add a flag to coerceToType() to control whether duplicated nodes should

be kept

python/PyQt6/core/auto_generated/geometry/qgsgeometry.sip.in

@@ -2310,7 +2310,7 @@ Exports the geometry to a GeoJSON string.
 %End
 
 
-    QVector< QgsGeometry > coerceToType( Qgis::WkbType type, double defaultZ = 0, double defaultM = 0 ) const;
+    QVector< QgsGeometry > coerceToType( Qgis::WkbType type, double defaultZ = 0, double defaultM = 0, bool avoidDuplicates = true ) const;
 %Docstring
 Attempts to coerce this geometry into the specified destination
 ``type``.
@@ -2333,6 +2333,10 @@ Since QGIS 3.24, the parameters ``defaultZ`` and ``defaultM`` control
 the dimension value added when promoting geometries to Z, M or ZM
 versions. By default 0.0 is used for Z and M.
 
+Since QGIS 3.44, the parameters ``avoidDuplicates`` controls whether to
+keep duplicated nodes (e.g. start/end of rings) when promoting polygon
+geometries to points. By default duplicated nodes are ignored.
+
 .. note::
 
    This method is much stricter than :py:func:`~QgsGeometry.convertToType`, as it considers the exact WKB type

python/core/auto_generated/geometry/qgsgeometry.sip.in

@@ -2310,7 +2310,7 @@ Exports the geometry to a GeoJSON string.
 %End
 
 
-    QVector< QgsGeometry > coerceToType( Qgis::WkbType type, double defaultZ = 0, double defaultM = 0 ) const;
+    QVector< QgsGeometry > coerceToType( Qgis::WkbType type, double defaultZ = 0, double defaultM = 0, bool avoidDuplicates = true ) const;
 %Docstring
 Attempts to coerce this geometry into the specified destination
 ``type``.
@@ -2333,6 +2333,10 @@ Since QGIS 3.24, the parameters ``defaultZ`` and ``defaultM`` control
 the dimension value added when promoting geometries to Z, M or ZM
 versions. By default 0.0 is used for Z and M.
 
+Since QGIS 3.44, the parameters ``avoidDuplicates`` controls whether to
+keep duplicated nodes (e.g. start/end of rings) when promoting polygon
+geometries to points. By default duplicated nodes are ignored.
+
 .. note::
 
    This method is much stricter than :py:func:`~QgsGeometry.convertToType`, as it considers the exact WKB type

src/analysis/processing/qgsalgorithmconvertgeometrytype.cpp

@@ -168,7 +168,7 @@ const QVector< QgsGeometry > QgsConvertGeometryTypeAlgorithm::convertGeometry( c
   }
   else
   {
-    geometries = geom.coerceToType( outputWkbType );
+    geometries = geom.coerceToType( outputWkbType, 0, 0, false );
   }
 
   return geometries;

src/core/geometry/qgsgeometry.cpp

@@ -1695,7 +1695,7 @@ json QgsGeometry::asJsonObject( int precision ) const
 
 }
 
-QVector<QgsGeometry> QgsGeometry::coerceToType( const Qgis::WkbType type, double defaultZ, double defaultM ) const
+QVector<QgsGeometry> QgsGeometry::coerceToType( const Qgis::WkbType type, double defaultZ, double defaultM, bool avoidDuplicates ) const
 {
   QVector< QgsGeometry > res;
   if ( isNull() )
@@ -1768,7 +1768,7 @@ QVector<QgsGeometry> QgsGeometry::coerceToType( const Qgis::WkbType type, double
     QSet< QgsPoint > added;
     for ( auto vertex = source.vertices_begin(); vertex != source.vertices_end(); ++vertex )
     {
-      if ( added.contains( *vertex ) )
+      if ( avoidDuplicates && added.contains( *vertex ) )
         continue; // avoid duplicate points, e.g. start/end of rings
       mp->addGeometry( ( *vertex ).clone() );
       added.insert( *vertex );

src/core/geometry/qgsgeometry.h

@@ -2223,6 +2223,10 @@ class CORE_EXPORT QgsGeometry
      * to Z, M or ZM versions.
      * By default 0.0 is used for Z and M.
      *
+     * Since QGIS 3.44, the parameters \a avoidDuplicates controls whether to keep duplicated nodes (e.g. start/end of rings)
+     * when promoting polygon geometries to points.
+     * By default duplicated nodes are ignored.
+     *
      * \note This method is much stricter than convertToType(), as it considers the exact WKB type
      * of geometries instead of the geometry family (point/line/polygon), and tries more exhaustively
      * to coerce geometries to the desired \a type. It also correctly maintains curves and z/m values
@@ -2230,7 +2234,7 @@ class CORE_EXPORT QgsGeometry
      *
      * \since QGIS 3.14
      */
-    QVector< QgsGeometry > coerceToType( Qgis::WkbType type, double defaultZ = 0, double defaultM = 0 ) const;
+    QVector< QgsGeometry > coerceToType( Qgis::WkbType type, double defaultZ = 0, double defaultM = 0, bool avoidDuplicates = true ) const;
 
