[clang][dataflow] Add a test to justify skipping past references in UO_Deref

This is part of the implementation of the dataflow analysis framework.
See "[RFC] A dataflow analysis framework for Clang AST" on cfe-dev.

Reviewed-by: xazax.hun

Differential Revision: https://reviews.llvm.org/D117567

clang/lib/Analysis/FlowSensitive/Transfer.cpp

@@ -182,6 +182,7 @@ public:
 
     switch (S->getOpcode()) {
     case UO_Deref: {
+      // Skip past a reference to handle dereference of a dependent pointer.
       const auto *SubExprVal = cast_or_null<PointerValue>(
           Env.getValue(*SubExpr, SkipPast::Reference));
       if (SubExprVal == nullptr)

clang/unittests/Analysis/FlowSensitive/TransferTest.cpp

@@ -1758,4 +1758,34 @@ TEST_F(TransferTest, AddrOfReference) {
               });
 }
 
+TEST_F(TransferTest, DerefDependentPtr) {
+  std::string Code = R"(
+    template <typename T>
+    void target(T *Foo) {
+      T &Bar = *Foo;
+      /*[[p]]*/
+    }
+  )";
+  runDataflow(
+      Code, [](llvm::ArrayRef<
+                   std::pair<std::string, DataflowAnalysisState<NoopLattice>>>
+                   Results,
+               ASTContext &ASTCtx) {
+        ASSERT_THAT(Results, ElementsAre(Pair("p", _)));
+        const Environment &Env = Results[0].second.Env;
+
+        const ValueDecl *FooDecl = findValueDecl(ASTCtx, "Foo");
+        ASSERT_THAT(FooDecl, NotNull());
+
+        const ValueDecl *BarDecl = findValueDecl(ASTCtx, "Bar");
+        ASSERT_THAT(BarDecl, NotNull());
+
+        const auto *FooVal =
+            cast<PointerValue>(Env.getValue(*FooDecl, SkipPast::None));
+        const auto *BarVal =
+            cast<ReferenceValue>(Env.getValue(*BarDecl, SkipPast::None));
+        EXPECT_EQ(&BarVal->getPointeeLoc(), &FooVal->getPointeeLoc());
+      });
+}
+
 } // namespace