[processing] Fix evaluation of vector layer references to compatible feature sources

Refs #18545

Author: nyalldawson

src/core/processing/qgsprocessingparameters.cpp

@@ -321,25 +321,45 @@ QString QgsProcessingParameters::parameterAsCompatibleSourceLayerPath( const Qgs
     val = fromVar.source;
   }
 
-  QString layerRef;
   if ( val.canConvert<QgsProperty>() )
   {
-    layerRef = val.value< QgsProperty >().valueAsString( context.expressionContext(), definition->defaultValue().toString() );
-  }
-  else if ( !val.isValid() || val.toString().isEmpty() )
-  {
-    // fall back to default
-    layerRef = definition->defaultValue().toString();
+    val = val.value< QgsProperty >().valueAsString( context.expressionContext(), definition->defaultValue().toString() );
   }
-  else
+
+  QgsVectorLayer *vl = nullptr;
+  vl = qobject_cast< QgsVectorLayer * >( qvariant_cast<QObject *>( val ) );
+
+  if ( !vl )
   {
-    layerRef = val.toString();
-  }
+    QString layerRef;
+    if ( val.canConvert<QgsProperty>() )
+    {
+      layerRef = val.value< QgsProperty >().valueAsString( context.expressionContext(), definition->defaultValue().toString() );
+    }
+    else if ( !val.isValid() || val.toString().isEmpty() )
+    {
+      // fall back to default
+      val = definition->defaultValue();
 
-  if ( layerRef.isEmpty() )
-    return QString();
+      // default value may be a vector layer
+      vl = qobject_cast< QgsVectorLayer * >( qvariant_cast<QObject *>( val ) );
+      if ( !vl )
+        layerRef = definition->defaultValue().toString();
+    }
+    else
+    {
+      layerRef = val.toString();
+    }
+
+    if ( !vl )
+    {
+      if ( layerRef.isEmpty() )
+        return QString();
+
+      vl = qobject_cast< QgsVectorLayer *>( QgsProcessingUtils::mapLayerFromString( layerRef, context ) );
+    }
+  }
 
-  QgsVectorLayer *vl = qobject_cast< QgsVectorLayer *>( QgsProcessingUtils::mapLayerFromString( layerRef, context ) );
   if ( !vl )
     return QString();
 

tests/src/analysis/testqgsprocessing.cpp

@@ -6391,6 +6391,18 @@ void TestQgsProcessing::convertCompatible()
   out = QgsProcessingParameters::parameterAsCompatibleSourceLayerPath( def.get(), params, context, QStringList() << "shp", QString( "shp" ), &feedback );
   QCOMPARE( out, layer->source() );
 
+  // incompatible format, will be converted
+  out = QgsProcessingParameters::parameterAsCompatibleSourceLayerPath( def.get(), params, context, QStringList() << "tab", QString( "tab" ), &feedback );
+  QVERIFY( out != layer->source() );
+  QVERIFY( out.endsWith( ".tab" ) );
+  QVERIFY( out.startsWith( QgsProcessingUtils::tempFolder() ) );
+
+  // layer as input
+  params.insert( QStringLiteral( "source" ), QVariant::fromValue( layer ) );
+  out = QgsProcessingParameters::parameterAsCompatibleSourceLayerPath( def.get(), params, context, QStringList() << "shp", QString( "shp" ), &feedback );
+  QCOMPARE( out, layer->source() );
+
+  // incompatible format, will be converted
   out = QgsProcessingParameters::parameterAsCompatibleSourceLayerPath( def.get(), params, context, QStringList() << "tab", QString( "tab" ), &feedback );
   QVERIFY( out != layer->source() );
   QVERIFY( out.endsWith( ".tab" ) );
@@ -6402,6 +6414,12 @@ void TestQgsProcessing::convertCompatible()
   QVERIFY( out != layer->source() );
   QVERIFY( out.endsWith( ".shp" ) );
   QVERIFY( out.startsWith( QgsProcessingUtils::tempFolder() ) );
+
+  // vector layer as default
+  def.reset( new QgsProcessingParameterFeatureSource( QStringLiteral( "source" ), QString(), QList<int>(), QVariant::fromValue( layer ) ) );
+  params.remove( QStringLiteral( "source" ) );
+  out = QgsProcessingParameters::parameterAsCompatibleSourceLayerPath( def.get(), params, context, QStringList() << "shp", QString( "shp" ), &feedback );
+  QCOMPARE( out, layer->source() );
 }
 
 void TestQgsProcessing::create()