diff --git a/.github/workflows/ubuntu.yml b/.github/workflows/ubuntu.yml
index 14b54348a..52e753436 100644
--- a/.github/workflows/ubuntu.yml
+++ b/.github/workflows/ubuntu.yml
@@ -146,6 +146,9 @@ jobs:
       - uses: actions/checkout@v3
       - name: Get latest CMake and ninja
         uses: lukka/get-cmake@latest
+      - name: Set env FORCE_STDCPPFS_FLAG for clang 7 / 8 / 9 / 10
+        run: echo "JSON_FORCED_GLOBAL_COMPILE_OPTIONS=-DJSON_HAS_FILESYSTEM=0;-DJSON_HAS_EXPERIMENTAL_FILESYSTEM=0" >> "$GITHUB_ENV"
+        if: ${{ matrix.compiler == '7' || matrix.compiler == '8' || matrix.compiler == '9' || matrix.compiler == '10' }}
       - name: Run CMake
         run: cmake -S . -B build -DJSON_CI=On
       - name: Build
diff --git a/cmake/ci.cmake b/cmake/ci.cmake
index f0b09f6d3..d07ab920c 100644
--- a/cmake/ci.cmake
+++ b/cmake/ci.cmake
@@ -901,12 +901,7 @@ add_custom_target(ci_cmake_flags
 foreach(COMPILER g++-4.8 g++-4.9 g++-5 g++-6 g++-7 g++-8 g++-9 g++-10 g++-11 clang++-3.5 clang++-3.6 clang++-3.7 clang++-3.8 clang++-3.9 clang++-4.0 clang++-5.0 clang++-6.0 clang++-7 clang++-8 clang++-9 clang++-10 clang++-11 clang++-12 clang++-13 clang++-14)
     find_program(COMPILER_TOOL NAMES ${COMPILER})
     if (COMPILER_TOOL)
-        if ("${COMPILER}" STREQUAL "clang++-9")
-            # fix for https://github.com/nlohmann/json/pull/3101#issuecomment-998788786 / https://stackoverflow.com/a/64051725/266378
-            set(ADDITIONAL_FLAGS "-DCMAKE_CXX_FLAGS=--gcc-toolchain=/root/gcc/9")
-        else()
-            unset(ADDITIONAL_FLAGS)
-        endif()
+        unset(ADDITIONAL_FLAGS)
 
         add_custom_target(ci_test_compiler_${COMPILER}
             COMMAND CXX=${COMPILER} ${CMAKE_COMMAND}
diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index 1afb000ae..3b9ae4d91 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -6,6 +6,15 @@ option(JSON_FastTests   "Skip expensive/slow tests." OFF)
 set(JSON_32bitTest     AUTO CACHE STRING "Enable the 32bit unit test (ON/OFF/AUTO/ONLY).")
 set(JSON_TestStandards "" CACHE STRING "The list of standards to test explicitly.")
 
+# using an env var, since this will also affect targets executing cmake (such as "ci_test_compiler_default")
+set(JSON_FORCED_GLOBAL_COMPILE_OPTIONS $ENV{JSON_FORCED_GLOBAL_COMPILE_OPTIONS})
+if (NOT "" STREQUAL "$ENV{JSON_FORCED_GLOBAL_COMPILE_OPTIONS}")
+    add_compile_options($ENV{JSON_FORCED_GLOBAL_COMPILE_OPTIONS})
+endif()
+if (NOT "" STREQUAL "$ENV{JSON_FORCED_GLOBAL_LINK_OPTIONS}")
+    add_link_options($ENV{JSON_FORCED_GLOBAL_LINK_OPTIONS})
+endif()
+
 include(test)
 
 #############################################################################
diff --git a/tests/src/unit-regression2.cpp b/tests/src/unit-regression2.cpp
index 308ceeef9..2bb9b32e5 100644
--- a/tests/src/unit-regression2.cpp
+++ b/tests/src/unit-regression2.cpp
@@ -37,7 +37,9 @@ using ordered_json = nlohmann::ordered_json;
 #endif
 
 #ifdef JSON_HAS_CPP_20
-    #include <span>
+    #if __has_include(<span>)
+        #include <span>
+    #endif
 #endif
 
 // NLOHMANN_JSON_SERIALIZE_ENUM uses a static std::pair
@@ -664,6 +666,7 @@ TEST_CASE("regression tests 2")
     }
 
 #ifdef JSON_HAS_CPP_20
+#if __has_include(<span>)
     SECTION("issue #2546 - parsing containers of std::byte")
     {
         const char DATA[] = R"("Hello, world!")"; // NOLINT(cppcoreguidelines-avoid-c-arrays,hicpp-avoid-c-arrays,modernize-avoid-c-arrays)
@@ -671,6 +674,7 @@ TEST_CASE("regression tests 2")
         const json j = json::parse(s);
         CHECK(j.dump() == "\"Hello, world!\"");
     }
+#endif
 #endif
 
     SECTION("issue #2574 - Deserialization to std::array, std::pair, and std::tuple with non-default constructable types fails")