From 544e621c0958006e5a513445dbc54118523a1add 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

(cherry picked from commit 1fed9020a21a3f3deb9cece03ebb533aeae8c6a2)
---
 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 fe2bed3e75e8..9336575030c2 100644
--- a/src/gui/processing/qgsprocessingwidgetwrapperimpl.cpp
+++ b/src/gui/processing/qgsprocessingwidgetwrapperimpl.cpp
@@ -1948,6 +1948,9 @@ 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 );