     /**
      * Try to convert the geometry to the requested type

tests/src/python/test_qgsgeometry.py

@@ -12002,15 +12002,22 @@ class TestQgsGeometry(QgisTestCase):
     def testCoerce(self):
         """Test coerce function"""
 
-        def coerce_to_wkt(wkt, type, defaultZ=None, defaultM=None):
+        def coerce_to_wkt(
+            wkt, type, defaultZ=None, defaultM=None, avoidDuplicates=True
+        ):
             geom = QgsGeometry.fromWkt(wkt)
             if defaultZ is not None or defaultM is not None:
                 return [
                     g.asWkt(2)
-                    for g in geom.coerceToType(type, defaultZ or 0, defaultM or 0)
+                    for g in geom.coerceToType(
+                        type, defaultZ or 0, defaultM or 0, avoidDuplicates
+                    )
                 ]
             else:
-                return [g.asWkt(2) for g in geom.coerceToType(type)]
+                return [
+                    g.asWkt(2)
+                    for g in geom.coerceToType(type, avoidDuplicates=avoidDuplicates)
+                ]
 
         self.assertEqual(
             coerce_to_wkt("Point (1 1)", QgsWkbTypes.Type.Point), ["Point (1 1)"]
@@ -12036,6 +12043,15 @@ class TestQgsGeometry(QgisTestCase):
             coerce_to_wkt("Polygon((1 1, 2 2, 1 2, 1 1))", QgsWkbTypes.Type.Point),
             ["Point (1 1)", "Point (2 2)", "Point (1 2)"],
         )
+        # keep duplicated nodes
+        self.assertEqual(
+            coerce_to_wkt(
+                "Polygon((1 1, 2 2, 1 2, 1 1))",
+                QgsWkbTypes.Type.Point,
+                avoidDuplicates=False,
+            ),
+            ["Point (1 1)", "Point (2 2)", "Point (1 2)", "Point (1 1)"],
+        )
         self.assertEqual(
             coerce_to_wkt("Polygon((1 1, 2 2, 1 2, 1 1))", QgsWkbTypes.Type.LineString),
             ["LineString (1 1, 2 2, 1 2, 1 1)"],
@@ -12290,6 +12306,14 @@ class TestQgsGeometry(QgisTestCase):
             coerce_to_wkt("Polygon ((1 1, 1 2, 2 2, 1 1))", QgsWkbTypes.Type.Point),
             ["Point (1 1)", "Point (1 2)", "Point (2 2)"],
         )
+        self.assertEqual(
+            coerce_to_wkt(
+                "Polygon ((1 1, 1 2, 2 2, 1 1))",
+                QgsWkbTypes.Type.Point,
+                avoidDuplicates=False,
+            ),
+            ["Point (1 1)", "Point (1 2)", "Point (2 2)", "Point (1 1)"],
+        )
 
         self.assertEqual(
             coerce_to_wkt(
@@ -12297,6 +12321,14 @@ class TestQgsGeometry(QgisTestCase):
             ),
             ["MultiPoint ((1 1),(1 2),(2 2))"],
         )
+        self.assertEqual(
+            coerce_to_wkt(
+                "Polygon ((1 1, 1 2, 2 2, 1 1))",
+                QgsWkbTypes.Type.MultiPoint,
+                avoidDuplicates=False,
+            ),
+            ["MultiPoint ((1 1),(1 2),(2 2),(1 1))"],
+        )
 
         self.assertEqual(
             coerce_to_wkt(
@@ -12312,6 +12344,23 @@ class TestQgsGeometry(QgisTestCase):
                 "Point (4 4)",
             ],
         )
+        self.assertEqual(
+            coerce_to_wkt(
+                "MultiPolygon (((1 1, 1 2, 2 2, 1 1)),((3 3, 4 3, 4 4, 3 3)))",
+                QgsWkbTypes.Type.Point,
+                avoidDuplicates=False,
+            ),
+            [
+                "Point (1 1)",
+                "Point (1 2)",
+                "Point (2 2)",
+                "Point (1 1)",
+                "Point (3 3)",
+                "Point (4 3)",
+                "Point (4 4)",
+                "Point (3 3)",
+            ],
+        )
 
         self.assertEqual(
             coerce_to_wkt(
@@ -12320,6 +12369,14 @@ class TestQgsGeometry(QgisTestCase):
             ),
             ["MultiPoint ((1 1),(1 2),(2 2),(3 3),(4 3),(4 4))"],
         )
+        self.assertEqual(
+            coerce_to_wkt(
+                "MultiPolygon (((1 1, 1 2, 2 2, 1 1)),((3 3, 4 3, 4 4, 3 3)))",
+                QgsWkbTypes.Type.MultiPoint,
+                avoidDuplicates=False,
+            ),
+            ["MultiPoint ((1 1),(1 2),(2 2),(1 1),(3 3),(4 3),(4 4),(3 3))"],
+        )
 
         # polygon -> lines
         self.assertEqual(