C++: Test to show lack of flow to crement operands

Author: jbj

cpp/ql/test/library-tests/dataflow/crement/crements.cpp

@@ -0,0 +1,16 @@
+void test_crement() {
+  int x1 = 0;
+  ++x1; // flow [NOT DETECTED]
+
+  int x2 = 0;
+  x2++; // flow [NOT DETECTED]
+
+  int x3 = 0;
+  x3 -= 1; // flow [NOT DETECTED]
+
+  int x4 = 0;
+  x4 |= 1; // flow [NOT DETECTED]
+
+  int x5 = 0;
+  x5 = x5 - 1; // flow (to RHS)
+}

cpp/ql/test/library-tests/dataflow/crement/from0.expected

@@ -0,0 +1 @@
+| crements.cpp:15:8:15:9 | x5 |

cpp/ql/test/library-tests/dataflow/crement/from0.ql

@@ -0,0 +1,14 @@
+import cpp
+import semmle.code.cpp.ir.dataflow.DataFlow
+
+class Cfg extends DataFlow::Configuration {
+  Cfg() { this = "from0::Cfg" }
+
+  override predicate isSource(DataFlow::Node source) { source.asExpr().getValue() = "0" }
+
+  override predicate isSink(DataFlow::Node sink) { sink.asExpr() instanceof VariableAccess }
+}
+
+from Cfg cfg, Expr sink
+where cfg.hasFlowToExpr(sink)
+select sink