[clang][dataflow] Fix unsupported types always being equal (#129502)

Previously when the framework encountered unsupported values (such as
enum classes), they were always treated as equal when comparing with
`==`, regardless of their actual values being different.
Now the two sides are only equal if there's a Value assigned to them.

Added a Value assignment for `nullptr`, to handle the special case of
`nullptr == nullptr`.

Author: Discookie

clang/lib/Analysis/FlowSensitive/Transfer.cpp

@@ -60,7 +60,9 @@ static BoolValue &evaluateBooleanEquality(const Expr &LHS, const Expr &RHS,
   Value *LHSValue = Env.getValue(LHS);
   Value *RHSValue = Env.getValue(RHS);
 
-  if (LHSValue == RHSValue)
+  // When two unsupported values are compared, both are nullptr. Only supported
+  // values should evaluate to equal.
+  if (LHSValue == RHSValue && LHSValue)
     return Env.getBoolLiteralValue(true);
 
   if (auto *LHSBool = dyn_cast_or_null<BoolValue>(LHSValue))
@@ -798,6 +800,14 @@ class TransferVisitor : public ConstStmtVisitor<TransferVisitor> {
     Env.setValue(*S, Env.getIntLiteralValue(S->getValue()));
   }
 
+  // Untyped nullptr's aren't handled by NullToPointer casts, so they need to be
+  // handled separately.
+  void VisitCXXNullPtrLiteralExpr(const CXXNullPtrLiteralExpr *S) {
+    auto &NullPointerVal =
+        Env.getOrCreateNullPointerValue(S->getType()->getPointeeType());
+    Env.setValue(*S, NullPointerVal);
+  }
+
   void VisitParenExpr(const ParenExpr *S) {
     // The CFG does not contain `ParenExpr` as top-level statements in basic
     // blocks, however manual traversal to sub-expressions may encounter them.

clang/unittests/Analysis/FlowSensitive/TransferTest.cpp

@@ -4974,6 +4974,41 @@ TEST(TransferTest, IntegerLiteralEquality) {
       });
 }
 
+TEST(TransferTest, UnsupportedValueEquality) {
+  std::string Code = R"(
+    // An explicitly unsupported type by the framework.
+    enum class EC {
+      A,
+      B
+    };
+  
+    void target() {
+      EC ec = EC::A;
+
+      bool unsupported_eq_same = (EC::A == EC::A);
+      bool unsupported_eq_other = (EC::A == EC::B);
+      bool unsupported_eq_var = (ec == EC::B);
+
+      (void)0; // [[p]]
+    }
+  )";
+  runDataflow(
+      Code,
+      [](const llvm::StringMap<DataflowAnalysisState<NoopLattice>> &Results,
+         ASTContext &ASTCtx) {
+        const Environment &Env = getEnvironmentAtAnnotation(Results, "p");
+
+        // We do not model the values of unsupported types, so this
+        // seemingly-trivial case will not be true either.
+        EXPECT_TRUE(isa<AtomicBoolValue>(
+            getValueForDecl<BoolValue>(ASTCtx, Env, "unsupported_eq_same")));
+        EXPECT_TRUE(isa<AtomicBoolValue>(
+            getValueForDecl<BoolValue>(ASTCtx, Env, "unsupported_eq_other")));
+        EXPECT_TRUE(isa<AtomicBoolValue>(
+            getValueForDecl<BoolValue>(ASTCtx, Env, "unsupported_eq_var")));
+      });
+}
+
 TEST(TransferTest, CorrelatedBranches) {
   std::string Code = R"(
     void target(bool B, bool C) {