fix: give an alias the region of its file

scripts/region.py

@@ -184,8 +184,12 @@ def build_region_index(profiles: dict) -> dict[str, dict]:
             # sees the union and is never dropped on ambiguity. The standalone
             # layout is a path of its own: GC/EUR/IPL.bin fell through to the
             # bare name and its five-region union, and was never reduced.
+            # An alias is another name the core reads for the same file:
+            # scph5552.bin, the name beetle_psx also opens for its European
+            # BIOS, stayed in a --region jp pack once scph5502.bin went.
+            aliases = {str(a) for a in f.get("aliases") or []}
             priority = f.get("priority")
-            for key in {path, f.get("standalone_path") or "", name} - {""}:
+            for key in {path, f.get("standalone_path") or "", name, *aliases} - {""}:
                 entry = index.setdefault(
                     key,
                     {"regions": set(), "has_untagged": False, "emulators": [],

tests/test_region.py

@@ -264,6 +264,18 @@ class TestStandaloneLayoutIsIndexed(unittest.TestCase):
         self.assertEqual(region.lookup_regions(index, "GC/EUR/IPL.bin", "IPL.bin"), {"europe"})
         self.assertIn("GC/EUR/IPL.bin", slot.build_slot_index(profiles))
 
+    def test_an_alias_carries_the_region_of_its_file(self):
+        """scph5552.bin, the name beetle_psx also opens for its European
+        BIOS, stayed in a --region jp pack once scph5502.bin went."""
+        index = region.build_region_index({"beetle_psx": {"files": [
+            {"name": "scph5502.bin", "aliases": ["scph5552.bin"],
+             "region": ["europe", "oceania"]},
+        ]}})
+        self.assertEqual(
+            region.lookup_regions(index, "scph5552.bin", "scph5552.bin"),
+            {"europe", "oceania"},
+        )
+
 
 class TestResolveRegionDrops(unittest.TestCase):
     def setUp(self):