[layouts] Fix missing layer scope for atlas enabled items

Fixes #18525

Author: nyalldawson

src/core/layout/qgslayout.cpp

@@ -393,6 +393,9 @@ QgsExpressionContext QgsLayout::createExpressionContext() const
   QgsExpressionContext context = QgsExpressionContext();
   context.appendScope( QgsExpressionContextUtils::globalScope() );
   context.appendScope( QgsExpressionContextUtils::projectScope( mProject ) );
+  if ( mReportContext->layer() )
+    context.appendScope( QgsExpressionContextUtils::layerScope( mReportContext->layer() ) );
+
   context.appendScope( QgsExpressionContextUtils::layoutScope( this ) );
   return context;
 }

tests/src/core/testqgslayoutcontext.cpp

@@ -23,6 +23,7 @@
 #include "qgsgeometry.h"
 #include "qgsproject.h"
 #include "qgslayout.h"
+#include "qgsexpressionutils.h"
 #include <QObject>
 #include "qgstest.h"
 #include <QtTest/QSignalSpy>
@@ -145,6 +146,12 @@ void TestQgsLayoutContext::layer()
   context.setLayer( nullptr );
   QVERIFY( !context.layer() );
 
+  QgsLayout l( QgsProject::instance() );
+  l.reportContext().setLayer( layer );
+  //test that expression context created for layout contains report context layer scope
+  QgsExpressionContext expContext  = l.createExpressionContext();
+  QCOMPARE( QgsExpressionUtils::getVectorLayer( expContext.variable( "layer" ), nullptr ), layer );
+
   delete layer;
 }