From 0374623f20e3f7501da4360e9a4e6fb23ead9529 Mon Sep 17 00:00:00 2001
From: Nyall Dawson <nyall.dawson@gmail.com>
Date: Wed, 20 Jan 2021 09:47:47 +1000
Subject: [PATCH] [processing] Don't default to input field for optional
 expression parameters

Fixes #41063
---
 src/gui/processing/qgsprocessingwidgetwrapperimpl.cpp | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/src/gui/processing/qgsprocessingwidgetwrapperimpl.cpp b/src/gui/processing/qgsprocessingwidgetwrapperimpl.cpp
index ebe3aa3c0777..a8ea8c08960b 100644
--- a/src/gui/processing/qgsprocessingwidgetwrapperimpl.cpp
+++ b/src/gui/processing/qgsprocessingwidgetwrapperimpl.cpp
@@ -1951,6 +1951,9 @@ QWidget *QgsProcessingExpressionWidgetWrapper::createWidget()
           mFieldExpWidget->setToolTip( parameterDefinition()->toolTip() );
           mFieldExpWidget->setExpressionDialogTitle( parameterDefinition()->description() );
           mFieldExpWidget->registerExpressionContextGenerator( this );
+          if ( expParam->flags() & QgsProcessingParameterDefinition::FlagOptional )
+            mFieldExpWidget->setAllowEmptyFieldName( true );
+
           connect( mFieldExpWidget, static_cast < void ( QgsFieldExpressionWidget::* )( const QString & ) >( &QgsFieldExpressionWidget::fieldChanged ), this, [ = ]( const QString & )
           {
             emit widgetValueHasChanged( this );