schema: resolve cross-module prefixes in if-feature expressions

And I have to type stuff here just to make CI happy, even though the
subject oneliner above has all the required info. Oh well.

Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>
Signed-off-by: Jan Kundrát <jan.kundrat@cesnet.cz>

Author: jktjkt

libyang/schema.py

@@ -1097,8 +1097,25 @@ def _get_operands_parsed(self):
         }
 
         def get_feature(name):
-            for feature in self.module_features:
-                if feature.name() == name:
+            prefix, _, local_name = name.rpartition(":")
+            candidates = self.module_features
+            pmod = self.cdata.mod
+            if prefix and pmod and pmod.mod and prefix != c2str(pmod.mod.prefix):
+                imp = next(
+                    (
+                        i
+                        for i in ly_array_iter(pmod.imports)
+                        if c2str(i.prefix) == prefix
+                    ),
+                    None,
+                )
+                if imp is None:
+                    raise LibyangError(
+                        "Unknown prefix %r in if-feature %r" % (prefix, name)
+                    )
+                candidates = Module(self.context, imp.module).features()
+            for feature in candidates:
+                if feature.name() == local_name:
                     return feature.cdata
             raise LibyangError("No feature %s in module" % name)
 

tests/test_schema.py

@@ -261,6 +261,13 @@ def test_iffeature_str(self):
         iff = next(self.leaf.if_features())
         self.assertEqual(str(iff), "turbo-boost OR networking")
 
+    def test_iffeature_str_cross_module(self):
+        self.ctx.load_module("yolo-featdep", None, ["*"])
+        self.ctx.load_module("yolo-featuser")
+        leaf = next(self.ctx.find_path("/yolo-featuser:gated"))
+        iff = next(leaf.if_features())
+        self.assertEqual(str(iff), "remote-feature")
+
     def test_iffeature_dump(self):
         iff = next(self.leaf.if_features())
         self.assertEqual(

tests/yang/yolo/yolo-featdep.yang

@@ -0,0 +1,15 @@
+module yolo-featdep {
+  yang-version 1.1;
+  namespace "urn:yang:yolo:featdep";
+  prefix featdep;
+
+  revision 2026-09-23 {
+    description
+      "Initial version.";
+  }
+
+  feature remote-feature {
+    description
+      "Defined here so yolo-featuser can reference it by prefix.";
+  }
+}

tests/yang/yolo/yolo-featuser.yang

@@ -0,0 +1,25 @@
+module yolo-featuser {
+  yang-version 1.1;
+  namespace "urn:yang:yolo:featuser";
+  prefix featuser;
+
+  import yolo-featdep { prefix dep; }
+
+  revision 2026-09-23 {
+    description
+      "Initial version.";
+  }
+
+  feature local-feature {
+    description
+      "Unused by `gated` - only here so this module has a non-empty
+       feature list of its own.";
+  }
+
+  leaf gated {
+    type string;
+    if-feature "dep:remote-feature";
+    description
+      "Gated by another module's feature, referenced by prefix.";
+  }
+}