<const char16_t *>(str));
     }
     static qsizetype lengthHelperPointer(const QChar *str) noexcept

src/corelib/text/qutf8stringview.h

@@ -134,14 +134,12 @@ private:
         return qsizetype(std::size(c));
     }
 
-    // Note: Do not replace with std::size(const Char (&)[N]), cause the result
+    // Note: Do not replace with std::size(const Char (&)[N]), because the result
     // will be of by one.
     template <typename Char, size_t N>
     static constexpr qsizetype lengthHelperContainer(const Char (&str)[N]) noexcept
     {
-        const auto it = std::char_traits<Char>::find(str, N, Char(0));
+        return QtPrivate::lengthHelperContainer(str);
-        const auto end = it ? it : std::next(str, N);
-        return qsizetype(std::distance(str, end));
     }
 
     template <typename Char>
@@ -174,8 +172,7 @@ public:
 #else
     template <typename Pointer, if_compatible_pointer<Pointer> = true>
     constexpr QBasicUtf8StringView(const Pointer &str) noexcept
-        : QBasicUtf8StringView(str,
+        : QBasicUtf8StringView(str, QtPrivate::lengthHelperPointer(str)) {}
-            str ? std::char_traits<std::remove_cv_t<std::remove_pointer_t<Pointer>>>::length(str) : 0) {}
 #endif
 
 #ifdef Q_QDOC