We were doing a check in a metaclass to ensure that Package classes weren't defining a function themselves; this check failed (erroneously) if that Package class subclassed another package class (because the check examined all superclasses and thought the definition we automatically add was an offender) (#29569)

lib/spack/spack/package.py

@@ -288,7 +288,7 @@ def determine_variants(cls, exes, version_str):
             # This function should not be overridden by subclasses,
             # as it is not designed for bespoke pkg detection but rather
             # on a per-platform basis
-            if hasattr(cls, 'platform_executables'):
+            if 'platform_executables' in cls.__dict__.keys():
                 raise PackageError("Packages should not override platform_executables")
             cls.platform_executables = platform_executables