diff --git a/CMake/CatchMiscFunctions.cmake b/CMake/CatchMiscFunctions.cmake
index 735c7868..44c87500 100644
--- a/CMake/CatchMiscFunctions.cmake
+++ b/CMake/CatchMiscFunctions.cmake
@@ -74,6 +74,7 @@ function(add_warnings_to_targets targets)
           "-Woverloaded-virtual"
           "-Wparentheses"
           "-Wpedantic"
+          "-Wredundant-decls"
           "-Wreorder"
           "-Wreturn-std-move"
           "-Wshadow"
diff --git a/src/catch2/catch_session.cpp b/src/catch2/catch_session.cpp
index af817017..86e215a9 100644
--- a/src/catch2/catch_session.cpp
+++ b/src/catch2/catch_session.cpp
@@ -13,9 +13,9 @@
 #include <catch2/internal/catch_run_context.hpp>
 #include <catch2/catch_test_spec.hpp>
 #include <catch2/catch_version.hpp>
-#include <catch2/interfaces/catch_interfaces_testcase.hpp>
 #include <catch2/internal/catch_startup_exception_registry.hpp>
 #include <catch2/internal/catch_sharding.hpp>
+#include <catch2/internal/catch_test_case_registry_impl.hpp>
 #include <catch2/internal/catch_textflow.hpp>
 #include <catch2/internal/catch_windows_h_proxy.hpp>
 #include <catch2/reporters/catch_reporter_multi.hpp>
diff --git a/src/catch2/catch_test_spec.cpp b/src/catch2/catch_test_spec.cpp
index 99d83792..f32f9864 100644
--- a/src/catch2/catch_test_spec.cpp
+++ b/src/catch2/catch_test_spec.cpp
@@ -7,6 +7,7 @@
 // SPDX-License-Identifier: BSL-1.0
 #include <catch2/catch_test_spec.hpp>
 #include <catch2/interfaces/catch_interfaces_testcase.hpp>
+#include <catch2/internal/catch_test_case_registry_impl.hpp>
 #include <catch2/internal/catch_reusable_string_stream.hpp>
 #include <catch2/internal/catch_string_manip.hpp>
 #include <catch2/catch_test_case_info.hpp>
diff --git a/src/catch2/interfaces/catch_interfaces_testcase.hpp b/src/catch2/interfaces/catch_interfaces_testcase.hpp
index f8e20351..daee8482 100644
--- a/src/catch2/interfaces/catch_interfaces_testcase.hpp
+++ b/src/catch2/interfaces/catch_interfaces_testcase.hpp
@@ -12,7 +12,6 @@
 
 namespace Catch {
 
-    class TestSpec;
     struct TestCaseInfo;
     class TestCaseHandle;
     class IConfig;
@@ -26,11 +25,6 @@ namespace Catch {
         virtual std::vector<TestCaseHandle> const& getAllTestsSorted( IConfig const& config ) const = 0;
     };
 
-    bool isThrowSafe( TestCaseHandle const& testCase, IConfig const& config );
-    bool matchTest( TestCaseHandle const& testCase, TestSpec const& testSpec, IConfig const& config );
-    std::vector<TestCaseHandle> filterTests( std::vector<TestCaseHandle> const& testCases, TestSpec const& testSpec, IConfig const& config );
-    std::vector<TestCaseHandle> const& getAllTestCasesSorted( IConfig const& config );
-
 }
 
 #endif // CATCH_INTERFACES_TESTCASE_HPP_INCLUDED
diff --git a/src/catch2/internal/catch_list.cpp b/src/catch2/internal/catch_list.cpp
index 4eb8bb7d..97e4c593 100644
--- a/src/catch2/internal/catch_list.cpp
+++ b/src/catch2/internal/catch_list.cpp
@@ -9,8 +9,8 @@
 
 #include <catch2/interfaces/catch_interfaces_registry_hub.hpp>
 #include <catch2/interfaces/catch_interfaces_reporter.hpp>
-#include <catch2/interfaces/catch_interfaces_testcase.hpp>
 #include <catch2/interfaces/catch_interfaces_reporter_factory.hpp>
+#include <catch2/internal/catch_test_case_registry_impl.hpp>
 #include <catch2/internal/catch_reporter_registry.hpp>
 #include <catch2/internal/catch_move_and_forward.hpp>
 #include <catch2/internal/catch_case_insensitive_comparisons.hpp>
diff --git a/src/catch2/internal/catch_test_case_registry_impl.cpp b/src/catch2/internal/catch_test_case_registry_impl.cpp
index b66469bc..f1702979 100644
--- a/src/catch2/internal/catch_test_case_registry_impl.cpp
+++ b/src/catch2/internal/catch_test_case_registry_impl.cpp
@@ -24,6 +24,38 @@
 
