Allow + for string concat in QgsExpression

Author: NathanW2

src/core/qgsexpression.cpp

@@ -2081,6 +2081,12 @@ QVariant QgsExpression::NodeBinaryOperator::eval( QgsExpression* parent, const Q
   switch ( mOp )
   {
     case boPlus:
+      if ( vL.type() == QVariant::String && vR.type() == QVariant::String )
+      {
+        QString sL = getStringValue( vL, parent ); ENSURE_NO_EVAL_ERROR;
+        QString sR = getStringValue( vR, parent ); ENSURE_NO_EVAL_ERROR;
+        return QVariant( sL + sR );
+      }
     case boMinus:
     case boMul:
     case boDiv:

tests/src/core/testqgsexpression.cpp

@@ -223,6 +223,7 @@ class TestQgsExpression: public QObject
       QTest::newRow( "regexp 3" ) << "'hello' ~ 'llo$'" << false << QVariant( 1 );
 
       // concatenation
+      QTest::newRow( "concat with plus" ) << "'a' + 'b'" << false << QVariant( "ab" );
       QTest::newRow( "concat" ) << "'a' || 'b'" << false << QVariant( "ab" );
       QTest::newRow( "concat with int" ) << "'a' || 1" << false << QVariant( "a1" );
       QTest::newRow( "concat with int" ) << "2 || 'b'" << false << QVariant( "2b" );