diff --git a/nullability/inference/collect_evidence.cc b/nullability/inference/collect_evidence.cc
index 44275f252..96a2893ac 100644
--- a/nullability/inference/collect_evidence.cc
+++ b/nullability/inference/collect_evidence.cc
@@ -3266,7 +3266,7 @@ static void saveCFGScopeVirtualMethodIndex(
                                 CFGVirtualMethodSummary);
 }
 
-llvm::Expected<CFGSummary> summarizeDefinition(
+llvm::Expected<std::optional<CFGSummary>> summarizeDefinition(
     const Decl& Definition, USRCache& USRCache,
     const NullabilityPragmas& Pragmas,
     const VirtualMethodIndex& VirtualMethodsInTU,
@@ -3290,7 +3290,7 @@ llvm::Expected<CFGSummary> summarizeDefinition(
   // functions.
   if (!TargetFunc && !isInferenceTarget(Definition) &&
       !hasAnyInferenceTargets(ReferencedDecls))
-    return Summary;
+    return std::nullopt;
 
   ASTContext &Ctx = Definition.getASTContext();
   llvm::Expected<dataflow::AdornedCFG> ACFG =
@@ -3340,10 +3340,10 @@ llvm::Expected<CFGSummary> summarizeDefinition(
                               .moveInto(Results))
     return Error;
 
-  // When `Results` are empty, the post-analysis callbacks should not have been
-  // called, so `Summary` should be empty. But, to be sure, explicitly return a
-  // fresh empty summary.
-  if (Results.empty()) return CFGSummary();
+  // When `Results` are empty, there's nothing to summarize and the
+  // post-analysis callbacks should not have been called, so `Summary` should be
+  // empty. Instead of returning an empty summary, don't return one.
+  if (Results.empty()) return std::nullopt;
 
   if (std::optional<dataflow::DataflowAnalysisState<PointerNullabilityLattice>>
           &ExitBlockResult = Results[ACFG->getCFG().getExit().getBlockID()]) {
@@ -3374,6 +3374,7 @@ llvm::Expected<CFGSummary> summarizeDefinition(
                                    "SAT solver reached iteration limit");
   }
 
+  if (Summary.behavior_summaries_size() <= 0) return std::nullopt;
   return Summary;
 }
 
diff --git a/nullability/inference/collect_evidence.h b/nullability/inference/collect_evidence.h
index 85a0a33b2..6ba530c92 100644
--- a/nullability/inference/collect_evidence.h
+++ b/nullability/inference/collect_evidence.h
@@ -7,6 +7,7 @@
 
 #include <algorithm>
 #include <memory>
+#include <optional>
 #include <ostream>
 #include <utility>
 #include <vector>
@@ -193,9 +194,9 @@ llvm::Error collectEvidenceFromDefinition(
     const SolverFactory &MakeSolver = makeDefaultSolverForInference);
 
 // Summarizes Nullability-relevant behaviors in and context for `Definition`.
-// If the resulting summary has no `behavior_summaries`, the analysis succeeded,
-// but there's no relevant content.
-llvm::Expected<CFGSummary> summarizeDefinition(
+// If std::nullopt is returned, the analysis succeeded, but there's no relevant
+// content.
+llvm::Expected<std::optional<CFGSummary>> summarizeDefinition(
     const Decl& Definition, USRCache& USRCache,
     const NullabilityPragmas& Pragmas,
     const VirtualMethodIndex& VirtualMethodsInTU,
diff --git a/nullability/inference/collect_evidence_test.cc b/nullability/inference/collect_evidence_test.cc
index 6b1c159a5..ce7dd7e84 100644
--- a/nullability/inference/collect_evidence_test.cc
+++ b/nullability/inference/collect_evidence_test.cc
@@ -6,6 +6,7 @@
 
 #include <cassert>
 #include <memory>
+#include <optional>
 #include <string>
 #include <string_view>
 #include <vector>
@@ -299,18 +300,21 @@ TEST(SummarizeDefinitionTest, Deref) {
     definition_location: "input.cc:2:10"
   )pb";
 
