C++: Remove abstract keyword from Access and Cast and create .dbscheme unions

Author: MathiasVP

cpp/ql/src/semmle/code/cpp/exprs/Access.qll

@@ -6,9 +6,13 @@ private import semmle.code.cpp.dataflow.EscapesTree
 /**
  * A C/C++ access expression. This refers to a function, variable, or enum constant.
  */
-abstract class Access extends Expr, NameQualifiableElement {
+class Access extends Expr, NameQualifiableElement, @access {
+  // As `@access` is a union type containing `@routineexpr` (which describes function accesses
+  // that are called), we need to exclude function calls.
+  Access() { this instanceof @routineexpr implies not iscall(underlyingElement(this), _) }
+
   /** Gets the accessed function, variable, or enum constant. */
-  abstract Declaration getTarget();
+  Declaration getTarget() { none() } // overridden in subclasses
 
   override predicate mayBeImpure() { none() }
 

cpp/ql/src/semmle/code/cpp/exprs/Cast.qll

@@ -7,7 +7,7 @@ private import semmle.code.cpp.internal.ResolveClass
  * Instances of this class are not present in the main AST which is navigated by parent/child links. Instead,
  * instances of this class are attached to nodes in the main AST via special conversion links.
  */
-abstract class Conversion extends Expr {
+class Conversion extends Expr, @conversion {
   /** Gets the expression being converted. */
   Expr getExpr() { result.getConversion() = this }
 

cpp/ql/src/semmlecode.cpp.dbscheme

@@ -1143,6 +1143,13 @@ conversionkinds(
     int kind: int ref
 );
 
+@conversion = @cast
+            | @array_to_pointer
+            | @parexpr
+            | @reference_to
+            | @ref_indirect
+            ;
+
 /*
 case @funbindexpr.kind of
   0 = @normal_call  // a normal call
@@ -1800,6 +1807,8 @@ lambda_capture(
 @addressable = @function | @variable ;
 @accessible = @addressable | @enumconstant ;
 
+@access = @varaccess | @routineexpr ;
+
 fold(
     int expr: @foldexpr ref,
     string operator: string ref,