Make sure the solver sees the correct subdir

conda_build/environ.py

@@ -888,7 +888,8 @@ def get_install_actions(
         with utils.LoggingContext(conda_log_level):
             with capture():
                 try:
-                    precs = _install_actions(prefix, index, specs)["LINK"]
+                    _actions = _install_actions(prefix, index, specs, subdir=subdir)
+                    precs = _actions["LINK"]
                 except (NoPackagesFoundError, UnsatisfiableError) as exc:
                     raise DependencyNeedsBuildingError(exc, subdir=subdir)
                 except (
@@ -1256,14 +1257,19 @@ def install_actions(
     prefix: str | os.PathLike | Path,
     index,
     specs: Iterable[str | MatchSpec],
+    subdir: str | None = None,
 ) -> InstallActionsType:
     # This is copied over from https://github.com/conda/conda/blob/23.11.0/conda/plan.py#L471
     # but reduced to only the functionality actually used within conda-build.
+    subdir_kwargs = {}
+    if subdir not in (None, "", "noarch"):
+        subdir_kwargs["CONDA_SUBDIR"] = subdir
 
     with env_vars(
         {
             "CONDA_ALLOW_NON_CHANNEL_URLS": "true",
             "CONDA_SOLVER_IGNORE_TIMESTAMPS": "false",
+            **subdir_kwargs,
         },
         callback=reset_context,
     ):

news/5350-subdir-cross

@@ -0,0 +1,19 @@
+### Enhancements
+
+* <news item>
+
+### Bug fixes
+
+* Ensure cross-building recipes select the correct noarch package variants. (#5341 via #5350)
+
+### Deprecations
+
+* <news item>
+
+### Docs
+
+* <news item>
+
+### Other
+
+* <news item>

tests/test-recipes/metadata/_cross_unix_windows_mingw/conda_build_config.yaml

@@ -0,0 +1,2 @@
+target_platform:
+  - win-64

tests/test-recipes/metadata/_cross_unix_windows_mingw/meta.yaml

@@ -0,0 +1,18 @@
+package:
+  name: foo
+  version: 0.0.1
+
+build:
+  number: 0
+  script:
+    - echo 'hello'
+    - ls $PREFIX
+    # this is the unix layout.
+    - test ! -d $PREFIX/x86_64-w64-mingw32
+    - test -d $PREFIX/Library
+
+requirements:
+  build:
+  host:
+    - m2w64-sysroot_win-64
+  run:

tests/test_api_build.py

@@ -1971,6 +1971,15 @@ def test_add_pip_as_python_dependency_from_condarc_file(
             api.build(testing_metadata)
 
 
+@pytest.mark.skipif(on_win, reason="Tests cross-compilation targeting Windows")
+def test_cross_unix_windows_mingw(testing_config):
+    recipe = os.path.join(metadata_dir, "_cross_unix_windows_mingw")
+    testing_config.channel_urls = [
+        "conda-forge",
+    ]
+    api.build(recipe, config=testing_config)
+
+
 @pytest.mark.parametrize(
     "recipe", sorted(Path(metadata_dir, "_build_script_errors").glob("*"))
 )