BUG: use 11.0 as minimum macOS platform ABI tag on arm64

Fixes mesonbuild#524.

mesonpy/_tags.py

@@ -129,6 +129,16 @@ def _get_macosx_platform_tag() -> str:
     # can therefore ignore the issue and generate the slightly
     # incorrect tag.
 
+    # The minimum macOS ABI version on arm64 is 11.0.  The macOS SDK
+    # or arm64 silently bumps any compatibility version specified via
+    # the MACOSX_DEPLOYMENT_TARGET environment variable to 11.0.
+    # Depsite the platform ABI tag being intended to be a minimum
+    # compatibiloty version, pip refuses to install wheels with a
+    # platform tag specifying ABI compatibility version lower than
+    # 11.0.  Use 11.0 as minimum ABI version on arm64.
+    if arch == 'arm64' and version < (11, 0):
+        version = (11, 0)
+
     major, minor = version
 
     if major >= 11:

tests/test_tags.py

@@ -54,6 +54,16 @@ def test_macos_platform_tag(monkeypatch):
             assert next(packaging.tags.mac_platforms((major, minor))) == mesonpy._tags.get_platform_tag()
 
 
+@pytest.mark.skipif(sys.platform != 'darwin', reason='macOS specific test')
+def test_macos_platform_tag_arm64(monkeypatch):
+    monkeypatch.setenv('_PYTHON_HOST_PLATFORM', 'macosx-12.0-arm64')
+    # Verify that the minimum platform ABI version on arm64 is 11.0.
+    monkeypatch.setenv('MACOSX_DEPLOYMENT_TARGET', '10.12')
+    assert mesonpy._tags.get_platform_tag() == 'macosx_11_0_arm64'
+    monkeypatch.setenv('MACOSX_DEPLOYMENT_TARGET', '12.34')
+    assert mesonpy._tags.get_platform_tag() == 'macosx_12_0_arm64'
+
+
 @pytest.mark.skipif(sys.platform != 'darwin', reason='macOS specific test')
 def test_python_host_platform(monkeypatch):
     monkeypatch.setenv('_PYTHON_HOST_PLATFORM', 'macosx-12.0-arm64')