qmljsreformatter: support nullish coalescing

qmljsscanner did interpret ?? and ?. as a single ? and thus as the
start of a ternary operator, breaking reformatting.

Fixes: QTCREATORBUG-27344
Change-Id: I0429d20aed0196743f1c20e6ceac11351d5a7a3b
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>

src/libs/qmljs/qmljsreformatter.cpp

@@ -957,10 +957,10 @@ protected:
         accept(ast->left);
 
         // in general, avoid splitting at the operator
-        // but && and || are ok
+        // but && || and ?? are ok
         qreal splitBadness = 30;
         if (ast->op == QSOperator::And
-                || ast->op == QSOperator::Or)
+                || ast->op == QSOperator::Or || ast->op == QSOperator::Coalesce)
             splitBadness = 0;
         addPossibleSplit(splitBadness);
 

src/libs/qmljs/qmljsscanner.cpp

@@ -498,6 +498,18 @@ QList<Token> Scanner::operator()(const QString &text, int startState)
             setRegexpMayFollow(&_state, true);
             break;
 
+        case '?':
+            switch (la.unicode()) {
+            case '?':
+            case '.':
+                tokens.append(Token(index, 2, Token::Delimiter));
+                index += 2;
+            default:
+                tokens.append(Token(index++, 1, Token::Delimiter));
+            }
+            setRegexpMayFollow(&_state, true);
+            break;
+
         default:
             if (ch.isSpace()) {
                 do {

tests/auto/qml/reformatter/nullishCoalescing.qml

@@ -0,0 +1,9 @@
+import QtQuick 2.15
+import QtQuick.Controls 2.15
+
+Item {
+    property string otherString: ""
+    property string aProp: otherString ?? "N/A"
+    property string bProp: otherString?.trim()
+    property int unrelatedProp: 10
+}