C++: Reintroduce 'StdBasicStringIterator'.

cpp/ql/lib/semmle/code/cpp/models/implementations/StdString.qll

@@ -15,6 +15,24 @@ private class StdBasicString extends ClassTemplateInstantiation {
   StdBasicString() { this.hasQualifiedName(["std", "bsl"], "basic_string") }
 }
 
+/**
+ * The `std::basic_string::iterator` declaration.
+ *
+ * Intuitively, this class shouldn't be necessary as it's already captured
+ * by the `StdIterator` class. However, this class ensures that the typedef inside the
+ * body of the `std::string` class is also seen as an iterator.
+ *
+ * Eventually, we should be consistent about which of the following should be recognized as iterators:
+ * 1. The typedef type.
+ * 2. The template class of the resolved type.
+ * 3. Any instantiation of the resolved type.
+ */
+private class StdBasicStringIterator extends Iterator, Type {
+  StdBasicStringIterator() {
+    this.getEnclosingElement() instanceof StdBasicString and this.hasName("iterator")
+  }
+}
+
 /**
  * A `std::string` function for which taint should be propagated.
  */