fix: reset the cached compiler when build_ext is reinitialized

setup.py

@@ -40,6 +40,7 @@ def initialize_options(self):
         # same command object, so make sure not to override previously
         # set options.
         if getattr(self, '_initialized', False):
+            self.compiler = None
             return
 
         super().initialize_options()

tests/test_build.py

@@ -0,0 +1,29 @@
+import runpy
+import unittest
+from pathlib import Path
+from unittest import mock
+
+from setuptools import Distribution, Extension
+from setuptools._distutils.ccompiler import new_compiler
+
+
+class TestBuildExt(unittest.TestCase):
+    def test_reinitialized_command_uses_fresh_compiler(self):
+        setup_py = Path(__file__).parents[1] / "setup.py"
+        with mock.patch("setuptools.setup") as setup:
+            runpy.run_path(str(setup_py))
+
+        build_ext = setup.call_args.kwargs["cmdclass"]["build_ext"]
+        distribution = Distribution({
+            "cmdclass": {"build_ext": build_ext},
+            "ext_modules": [Extension("test", ["test.c"])],
+        })
+        command = distribution.get_command_obj("build_ext")
+        command.ensure_finalized()
+        command.compiler = new_compiler()
+
+        distribution.reinitialize_command("build_ext")
+        command.build_extensions = mock.Mock()
+        command.run()
+
+        command.build_extensions.assert_called_once_with()