Reland [clang][dataflow] Fix unsupported types always being equal (#131575)

Relands #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 handling for the special case of `nullptr == nullptr`, to preserve
the behavior of untyped `nullptr` having no value.

Author: Discookie

clang/lib/Analysis/FlowSensitive/Transfer.cpp

@@ -60,7 +60,14 @@ 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);
+
+  // Special case: `NullPtrLiteralExpr == itself`. When both sides are untyped
+  // nullptr, they do not have an assigned Value, but they compare equal.
+  if (LHS.getType()->isNullPtrType() && RHS.getType()->isNullPtrType())
     return Env.getBoolLiteralValue(true);
 
   if (auto *LHSBool = dyn_cast_or_null<BoolValue>(LHSValue))

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) {