-  llvm::Expected<CFGSummary> Summary = summarizeTargetFuncDefinition(Src);
-  ASSERT_THAT_EXPECTED(Summary, llvm::Succeeded());
+  llvm::Expected<std::optional<CFGSummary>> SummaryResult =
+      summarizeTargetFuncDefinition(Src);
+  ASSERT_THAT_EXPECTED(SummaryResult, llvm::Succeeded());
+  ASSERT_TRUE(SummaryResult->has_value());
+  CFGSummary& Summary = **SummaryResult;
   // Given our reliance on particular atoms, we verify that the atom maps are
   // not empty. It is difficult to meaningfully connect the input code to more
   // detailed aspects of these maps.
-  EXPECT_THAT(Summary->logical_context().atom_defs(), Not(IsEmpty()));
-  Summary->mutable_logical_context()->clear_atom_defs();
+  EXPECT_THAT(Summary.logical_context().atom_defs(), Not(IsEmpty()));
+  Summary.mutable_logical_context()->clear_atom_defs();
 
-  EXPECT_THAT(Summary->logical_context().atom_deps(), Not(IsEmpty()));
-  Summary->mutable_logical_context()->clear_atom_deps();
+  EXPECT_THAT(Summary.logical_context().atom_deps(), Not(IsEmpty()));
+  Summary.mutable_logical_context()->clear_atom_deps();
 
-  EXPECT_THAT(*Summary, EqualsProto(Proto));
+  EXPECT_THAT(Summary, EqualsProto(Proto));
 }
 
 TEST(SummarizeDefinitionTest, NullableArgPassed) {
@@ -340,18 +344,21 @@ TEST(SummarizeDefinitionTest, NullableArgPassed) {
     definition_location: "input.cc:3:10"
   )pb";
 
-  llvm::Expected<CFGSummary> Summary = summarizeTargetFuncDefinition(Src);
-  ASSERT_THAT_EXPECTED(Summary, llvm::Succeeded());
+  llvm::Expected<std::optional<CFGSummary>> SummaryResult =
+      summarizeTargetFuncDefinition(Src);
+  ASSERT_THAT_EXPECTED(SummaryResult, llvm::Succeeded());
+  ASSERT_TRUE(SummaryResult->has_value());
+  CFGSummary& Summary = **SummaryResult;
   // Given our reliance on particular atoms, we verify that the atom maps are
   // not empty. It is difficult to meaningfully connect the input code to more
   // detailed aspects of these maps.
-  EXPECT_THAT(Summary->logical_context().atom_defs(), Not(IsEmpty()));
-  Summary->mutable_logical_context()->clear_atom_defs();
+  EXPECT_THAT(Summary.logical_context().atom_defs(), Not(IsEmpty()));
+  Summary.mutable_logical_context()->clear_atom_defs();
 
-  EXPECT_THAT(Summary->logical_context().atom_deps(), Not(IsEmpty()));
-  Summary->mutable_logical_context()->clear_atom_deps();
+  EXPECT_THAT(Summary.logical_context().atom_deps(), Not(IsEmpty()));
+  Summary.mutable_logical_context()->clear_atom_deps();
 
-  EXPECT_THAT(*Summary, EqualsProto(Proto));
+  EXPECT_THAT(Summary, EqualsProto(Proto));
 }
 
 class CollectEvidenceFromDefinitionTest
diff --git a/nullability/inference/collect_evidence_test_utilities.cc b/nullability/inference/collect_evidence_test_utilities.cc
index 23affe0d2..82223ff85 100644
--- a/nullability/inference/collect_evidence_test_utilities.cc
+++ b/nullability/inference/collect_evidence_test_utilities.cc
@@ -5,6 +5,7 @@
 #include "nullability/inference/collect_evidence_test_utilities.h"
 
 #include <memory>
+#include <optional>
 #include <string>
 #include <utility>
 #include <vector>
@@ -73,7 +74,7 @@ std::string printToString(DefinitionCollectionMode Mode) {
   llvm_unreachable("Unknown CollectionMode");
 }
 
-static llvm::Expected<CFGSummary> summarizeDefinitionNamed(
+static llvm::Expected<std::optional<CFGSummary>> summarizeDefinitionNamed(
     llvm::StringRef TargetName, llvm::StringRef Source) {
   USRCache UsrCache;
   NullabilityPragmas Pragmas;
@@ -84,7 +85,7 @@ static llvm::Expected<CFGSummary> summarizeDefinitionNamed(
                              getVirtualMethodIndex(AST.context(), UsrCache));
 }
 
