diff --git a/core/clingutils/res/TClingUtils.h b/core/clingutils/res/TClingUtils.h
index afb8b6252dce8..467f271ebe03d 100644
--- a/core/clingutils/res/TClingUtils.h
+++ b/core/clingutils/res/TClingUtils.h
@@ -12,8 +12,6 @@
 #ifndef ROOT_TMetaUtils
 #define ROOT_TMetaUtils
 
-#include "RConversionRuleParser.h"
-
 #include <functional>
 #include <set>
 #include <string>
diff --git a/core/clingutils/src/TClingUtils.cxx b/core/clingutils/src/TClingUtils.cxx
index 699757af88fc8..d8ca763e52866 100644
--- a/core/clingutils/src/TClingUtils.cxx
+++ b/core/clingutils/src/TClingUtils.cxx
@@ -25,6 +25,7 @@
 #include <cctype>
 
 #include "RConfigure.h"
+#include "RConversionRuleParser.h"
 #include <ROOT/RConfig.hxx>
 #include <ROOT/FoundationUtils.hxx>
 #include "Rtypes.h"
diff --git a/core/clingutils/test/CMakeLists.txt b/core/clingutils/test/CMakeLists.txt
index 15cc3ff48ad0d..a2d5bd79c4a28 100644
--- a/core/clingutils/test/CMakeLists.txt
+++ b/core/clingutils/test/CMakeLists.txt
@@ -30,4 +30,4 @@ if(NOT builtin_clang)
   link_directories("${LLVM_LIBRARY_DIR}")
 endif()
 
-ROOT_ADD_UNITTEST_DIR(Core RIO ${CLING_LIBRARIES} $<TARGET_OBJECTS:ClingUtils>)
+ROOT_ADD_UNITTEST_DIR(Core ${CLING_LIBRARIES} $<TARGET_OBJECTS:ClingUtils>)
diff --git a/core/clingutils/test/TClingUtilsTests.cxx b/core/clingutils/test/TClingUtilsTests.cxx
index 7380886e60e55..311e124f7953a 100644
--- a/core/clingutils/test/TClingUtilsTests.cxx
+++ b/core/clingutils/test/TClingUtilsTests.cxx
@@ -15,15 +15,18 @@
  *************************************************************************/
 
 #include <TClingUtils.h>
-#include <TClass.h>
-#include <TInterpreter.h>
 
+// This test must under no circumstances include other ROOT headers. It must not call into Cling, TInterpreter, TClass.
+// It is meant for unit testing of TClingUtils and statically links ClingUtils and Cling/Clang/LLVM libraries. If the
+// test was to do any of the above, there would be two copies of functions around (in libCling.so and the test binary)
+// with not much guarantees which ones are called at which moment.
+
+#include <DllImport.h>
 #include <ROOT/FoundationUtils.hxx>
 
 #include "gtest/gtest.h"
 
 #include <fstream>
-#include <deque>
 
 TEST(TClingUtilsTests, GetCppName)
 {
@@ -89,24 +92,12 @@ TEST(TClingUtilsTests, GetRealPath)
 #endif // not R__WIN32
 }
 
-TEST(TClingUtilsTests, CollectionSizeof)
-{
-   // https://its.cern.ch/jira/browse/ROOT-9889
-   EXPECT_EQ(sizeof(std::deque<short>), TClass::GetClass("std::deque<short>")->GetClassSize());
-   EXPECT_EQ(sizeof(std::deque<unsigned short>), TClass::GetClass("std::deque<unsigned short>")->GetClassSize());
-   EXPECT_EQ(sizeof(std::deque<int>), TClass::GetClass("std::deque<int>")->GetClassSize());
-   EXPECT_EQ(sizeof(std::deque<unsigned int>), TClass::GetClass("std::deque<unsigned int>")->GetClassSize());
-   EXPECT_EQ(sizeof(std::deque<long>), TClass::GetClass("std::deque<long>")->GetClassSize());
-   EXPECT_EQ(sizeof(std::deque<unsigned long>), TClass::GetClass("std::deque<unsigned long>")->GetClassSize());
-}
+// Forward-declare gCling to not include TInterpreter.h just for checking that the interpreter has not been initialized.
+class TInterpreter;
+R__EXTERN TInterpreter *gCling;
 
