fix: universal2 tags computation was incorrect

src/scikit_build_core/builder/builder.py

@@ -39,26 +39,31 @@ def get_archs(self) -> list[str]:
         Example (macOS):
             ARCHFLAGS="-arch x86_64" -> ["x86_64"]
             ARCHFLAGS="-arch x86_64 -arch arm64" -> ["x86_64", "arm64"]
-            ARCHFLAGS="-arch universal2" -> ["universal2"]
 
         Returns an empty list otherwise or if ARCHFLAGS is not set.
-
-        If the extra_tags setting is True, then the list of platforms is
-        expanded to include extra matching platforms (universal2 also produces
-        x86_64 and arm64).
         """
 
         if sys.platform.startswith("darwin"):
             archs = re.findall(r"-arch (\S+)", self.config.env.get("ARCHFLAGS", ""))
-            if (
-                self.settings.wheel.expand_macos_universal_tags
-                and ["universal2"] == archs
-            ):
-                archs += ["x86_64", "arm64"]
             return archs
 
         return []
 
+    def get_arch_tags(self) -> list[str]:
+        """
+        This function returns tags suitable for use in wheels. The main
+        difference between this method and get_archs() is that this returns
+        universal2 instead of separate tags for x86_64 and arm64, or all of them
+        if wheel.expand-macos-universal-tags is true.
+        """
+
+        archs = self.get_archs()
+        if sys.platform.startswith("darwin") and set(archs) == {"arm64", "x86_64"}:
+            if self.settings.wheel.expand_macos_universal_tags:
+                return ["universal2", "x86_64", "arm64"]
+            return ["universal2"]
+        return archs
+
     def configure(
         self,
         *,

src/scikit_build_core/pyproject/wheel.py

@@ -198,7 +198,7 @@ def build_wheel(
 
         _write_wheel_metadata(packages_dir=wheel_dirs["platlib"], metadata=metadata)
 
-        tags = WheelTag.compute_best(builder.get_archs(), settings.wheel.py_api)
+        tags = WheelTag.compute_best(builder.get_arch_tags(), settings.wheel.py_api)
         wheel.build({k: str(v) for k, v in wheel_dirs.items()}, tags=tags.tags_dict())
 
     wheel_filename: str = wheel.filename

tests/test_builder.py

@@ -15,6 +15,7 @@
     get_python_library,
 )
 from scikit_build_core.builder.wheel_tag import WheelTag
+from scikit_build_core.cmake import CMaker
 from scikit_build_core.settings.skbuild_model import ScikitBuildSettings, WheelSettings
 
 
@@ -97,20 +98,24 @@ def test_wheel_tag(monkeypatch, minver, archs, answer):
 
 
 def test_builder_macos_arch_extra(monkeypatch):
-    archflags = "-arch universal2"
+    archflags = "-arch arm64 -arch x86_64"
     monkeypatch.setattr(sys, "platform", "darwin")
     monkeypatch.setenv("ARCHFLAGS", archflags)
-    tmpcfg = SimpleNamespace(env=os.environ.copy())
-    tmpbuilder = typing.cast(
-        Builder,
-        SimpleNamespace(
-            config=tmpcfg,
-            settings=ScikitBuildSettings(
-                wheel=WheelSettings(expand_macos_universal_tags=True)
-            ),
+    tmpcfg = typing.cast(CMaker, SimpleNamespace(env=os.environ.copy()))
+
+    tmpbuilder = Builder(
+        settings=ScikitBuildSettings(wheel=WheelSettings()),
+        config=tmpcfg,
+    )
+    assert tmpbuilder.get_arch_tags() == ["universal2"]
+
+    tmpbuilder_ex = Builder(
+        settings=ScikitBuildSettings(
+            wheel=WheelSettings(expand_macos_universal_tags=True)
         ),
+        config=tmpcfg,
     )
-    assert Builder.get_archs(tmpbuilder) == ["universal2", "x86_64", "arm64"]
+    assert tmpbuilder_ex.get_arch_tags() == ["universal2", "x86_64", "arm64"]
 
 
 def test_wheel_tag_with_abi_darwin(monkeypatch):