Use an env variable.

.github/workflows/build-test-ubuntu-arm64.yml

@@ -177,14 +177,10 @@ jobs:
         if: ${{ github.event_name != 'pull_request' || github.event.pull_request.head.repo.full_name == github.repository}}
         run: aws s3 sync ${{inputs.autotest_data_s3_url}} ./test_data --delete --no-sign-request --size-only
 
-        # See the comment in the patch file for an explanation.
-      - name: Disable broken Python regression tests
-        if: ${{ matrix.os == 'ubuntu20' && inputs.mrbind }}
-        run: git am scripts/mrbind/0001-Disable-tests-that-seem-to-expose-an-ABI-incompatibility.patch
-
       - name: Python Regression Tests
         env:
           SMOKE: ${{ env.INTERNAL_BUILD == 'true' && inputs.full_config_build == false && matrix.config == 'Debug' }}
+          MR_REGRESSION_TESTS_UBUNTUARM2004_MRBIND_ABI_ISSUES: ${{ matrix.os == 'ubuntu20' && inputs.mrbind }}
         if: ${{ env.INTERNAL_BUILD == 'true' }}
         working-directory: ./build/${{ matrix.config }}/bin
         run: |

test_regression/test_algorithms/test_decimate.py

@@ -15,6 +15,7 @@
                   }},
                  id="maxError_0.05"),
     pytest.param({"name": "maxError_0.25",
+                  "hasAbiIssuesOnUbuntuArm2004Mrbind": True,
                   "params": {
                       "maxError": 0.2
                   }},
@@ -35,6 +36,7 @@
                       "maxError": 0.05
                   }}, id="maxEdgeLen_0.4"),
     pytest.param({"name": "maxTriangleAspectRatio_5",
+                  "hasAbiIssuesOnUbuntuArm2004Mrbind": True,
                   "params": {
                       "maxTriangleAspectRatio": 5,
                       "maxError": 0.15
@@ -85,6 +87,16 @@ def test_decimate(tmp_path, dec_params):
     """
     Test decimate algorithm with all settings, avaliable in UI
     """
+
+    # Those tests fail (seemingly due to an unknown ABI incompatibility) on Arm Ubuntu 20.04 when the MRBind bindings
+    # are compiled with Clang 18 AND MeshLib is compiled with Clang 12 or older (14 is fine on Ubuntu 22.04, 13 wasn't tested).
+    # So instead we build the whole MeshLib with Clang 18 (only on ubuntu 20.04 arm) when building the wheels,
+    # but when building ML we simply disable the offending tests (makes no sense to use Clang 18 for that,
+    # since the library users will then face this ABI incompatibility).
+    if os.getenv("MR_REGRESSION_TESTS_UBUNTUARM2004_MRBIND_ABI_ISSUES","0") == "1" and getattr(dec_params, "hasAbiIssuesOnUbuntuArm2004Mrbind", False):
+        print('Skipping this configuration on Ubuntu Arm 20.04')
+        return
+
     #  Load input meshes
     input_folder = Path(test_files_path) / "algorithms" / "decimate" / "R0003C_V4-16aug19"
     case_name = dec_params["name"]