QXmlStreamReader: use std::optional

The optional-like FastScanNameResult was used to make some previous
changes backport-able to Qt 5.15 (std::optional is C++17 whereas Qt 5.15
requires C++14).

Amends commit 6326bec46a618c72feba4a2bb994c4d475050aed.

Change-Id: I409e1da83f82927c1eb24c47c1414c0c7ab1bf5b
Reviewed-by: Marc Mutz <marc.mutz@qt.io>
(cherry picked from commit effb3bddf63eb6c53d81ce6b0a30be6e3e80e189)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/serialization/qxmlstream.cpp

@@ -1297,8 +1297,7 @@ inline qsizetype QXmlStreamReaderPrivate::fastScanContentCharList()
 }
 
 // Fast scan an XML attribute name (e.g. "xml:lang").
-inline QXmlStreamReaderPrivate::FastScanNameResult
-QXmlStreamReaderPrivate::fastScanName(Value *val)
+inline std::optional<qsizetype> QXmlStreamReaderPrivate::fastScanName(Value *val)
 {
     qsizetype n = 0;
     uint c;
@@ -1307,7 +1306,7 @@ QXmlStreamReaderPrivate::fastScanName(Value *val)
             // This is too long to be a sensible name, and
             // can exhaust memory, or the range of decltype(*prefix)
             raiseNamePrefixTooLongError();
-            return {};
+            return std::nullopt;
         }
         switch (c) {
         case '\n':
@@ -1341,18 +1340,18 @@ QXmlStreamReaderPrivate::fastScanName(Value *val)
                 putChar(':');
                 --n;
             }
-            return FastScanNameResult(n);
+            return n;
         case ':':
             if (val) {
                 if (val->prefix == 0) {
                     val->prefix = qint16(n + 2);
                 } else { // only one colon allowed according to the namespace spec.
                     putChar(c);
-                    return FastScanNameResult(n);
+                    return n;
                 }
             } else {
                 putChar(c);
-                return FastScanNameResult(n);
+                return n;
             }
             Q_FALLTHROUGH();
         default:
@@ -1366,7 +1365,7 @@ QXmlStreamReaderPrivate::fastScanName(Value *val)
     qsizetype pos = textBuffer.size() - n;
     putString(textBuffer, pos);
     textBuffer.resize(pos);
-    return FastScanNameResult(0);
+    return 0;
 }
 
 enum NameChar { NameBeginning, NameNotBeginning, NotName };

src/corelib/serialization/qxmlstream_p.h

@@ -21,6 +21,7 @@
 
 
 #include <memory>
+#include <optional>
 
 #ifndef QXMLSTREAM_P_H
 #define QXMLSTREAM_P_H
@@ -498,16 +499,7 @@ public:
     qsizetype fastScanLiteralContent();
     qsizetype fastScanSpace();
     qsizetype fastScanContentCharList();
-
-    struct FastScanNameResult {
-        FastScanNameResult() : ok(false) {}
-        explicit FastScanNameResult(qsizetype len) : addToLen(len), ok(true) { }
-        operator bool() { return ok; }
-        qsizetype operator*() { Q_ASSERT(ok); return addToLen; }
-        qsizetype addToLen;
-        bool ok;
-    };
-    FastScanNameResult fastScanName(Value *val = nullptr);
+    std::optional<qsizetype> fastScanName(Value *val = nullptr);
     inline qsizetype fastScanNMTOKEN();