-llvm::Expected<CFGSummary> summarizeTargetFuncDefinition(
+llvm::Expected<std::optional<CFGSummary>> summarizeTargetFuncDefinition(
     llvm::StringRef Source) {
   return summarizeDefinitionNamed("target", Source);
 }
@@ -98,17 +99,20 @@ collectFromDefinitionViaSummaryWithErrors(
   USRCache UsrCache;
   std::vector<Evidence> Results;
   VirtualMethodIndex TUScopeVMI = getVirtualMethodIndex(ASTCtx, UsrCache);
-  llvm::Expected<CFGSummary> Summary = summarizeDefinition(
+  llvm::Expected<std::optional<CFGSummary>> SummaryResult = summarizeDefinition(
       Definition, UsrCache, Pragmas, TUScopeVMI, MakeSolver);
-  if (!Summary) return {Summary.takeError(), Results};
+  if (!SummaryResult) return {SummaryResult.takeError(), Results};
+  if (!SummaryResult->has_value())
+    return {llvm::Error::success(), std::vector<Evidence>{}};
+  CFGSummary& Summary = **SummaryResult;
 
   auto VMIForPropagation = std::make_shared<VirtualMethodIndex>(
-      loadVirtualMethodsIndex(Summary->virtual_method_index()));
+      loadVirtualMethodsIndex(Summary.virtual_method_index()));
   // All overrides from anywhere in the TU are relevant for propagating
   // evidence, so we use the entire TU-scoped collection for this direction.
   VMIForPropagation->Overrides = std::move(TUScopeVMI.Overrides);
   return {collectEvidenceFromSummary(
-              *Summary,
+              Summary,
               evidenceEmitterWithPropagation(
                   [&Results](const Evidence& E) { Results.push_back(E); },
                   VMIForPropagation),
diff --git a/nullability/inference/collect_evidence_test_utilities.h b/nullability/inference/collect_evidence_test_utilities.h
index 3076b6859..db00f8182 100644
--- a/nullability/inference/collect_evidence_test_utilities.h
+++ b/nullability/inference/collect_evidence_test_utilities.h
@@ -5,6 +5,7 @@
 #ifndef THIRD_PARTY_CRUBIT_NULLABILITY_INFERENCE_COLLECT_EVIDENCE_TEST_UTILITIES_H_
 #define THIRD_PARTY_CRUBIT_NULLABILITY_INFERENCE_COLLECT_EVIDENCE_TEST_UTILITIES_H_
 
+#include <optional>
 #include <string>
 #include <utility>
 #include <vector>
@@ -105,7 +106,7 @@ std::string printToString(DefinitionCollectionMode Mode);
 
 /// Summarizes the definition in `Source` with the name "target" (a generic name
 /// using the LLVM-style capitalization for function names).
-llvm::Expected<CFGSummary> summarizeTargetFuncDefinition(
+llvm::Expected<std::optional<CFGSummary>> summarizeTargetFuncDefinition(
     llvm::StringRef Source);
 
 //// Returns both an error and a vector to represent partial computations --
diff --git a/nullability/inference/infer_tu.cc b/nullability/inference/infer_tu.cc
index 2418e1a1e..b9e85b3f3 100644
--- a/nullability/inference/infer_tu.cc
+++ b/nullability/inference/infer_tu.cc
@@ -5,6 +5,7 @@
 #include "nullability/inference/infer_tu.h"
 
 #include <memory>
+#include <optional>
 #include <utility>
 #include <vector>
 
@@ -114,9 +115,11 @@ class InferenceManager {
     for (const auto* Impl : Sites.Definitions) {
       if (Filter && !Filter(*Impl)) continue;
 
-      if (llvm::Expected<CFGSummary> Summary =
+      if (llvm::Expected<std::optional<CFGSummary>> Summary =
               summarizeDefinition(*Impl, USRCache, Pragmas, *VMI)) {
-        Result.Summaries.push_back(*std::move(Summary));
+        if (Summary->has_value()) {
+          Result.Summaries.push_back(**std::move(Summary));
+        }
       } else {
         llvm::errs() << "Error summarizing definition: " << Summary.takeError()
                      << "\n";