C#: Add ExtensionType to the QL library.

Author: michaelnebel

csharp/ql/lib/semmle/code/csharp/Member.qll

@@ -469,7 +469,7 @@ class Virtualizable extends Overridable, Member, @virtualizable {
 
 /**
  * A parameterizable declaration. Either a callable (`Callable`), a delegate
- * type (`DelegateType`), or an indexer (`Indexer`).
+ * type (`DelegateType`), an indexer (`Indexer`), or an extension (`ExtensionType`).
  */
 class Parameterizable extends Declaration, @parameterizable {
   /** Gets raw parameter `i`, including the `this` parameter at index 0. */

csharp/ql/lib/semmle/code/csharp/Type.qll

@@ -17,7 +17,8 @@ private import semmle.code.csharp.frameworks.system.runtime.CompilerServices
  *
  * Either a value or reference type (`ValueOrRefType`), the `void` type (`VoidType`),
  * a pointer type (`PointerType`), the arglist type (`ArglistType`), an unknown
- * type (`UnknownType`), or a type parameter (`TypeParameter`).
+ * type (`UnknownType`), a type parameter (`TypeParameter`) or
+ * an extension type (`ExtensionType`).
  */
 class Type extends Member, TypeContainer, @type {
   /** Gets the name of this type without additional syntax such as `[]` or `*`. */
@@ -1326,3 +1327,35 @@ class TypeMention extends @type_mention {
   /** Gets the location of this type mention. */
   Location getLocation() { type_mention_location(this, result) }
 }
+
+/**
+ * A type extension declaration, for example `extension(string s) { ... }` in
+ *
+ * ```csharp
+ * static class MyExtensions {
+ *   extension(string s) { ... }
+ * ```
+ */
+class ExtensionType extends Parameterizable, @extension_type {
+  /**
+   * Gets the receiver parameter of this extension type, if any.
+   */
+  Parameter getReceiverParameter() { result = this.getParameter(0) }
+
+  /**
+   * Holds if this extension type has a receiver parameter.
+   */
+  predicate hasReceiverParameter() { exists(this.getReceiverParameter()) }
+
+  /**
+   * Gets the type being extended by this extension type.
+   */
+  Type getExtendedType() {
+    extension_receiver_type(this, result)
+    or
+    not extension_receiver_type(this, any(Type t)) and
+    extension_receiver_type(this, getTypeRef(result))
+  }
+
+  override string getAPrimaryQlClass() { result = "ExtensionType" }
+}