diff --git a/.github/workflows/iwyu.yml b/.github/workflows/iwyu.yml
index 667c0ee3f..d27022ce6 100644
--- a/.github/workflows/iwyu.yml
+++ b/.github/workflows/iwyu.yml
@@ -28,7 +28,7 @@ jobs:
 
       - name: Prepare CMake
         run: |
-          cmake -S . -B cmake.output -G "Unix Makefiles" -DCMAKE_BUILD_TYPE=Release -DHAVE_RULES=On -DBUILD_TESTS=On -DBUILD_GUI=On -DBUILD_TESTS=On -DBUILD_GUI=On -DWITH_QCHART=On -DCMAKE_GLOBAL_AUTOGEN_TARGET=On -DCMAKE_DISABLE_PRECOMPILE_HEADERS=On -DCPPCHK_GLIBCXX_DEBUG=Off -DUSE_MATCHCOMPILER=Off
+          cmake -S . -B cmake.output -G "Unix Makefiles" -DCMAKE_BUILD_TYPE=Release -DHAVE_RULES=On -DBUILD_TESTS=On -DBUILD_GUI=On -DBUILD_TESTS=On -DBUILD_GUI=On -DWITH_QCHART=On -DENABLE_CHECK_INTERNAL=On -DCMAKE_GLOBAL_AUTOGEN_TARGET=On -DCMAKE_DISABLE_PRECOMPILE_HEADERS=On -DCPPCHK_GLIBCXX_DEBUG=Off -DUSE_MATCHCOMPILER=Off
 
       - name: Prepare CMake dependencies
         run: |
diff --git a/lib/checkinternal.h b/lib/checkinternal.h
index 939e7327b..5cd875e42 100644
--- a/lib/checkinternal.h
+++ b/lib/checkinternal.h
@@ -24,10 +24,15 @@
 
 #include "check.h"
 #include "config.h"
+#include "errortypes.h"
 #include "settings.h"
 
 #include <string>
 
+class ErrorLogger;
+class Token;
+class Tokenizer;
+
 /// @addtogroup Checks
 /// @{
 
diff --git a/lib/cppcheck.h b/lib/cppcheck.h
index 560823203..5889b63dd 100644
--- a/lib/cppcheck.h
+++ b/lib/cppcheck.h
@@ -32,7 +32,7 @@
 #include <cstddef>
 #include <fstream> // IWYU pragma: keep
 #include <functional>
-#include <istream>
+#include <iosfwd>
 #include <list>
 #include <map>
 #include <string>
diff --git a/lib/smallvector.h b/lib/smallvector.h
index 738e53dae..0079554f7 100644
--- a/lib/smallvector.h
+++ b/lib/smallvector.h
@@ -29,6 +29,7 @@ static constexpr std::size_t DefaultSmallVectorSize = 0;
 template<typename T, std::size_t N = DefaultSmallVectorSize>
 using SmallVector = boost::container::small_vector<T, N>;
 #else
+#include <utility>
 #include <vector>
 
 template<class T, std::size_t N>
diff --git a/lib/tokenrange.h b/lib/tokenrange.h
index b1405821d..0498ceecf 100644
--- a/lib/tokenrange.h
+++ b/lib/tokenrange.h
@@ -23,6 +23,12 @@
 
 #include "config.h"
 
+#include <cstddef>
+#include <iterator>
+#include <type_traits>
+
+class Token;
+
 template<typename T, REQUIRES("T must be a Token class", std::is_convertible<T*, const Token*> )>
 class TokenRangeBase {
     T* mFront;
diff --git a/lib/valueflow.h b/lib/valueflow.h
index cddbbf8ea..e05077498 100644
--- a/lib/valueflow.h
+++ b/lib/valueflow.h
@@ -24,7 +24,6 @@
 #include "config.h"
 #include "mathlib.h"
 
-#include <algorithm>
 #include <cassert>
 #include <cstdlib>
 #include <functional>
diff --git a/lib/valueptr.h b/lib/valueptr.h
index 45ca7d94e..7d65f976d 100644
--- a/lib/valueptr.h
+++ b/lib/valueptr.h
@@ -22,7 +22,6 @@
 //---------------------------------------------------------------------------
 
 #include "config.h"
-#include <functional>
 #include <memory>
 
 template<class T>