diff --git a/cmake/ConfigureCompiler.cmake b/cmake/ConfigureCompiler.cmake
index 61d800c30c..9343ee402f 100644
--- a/cmake/ConfigureCompiler.cmake
+++ b/cmake/ConfigureCompiler.cmake
@@ -285,7 +285,6 @@ else()
       -Wno-comment
       -Wno-delete-non-virtual-dtor
       -Wno-format
-      -Wno-mismatched-tags
       -Wno-sign-compare
       -Wno-tautological-pointer-compare
       -Wno-undefined-var-template
diff --git a/fdbclient/ActorLineageProfiler.h b/fdbclient/ActorLineageProfiler.h
index a55d1541e1..07b7c30966 100644
--- a/fdbclient/ActorLineageProfiler.h
+++ b/fdbclient/ActorLineageProfiler.h
@@ -96,7 +96,7 @@ struct ConfigError {
 class ProfilerConfigT {
 private: // private types
 	using Lock = std::unique_lock<std::mutex>;
-	friend class crossbow::create_static<ProfilerConfigT>;
+	friend struct crossbow::create_static<ProfilerConfigT>;
 
 private: // members
 	std::shared_ptr<SampleIngestor> ingestor = std::make_shared<NoneIngestor>();
diff --git a/fdbclient/ConfigTransactionInterface.h b/fdbclient/ConfigTransactionInterface.h
index 6e71b72457..c6f2aa920f 100644
--- a/fdbclient/ConfigTransactionInterface.h
+++ b/fdbclient/ConfigTransactionInterface.h
@@ -188,11 +188,11 @@ struct ConfigTransactionInterface {
 
 public:
 	static constexpr FileIdentifier file_identifier = 982485;
-	struct RequestStream<ConfigTransactionGetGenerationRequest> getGeneration;
-	struct RequestStream<ConfigTransactionGetRequest> get;
-	struct RequestStream<ConfigTransactionGetConfigClassesRequest> getClasses;
-	struct RequestStream<ConfigTransactionGetKnobsRequest> getKnobs;
-	struct RequestStream<ConfigTransactionCommitRequest> commit;
+	class RequestStream<ConfigTransactionGetGenerationRequest> getGeneration;
+	class RequestStream<ConfigTransactionGetRequest> get;
+	class RequestStream<ConfigTransactionGetConfigClassesRequest> getClasses;
+	class RequestStream<ConfigTransactionGetKnobsRequest> getKnobs;
+	class RequestStream<ConfigTransactionCommitRequest> commit;
 
 	ConfigTransactionInterface();
 	void setupWellKnownEndpoints();
diff --git a/fdbrpc/FlowTransport.h b/fdbrpc/FlowTransport.h
index abc42e9d70..24daae400a 100644
--- a/fdbrpc/FlowTransport.h
+++ b/fdbrpc/FlowTransport.h
@@ -134,7 +134,7 @@ public:
 	}
 };
 
-struct TransportData;
+class TransportData;
 
 struct Peer : public ReferenceCounted<Peer> {
 	TransportData* transport;
diff --git a/fdbserver/DataDistribution.actor.cpp b/fdbserver/DataDistribution.actor.cpp
index 7249934267..21697dedc1 100644
--- a/fdbserver/DataDistribution.actor.cpp
+++ b/fdbserver/DataDistribution.actor.cpp
@@ -47,7 +47,7 @@
 #include "flow/serialize.h"
 
 class TCTeamInfo;
-struct TCMachineInfo;
+class TCMachineInfo;
 class TCMachineTeamInfo;
 
 namespace {
diff --git a/fdbserver/DeltaTree.h b/fdbserver/DeltaTree.h
index 2c1f7ea91f..4a2f77c52f 100644
--- a/fdbserver/DeltaTree.h
+++ b/fdbserver/DeltaTree.h
@@ -349,7 +349,7 @@ public:
 		}
 	};
 
-	struct Cursor;
+	class Cursor;
 
 	// A Mirror is an accessor for a DeltaTree which allows insertion and reading.  Both operations are done
 	// using cursors which point to and share nodes in an tree that is built on-demand and mirrors the compressed
@@ -515,7 +515,8 @@ public:
 
 	// Cursor provides a way to seek into a DeltaTree and iterate over its contents
 	// All Cursors from a Mirror share the same decoded node 'cache' (tree of DecodedNodes)
-	struct Cursor {
+	class Cursor {
+	public:
 		Cursor() : mirror(nullptr), node(nullptr) {}
 
 		Cursor(Mirror* r) : mirror(r), node(mirror->root) {}
diff --git a/flow/Trace.h b/flow/Trace.h
index aeaabb4373..52dc94aab7 100644
--- a/flow/Trace.h
+++ b/flow/Trace.h
@@ -588,8 +588,8 @@ void removeTraceRole(std::string const& role);
 void retrieveTraceLogIssues(std::set<std::string>& out);
 void setTraceLogGroup(const std::string& role);
 template <class T>
-struct Future;
-struct Void;
+class Future;
+class Void;
 Future<Void> pingTraceLogWriterThread();
 
 enum trace_clock_t { TRACE_CLOCK_NOW, TRACE_CLOCK_REALTIME };
diff --git a/flow/flow.h b/flow/flow.h
index 366ac01175..b331107ee5 100644
--- a/flow/flow.h
+++ b/flow/flow.h
@@ -445,7 +445,8 @@ struct LineageProperties : LineagePropertiesBase {
 	}
 };
 
-struct ActorLineage : ThreadSafeReferenceCounted<ActorLineage> {
+class ActorLineage : public ThreadSafeReferenceCounted<ActorLineage> {
+public:
 	friend class LineageReference;
 
 	struct Property {
diff --git a/flow/network.h b/flow/network.h
index 60a190c0af..a0710f5ce4 100644
--- a/flow/network.h
+++ b/flow/network.h
@@ -407,7 +407,7 @@ public:
 };
 
 // forward declare SendBuffer, declared in serialize.h
-struct SendBuffer;
+class SendBuffer;
 
 class IConnection {
 public:
diff --git a/flow/serialize.h b/flow/serialize.h
index 07f70b1f24..ac48fa9740 100644
--- a/flow/serialize.h
+++ b/flow/serialize.h
@@ -851,7 +851,8 @@ struct ISerializeSource {
 };
 
 template <class T, class V>
-struct MakeSerializeSource : ISerializeSource {
+class MakeSerializeSource : public ISerializeSource {
+public:
 	using value_type = V;
 	void serializePacketWriter(PacketWriter& w) const override {
 		ObjectWriter writer([&](size_t size) { return w.writeBytes(size); }, AssumeVersion(w.protocolVersion()));
diff --git a/flow/singleton.h b/flow/singleton.h
index b565604674..7d193e8dde 100644
--- a/flow/singleton.h
+++ b/flow/singleton.h
@@ -272,4 +272,4 @@ typename singleton<T, C, L, M>::pointer singleton<T, C, L, M>::instance_ = nullp
 template <typename T, typename C, typename L, typename M>
 M singleton<T, C, L, M>::mutex_;
 
-} // namespace crossbow
\ No newline at end of file
+} // namespace crossbow