Add API for specifying fallback CRS and layer WKB type to use

for QgsVectorLayer, when constructing a layer with a URI which
may be invalid

These may be used for layers where the geometry type/CRS is
known in advance, and where the layer path may not be
initially resolvable. (E.g. layers with a URI pointing to a
non-existant file). It is only ever used if the layer cannot
be resolved, otherwise the actual layer geometry type will be
detected and used for the layer.

python/core/auto_generated/qgsattributeeditorelement.sip.in

@@ -406,7 +406,7 @@ class QgsAttributeEditorHtmlElement : QgsAttributeEditorElement
 %Docstring
 An attribute editor widget that will represent arbitrary HTML code.
 
-.. versionadded:: 3.10
+.. versionadded:: 3.8
 %End
 
 %TypeHeaderCode

python/core/auto_generated/qgsreadwritecontext.sip.in

@@ -119,7 +119,7 @@ Returns data provider coordinate transform context
 
 .. seealso:: :py:func:`setTransformContext`
 
-.. versionadded:: 3.10
+.. versionadded:: 3.8
 %End
 
     void setTransformContext( const QgsCoordinateTransformContext &transformContext );
@@ -128,7 +128,7 @@ Sets data coordinate transform context to ``transformContext``
 
 .. seealso:: :py:func:`transformContext`
 
-.. versionadded:: 3.10
+.. versionadded:: 3.8
 %End
 
       public:

python/core/auto_generated/qgsvectorlayer.sip.in

@@ -339,7 +339,7 @@ Constructor for LayerOptions.
 %Docstring
 Constructor for LayerOptions.
 
-.. versionadded:: 3.10
+.. versionadded:: 3.8
 %End
 
       bool loadDefaultStyle;
@@ -348,8 +348,11 @@ Constructor for LayerOptions.
 
       QgsCoordinateTransformContext transformContext;
 
-    };
+      QgsWkbTypes::Type fallbackWkbType;
 
+      QgsCoordinateReferenceSystem fallbackCrs;
+
+    };
 
     explicit QgsVectorLayer( const QString &path = QString(), const QString &baseName = QString(),
                              const QString &providerLib = "ogr", const QgsVectorLayer::LayerOptions &options = QgsVectorLayer::LayerOptions() );

src/core/qgsattributeeditorelement.h

@@ -474,7 +474,7 @@ class CORE_EXPORT QgsAttributeEditorQmlElement : public QgsAttributeEditorElemen
  * \ingroup core
  * An attribute editor widget that will represent arbitrary HTML code.
  *
- * \since QGIS 3.10
+ * \since QGIS 3.8
  */
 class CORE_EXPORT QgsAttributeEditorHtmlElement : public QgsAttributeEditorElement
 {

src/core/qgsreadwritecontext.h

@@ -120,7 +120,7 @@ class CORE_EXPORT QgsReadWriteContext
      *
      * \see setTransformContext()
      *
-     * \since QGIS 3.10
+     * \since QGIS 3.8
      */
     QgsCoordinateTransformContext transformContext() const;
 
@@ -129,7 +129,7 @@ class CORE_EXPORT QgsReadWriteContext
      *
      * \see transformContext()
      *
-     * \since QGIS 3.10
+     * \since QGIS 3.8
      */
     void setTransformContext( const QgsCoordinateTransformContext &transformContext );
 

src/core/qgsvectorlayer.cpp

@@ -148,6 +148,10 @@ QgsVectorLayer::QgsVectorLayer( const QString &vectorLayerPath,
   , mAuxiliaryLayerKey( QString() )
   , mReadExtentFromXml( options.readExtentFromXml )
 {
+  if ( options.fallbackCrs.isValid() )
+    setCrs( options.fallbackCrs, false );
+  mWkbType = options.fallbackWkbType;
+
   setProviderType( providerKey );
 
   mGeometryOptions = qgis::make_unique<QgsGeometryOptions>();

src/core/qgsvectorlayer.h

@@ -409,7 +409,7 @@ class CORE_EXPORT QgsVectorLayer : public QgsMapLayer, public QgsExpressionConte
 
       /**
        * Constructor for LayerOptions.
-       * \since QGIS 3.10
+       * \since QGIS 3.8
        */
       explicit LayerOptions( const QgsCoordinateTransformContext &transformContext,
                              bool loadDefaultStyle = true,
@@ -431,12 +431,37 @@ class CORE_EXPORT QgsVectorLayer : public QgsMapLayer, public QgsExpressionConte
 
       /**
        * Coordinate transform context
-       * \since QGIS 3.10
+       * \since QGIS 3.8
        */
       QgsCoordinateTransformContext transformContext = QgsCoordinateTransformContext();
 
-    };
+      /**
+       * Fallback geometry type.
+       *
+       * This may be set for layers where the geometry type is known in advance, and where
+       * the layer path may not be initially resolvable. (E.g. layers with a URI pointing
+       * to a non-existent file). It is only ever used if the layer cannot be resolved,
+       * otherwise the actual layer geometry type will be detected and used for the layer.
+       *
+       * \see fallbackCrs
+       * \since QGIS 3.8
+       */
+      QgsWkbTypes::Type fallbackWkbType = QgsWkbTypes::Unknown;
 
+      /**
+       * Fallback layer coordinate reference system.
+       *
+       * This may be set for layers where the coordinate reference system is known in advance, and where
+       * the layer path may not be initially resolvable. (E.g. layers with a URI pointing
+       * to a non-existent file). It is only ever used if the layer cannot be resolved,
+       * otherwise the actual layer CRS will be detected and used for the layer.
+       *
+       * \see fallbackWkbType
+       * \since QGIS 3.8
+       */
+      QgsCoordinateReferenceSystem fallbackCrs;
+
+    };
 
     /**
      * Constructor - creates a vector layer

src/gui/editorwidgets/qgshtmlwidgetwrapper.h

@@ -73,7 +73,7 @@ class GUI_EXPORT QgsHtmlWidgetWrapper : public QgsWidgetWrapper
 /**
  * \ingroup gui
  * To pass the QgsExpression functionality and it's context to the context of the QWebView
- * \since QGIS 3.10
+ * \since QGIS 3.8
  */
 class HtmlExpression : public QObject
 {

tests/src/python/test_qgsvectorlayer.py

@@ -434,6 +434,24 @@ class TestQgsVectorLayer(unittest.TestCase, FeatureSourceTestCase):
 
         shutil.rmtree(temp_path, True)
 
+    def testFallbackCrsWkbType(self):
+        """
+        Test fallback CRS and WKB types are used when layer path is invalid
+        """
+        vl = QgsVectorLayer('this is an outrage!!!')
+        self.assertFalse(vl.isValid()) # i'd certainly hope so...
+        self.assertEqual(vl.wkbType(), QgsWkbTypes.Unknown)
+        self.assertFalse(vl.crs().isValid())
+
+        # with fallback
+        options = QgsVectorLayer.LayerOptions()
+        options.fallbackWkbType = QgsWkbTypes.CircularString
+        options.fallbackCrs = QgsCoordinateReferenceSystem('EPSG:3111')
+        vl = QgsVectorLayer("i'm the moon", options=options)
+        self.assertFalse(vl.isValid())
+        self.assertEqual(vl.wkbType(), QgsWkbTypes.CircularString)
+        self.assertEqual(vl.crs().authid(), 'EPSG:3111')
+
     def test_layer_crs(self):
         """
         Test that spatial layers have CRS, and non-spatial don't