C#: Extend the CFG succ relation for pre incr/decr operations.

Author: michaelnebel

csharp/ql/lib/semmle/code/csharp/controlflow/internal/ControlFlowGraphImpl.qll

@@ -357,7 +357,9 @@ module Expressions {
       not this instanceof AndPatternExpr and
       not this instanceof RecursivePatternExpr and
       not this instanceof PositionalPatternExpr and
-      not this instanceof PropertyPatternExpr
+      not this instanceof PropertyPatternExpr and
+      not this instanceof PreIncrExpr and
+      not this instanceof PreDecrExpr
     }
 
     final override AstNode getChildNode(int i) { result = getExprChild(this, i) }
@@ -919,6 +921,54 @@ module Expressions {
       succ = this
     }
   }
+
+  private class PreIncrExprTree extends PostOrderTree instanceof PreIncrExpr {
+    private Expr operand;
+
+    PreIncrExprTree() { operand = this.getOperand() }
+
+    final override predicate propagatesAbnormal(AstNode child) { child = operand }
+
+    final override predicate first(AstNode first) { first(operand, first) }
+
+    final override predicate succ(AstNode pred, AstNode succ, Completion c) {
+      (
+        // Post-order: flow from last element of the operand to the element itself
+        succ = this and
+        last(operand, pred, c) and
+        c instanceof NormalCompletion
+        or
+        // Flow from the last element in ++x to x
+        succ = operand and
+        last(this, pred, c) and
+        c instanceof NormalCompletion
+      )
+    }
+  }
+
+  private class PreDecrExprTree extends PostOrderTree instanceof PreDecrExpr {
+    private Expr operand;
+
+    PreDecrExprTree() { operand = this.getOperand() }
+
+    final override predicate propagatesAbnormal(AstNode child) { child = operand }
+
+    final override predicate first(AstNode first) { first(operand, first) }
+
+    final override predicate succ(AstNode pred, AstNode succ, Completion c) {
+      (
+        // Post-order: flow from last element of the operand to the element itself
+        succ = this and
+        last(operand, pred, c) and
+        c instanceof NormalCompletion
+        or
+        // Flow from the last element in --x to x
+        succ = operand and
+        last(this, pred, c) and
+        c instanceof NormalCompletion
+      )
+    }
+  }
 }
 
 private class RecursivePatternExprTree extends PostOrderTree instanceof RecursivePatternExpr {