diff --git a/.github/workflows/CI-unixish.yml b/.github/workflows/CI-unixish.yml
index ff7c3f65..b0523507 100644
--- a/.github/workflows/CI-unixish.yml
+++ b/.github/workflows/CI-unixish.yml
@@ -7,8 +7,13 @@ jobs:
 
     strategy:
       matrix:
-        compiler: [clang++, g++]
         os: [ubuntu-22.04, ubuntu-24.04, macos-13, macos-14, macos-15]
+        compiler: [clang++]
+        include:
+          - os: ubuntu-22.04
+            compiler: g++
+          - os: ubuntu-24.04
+            compiler: g++
       fail-fast: false
 
     runs-on: ${{ matrix.os }}
@@ -26,15 +31,16 @@ jobs:
           sudo apt-get install valgrind
 
     - name: Install missing software on ubuntu (clang++)
-      if: matrix.os == 'ubuntu-24.04' && matrix.compiler == 'clang++'
+      if: contains(matrix.os, 'ubuntu') && matrix.compiler == 'clang++'
       run: |
         sudo apt-get update
-        sudo apt-get install libc++-18-dev
+        sudo apt-get install libc++-dev
 
+    # coreutils contains "nproc"
     - name: Install missing software on macos
       if: contains(matrix.os, 'macos')
       run: |
-        brew install python3
+        brew install coreutils
 
     - name: Install missing Python packages
       run: |
diff --git a/Makefile b/Makefile
index 4a6ae6b7..525669eb 100644
--- a/Makefile
+++ b/Makefile
@@ -16,7 +16,7 @@ test:	testrunner	simplecpp
 	python3 -m pytest integration_test.py -vv
 
 selfcheck:	simplecpp
-	./selfcheck.sh
+	CXX=$(CXX) ./selfcheck.sh
 
 simplecpp:	main.o simplecpp.o
 	$(CXX) $(LDFLAGS) main.o simplecpp.o -o simplecpp
diff --git a/main.cpp b/main.cpp
index a6d14386..747f041b 100644
--- a/main.cpp
+++ b/main.cpp
@@ -43,6 +43,11 @@ int main(int argc, char **argv)
             }
             case 'I': { // include path
                 const char * const value = arg[2] ? (argv[i] + 2) : argv[++i];
+                std::ifstream f(value);
+                if (!f.is_open()) {
+                    std::cout << "error: include path '" << value << "' does not exist" << std::endl;
+                    std::exit(1);
+                }
                 dui.includePaths.push_back(value);
                 found = true;
                 break;
diff --git a/selfcheck.sh b/selfcheck.sh
index a43ef8c5..5cf7beb1 100755
--- a/selfcheck.sh
+++ b/selfcheck.sh
@@ -4,8 +4,52 @@ output=$(./simplecpp simplecpp.cpp -e -f 2>&1)
 ec=$?
 errors=$(echo "$output" | grep -v 'Header not found: <')
 if [ $ec -ne 0 ]; then
-  # only fail if got errors which do not refer to missing system includes
+  # only fail if we got errors which do not refer to missing system includes
   if [ ! -z "$errors" ]; then
     exit $ec
   fi
-fi
\ No newline at end of file
+fi
+
+if [ -z "$CXX" ]; then
+  exit 0
+fi
+
+cxx_type=$($CXX --version | head -1 | cut -d' ' -f1)
+if [ "$cxx_type" = "Ubuntu" ]; then
+  cxx_type=$($CXX --version | head -1 | cut -d' ' -f2)
+fi
+if [ "$cxx_type" = "g++" ]; then
+  gcc_ver=$($CXX -dumpversion)
+  ./simplecpp simplecpp.cpp -e -f -std=gnu++11 -D__GNUC__ -D__STDC__ -D__STDC_HOSTED__ -D__CHAR_BIT__=8 -I"/usr/include" -I"/usr/include/linux" -I"/usr/include/c++/$gcc_ver" -I"/usr/include/x86_64-linux-gnu/c++/$gcc_ver"
+  ec=$?
+  if [ $ec -ne 0 ]; then
+    exit $ec
+  fi
+elif [ "$cxx_type" = "clang" ]; then
+  clang_ver=$($CXX -dumpversion)
+  clang_ver=${clang_ver%%.*}
+  find /usr/include -name stubs-32.h
+  cxx_inc="/usr/include/c++/v1"
+  if [ ! -d "$cxx_inc" ]; then
+    cxx_inc="/usr/lib/llvm-$clang_ver/include/c++/v1"
+  fi
+  ./simplecpp simplecpp.cpp -e -f -std=gnu++11 -D__BYTE_ORDER__ -D__linux__ -I"$cxx_inc" -I"/usr/include" -I"/usr/include/x86_64-linux-gnu"
+  ec=$?
+  if [ $ec -ne 0 ]; then
+    exit $ec
+  fi
+elif [ "$cxx_type" = "Apple" ]; then
+  find /Applications -name endian.h
+  xcode_path="/Applications/Xcode_16.4.app"
+  if [ ! -d "$xcode_path" ]; then
+    xcode_path="/Applications/Xcode_15.2.app"
+  fi
+  ./simplecpp simplecpp.cpp -e -f -std=gnu++11 -D__BYTE_ORDER__ -D__APPLE__ -I"$xcode_path/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include" -I"$xcode_path/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/c++/v1"
+  ec=$?
+  if [ $ec -ne 0 ]; then
+    exit $ec
+  fi
+else
+  echo "unknown compiler '$cxx_type'"
+  exit 1
+fi