[FEATURE][expressions] If a feature is passed to a function

expecting geometry inputs, then use that feature's geometry
instead of throwing an error

eg

buffer(@atlas_feature,100)

or

buffer(get_feature('my_layer','id=54'))

src/core/qgsexpression.cpp

@@ -283,6 +283,15 @@ static QgsGeometry getGeometry( const QVariant& value, QgsExpression* parent )
 {
   if ( value.canConvert<QgsGeometry>() )
     return value.value<QgsGeometry>();
+  else if ( value.canConvert<QgsFeature>() )
+  {
+    //try to grab geometry from feature
+    QgsFeature f = value.value<QgsFeature>();
+    if ( f.constGeometry() )
+    {
+      return *f.constGeometry();
+    }
+  }
 
   parent->setEvalErrorString( "Cannot convert to QgsGeometry" );
   return QgsGeometry();
@@ -1152,7 +1161,7 @@ static QVariant fcnWordwrap( const QVariantList& values, const QgsExpressionCont
 static QVariant fcnLength( const QVariantList& values, const QgsExpressionContext*, QgsExpression* parent )
 {
   // two variants, one for geometry, one for string
-  if ( values.at( 0 ).canConvert<QgsGeometry>() )
+  if ( values.at( 0 ).canConvert<QgsGeometry>() || values.at( 0 ).canConvert<QgsFeature>() )
   {
     //geometry variant
     QgsGeometry geom = getGeometry( values.at( 0 ), parent );

tests/src/core/testqgsexpression.cpp

@@ -1548,6 +1548,31 @@ class TestQgsExpression: public QObject
       QCOMPARE( out.toDouble(), result );
     }
 
+    void implicit_feature_to_geometry()
+    {
+      // test implicit feature to geometry cast
+      QgsPolyline polyline;
+      polyline << QgsPoint( 0, 0 ) << QgsPoint( 10, 0 );
+      QgsFeature feat;
+      feat.setGeometry( QgsGeometry::fromPolyline( polyline ) );
+
+      QgsExpressionContext context;
+      context.setFeature( feat );
+      QgsExpression exp1( "length($currentfeature)" );
+      QVariant result = exp1.evaluate( &context );
+      QCOMPARE( result.toDouble(), 10. );
+
+      QgsExpression exp2( "x(end_point($currentfeature))" );
+      result = exp2.evaluate( &context );
+      QCOMPARE( result.toDouble(), 10. );
+
+      // feature without geometry
+      QgsFeature feat2;
+      context.setFeature( feat2 );
+      result = exp2.evaluate( &context );
+      QVERIFY( !result.isValid() );
+    }
+
     void eval_geometry_calc()
     {
       QgsPolyline polyline, polygon_ring;