Ruby: Add regression test

ruby/ql/test/library-tests/dataflow/regressions/Regressions.expected

@@ -0,0 +1 @@
+| regressions.rb:2:1:2:9 | [post] call to reverse | regressions.rb:3:6:3:6 | x |

ruby/ql/test/library-tests/dataflow/regressions/Regressions.ql

@@ -0,0 +1,39 @@
+private import codeql.ruby.dataflow.FlowSummary
+
+private class ReverseSummary extends SimpleSummarizedCallable {
+  ReverseSummary() { this = "reverse" }
+
+  override predicate propagatesFlow(string input, string output, boolean preservesValue) {
+    input = "Argument[self].WithElement[any]" and
+    output = "ReturnValue" and
+    preservesValue = true
+  }
+}
+
+private module Config implements DataFlow::ConfigSig {
+  predicate isSource(DataFlow::Node source) {
+    source
+        .(DataFlow::PostUpdateNode)
+        .getPreUpdateNode()
+        .asExpr()
+        .getExpr()
+        .(MethodCall)
+        .getMethodName() = "reverse"
+  }
+
+  predicate isSink(DataFlow::Node sink) {
+    exists(MethodCall mc |
+      mc.getMethodName() = "sink" and
+      sink.asExpr().getExpr() = mc.getAnArgument()
+    )
+  }
+}
+
+/**
+ * This predicate should not have a result. We check that the flow summary for
+ * `reverse` does not get picked up by the `reverseStepThroughInputOutputAlias`
+ * logic in `DataFlowImplCommon.qll`.
+ */
+query predicate noReverseStepThroughInputOutputAlias(DataFlow::Node source, DataFlow::Node sink) {
+  DataFlow::Global<Config>::flow(source, sink)
+}

ruby/ql/test/library-tests/dataflow/regressions/regressions.rb

@@ -0,0 +1,3 @@
+x = foo
+x.reverse.bar
+sink(x)