 namespace Catch {
 
+    namespace {
+        static void enforceNoDuplicateTestCases(
+            std::vector<TestCaseHandle> const& tests ) {
+            auto testInfoCmp = []( TestCaseInfo const* lhs,
+                                   TestCaseInfo const* rhs ) {
+                return *lhs < *rhs;
+            };
+            std::set<TestCaseInfo const*, decltype( testInfoCmp )&> seenTests(
+                testInfoCmp );
+            for ( auto const& test : tests ) {
+                const auto infoPtr = &test.getTestCaseInfo();
+                const auto prev = seenTests.insert( infoPtr );
+                CATCH_ENFORCE( prev.second,
+                               "error: test case \""
+                                   << infoPtr->name << "\", with tags \""
+                                   << infoPtr->tagsAsString()
+                                   << "\" already defined.\n"
+                                   << "\tFirst seen at "
+                                   << ( *prev.first )->lineInfo << "\n"
+                                   << "\tRedefined at " << infoPtr->lineInfo );
+            }
+        }
+
+        static bool matchTest( TestCaseHandle const& testCase,
+                               TestSpec const& testSpec,
+                               IConfig const& config ) {
+            return testSpec.matches( testCase.getTestCaseInfo() ) &&
+                   isThrowSafe( testCase, config );
+        }
+
+    } // end unnamed namespace
+
     std::vector<TestCaseHandle> sortTests( IConfig const& config, std::vector<TestCaseHandle> const& unsortedTestCases ) {
         switch (config.runOrder()) {
         case TestRunOrder::Declared:
@@ -80,29 +112,6 @@ namespace Catch {
         return !testCase.getTestCaseInfo().throws() || config.allowThrows();
     }
 
-    bool matchTest( TestCaseHandle const& testCase, TestSpec const& testSpec, IConfig const& config ) {
-        return testSpec.matches( testCase.getTestCaseInfo() ) && isThrowSafe( testCase, config );
-    }
-
-    void
-    enforceNoDuplicateTestCases( std::vector<TestCaseHandle> const& tests ) {
-        auto testInfoCmp = []( TestCaseInfo const* lhs,
-                               TestCaseInfo const* rhs ) {
-            return *lhs < *rhs;
-        };
-        std::set<TestCaseInfo const*, decltype(testInfoCmp) &> seenTests(testInfoCmp);
-        for ( auto const& test : tests ) {
-            const auto infoPtr = &test.getTestCaseInfo();
-            const auto prev = seenTests.insert( infoPtr );
-            CATCH_ENFORCE(
-                prev.second,
-                "error: test case \"" << infoPtr->name << "\", with tags \""
-                    << infoPtr->tagsAsString() << "\" already defined.\n"
-                    << "\tFirst seen at " << ( *prev.first )->lineInfo << "\n"
-                    << "\tRedefined at " << infoPtr->lineInfo );
-        }
-    }
-
     std::vector<TestCaseHandle> filterTests( std::vector<TestCaseHandle> const& testCases, TestSpec const& testSpec, IConfig const& config ) {
         std::vector<TestCaseHandle> filtered;
         filtered.reserve( testCases.size() );
diff --git a/src/catch2/internal/catch_test_case_registry_impl.hpp b/src/catch2/internal/catch_test_case_registry_impl.hpp
index 6784e034..a4a27ed1 100644
--- a/src/catch2/internal/catch_test_case_registry_impl.hpp
+++ b/src/catch2/internal/catch_test_case_registry_impl.hpp
@@ -24,9 +24,6 @@ namespace Catch {
     std::vector<TestCaseHandle> sortTests( IConfig const& config, std::vector<TestCaseHandle> const& unsortedTestCases );
 
     bool isThrowSafe( TestCaseHandle const& testCase, IConfig const& config );
-    bool matchTest( TestCaseHandle const& testCase, TestSpec const& testSpec, IConfig const& config );
-
-    void enforceNoDuplicateTestCases( std::vector<TestCaseHandle> const& functions );
 
     std::vector<TestCaseHandle> filterTests( std::vector<TestCaseHandle> const& testCases, TestSpec const& testSpec, IConfig const& config );
     std::vector<TestCaseHandle> const& getAllTestCasesSorted( IConfig const& config );