-TEST(TClingUtilsTests, ReSubstTemplateArg)
-{
-   // #18811
-   gInterpreter->Declare("template <typename T> struct S {};"
-                         "template <typename T1, typename T2> struct Two { using value_type = S<T2>; };"
-                         "template <typename T> struct One { Two<int, int>::value_type *t; };");
+class InterpreterCheck : public testing::Environment {
+   void TearDown() override { ASSERT_EQ(gCling, nullptr); }
+};
 
-   auto c = TClass::GetClass("One<std::string>");
-   c->BuildRealData();
-}
+testing::Environment *gInterpreterCheck = testing::AddGlobalTestEnvironment(new InterpreterCheck);
diff --git a/core/dictgen/src/XMLReader.cxx b/core/dictgen/src/XMLReader.cxx
index f4a28eae56543..a65b4c633a393 100644
--- a/core/dictgen/src/XMLReader.cxx
+++ b/core/dictgen/src/XMLReader.cxx
@@ -18,6 +18,7 @@
 
 
 #include "XMLReader.h"
+#include "RConversionRuleParser.h"
 #include "SelectionRules.h"
 #include "TClingUtils.h"
 
diff --git a/core/dictgen/src/rootcling_impl.cxx b/core/dictgen/src/rootcling_impl.cxx
index cea4c0bbff909..fb5961732e7a1 100644
--- a/core/dictgen/src/rootcling_impl.cxx
+++ b/core/dictgen/src/rootcling_impl.cxx
@@ -12,6 +12,7 @@
 #include "rootclingCommandLineOptionsHelp.h"
 
 #include "RConfigure.h"
+#include "RConversionRuleParser.h"
 #include <ROOT/RConfig.hxx>
 #include <ROOT/FoundationUtils.hxx>
 #include "snprintf.h"
diff --git a/core/meta/test/testTClass.cxx b/core/meta/test/testTClass.cxx
index fd6b09e7a0b09..7fc7231eed3c5 100644
--- a/core/meta/test/testTClass.cxx
+++ b/core/meta/test/testTClass.cxx
@@ -7,6 +7,8 @@
 
 #include "gtest/gtest.h"
 
+#include <deque>
+
 TEST(TClass, DictCheck)
 {
    gInterpreter->ProcessLine(".L stlDictCheck.h+");
@@ -80,3 +82,25 @@ TEST(TClass, ConsistentSTLLookup)
    auto second = first->GetActualClass(&map);
    EXPECT_EQ(first, second);
 }
+
+TEST(TClass, CollectionSizeof)
+{
+   // https://its.cern.ch/jira/browse/ROOT-9889
+   EXPECT_EQ(sizeof(std::deque<short>), TClass::GetClass("std::deque<short>")->GetClassSize());
+   EXPECT_EQ(sizeof(std::deque<unsigned short>), TClass::GetClass("std::deque<unsigned short>")->GetClassSize());
+   EXPECT_EQ(sizeof(std::deque<int>), TClass::GetClass("std::deque<int>")->GetClassSize());
+   EXPECT_EQ(sizeof(std::deque<unsigned int>), TClass::GetClass("std::deque<unsigned int>")->GetClassSize());
+   EXPECT_EQ(sizeof(std::deque<long>), TClass::GetClass("std::deque<long>")->GetClassSize());
+   EXPECT_EQ(sizeof(std::deque<unsigned long>), TClass::GetClass("std::deque<unsigned long>")->GetClassSize());
+}
+
+TEST(TClass, ReSubstTemplateArg)
+{
+   // #18811
+   gInterpreter->Declare("template <typename T> struct S {};"
+                         "template <typename T1, typename T2> struct Two { using value_type = S<T2>; };"
+                         "template <typename T> struct One { Two<int, int>::value_type *t; };");
+
+   auto c = TClass::GetClass("One<std::string>");
+   c->BuildRealData();
+}
diff --git a/core/metacling/src/TClingCallbacks.cxx b/core/metacling/src/TClingCallbacks.cxx
index b3d30495251f7..5f18e0da5b85b 100644
--- a/core/metacling/src/TClingCallbacks.cxx
+++ b/core/metacling/src/TClingCallbacks.cxx
@@ -11,6 +11,7 @@
 
 #include "TClingCallbacks.h"
 
+#include <DllImport.h> // for R__EXTERN
 #include <ROOT/FoundationUtils.hxx>
 
 #include "cling/Interpreter/DynamicLibraryManager.h"