[ApiModel] Ensure no merging of methods with UNKNOWN api-level

Fixes: b/268605608
Change-Id: Ibf8b8385f3ddd46a70811b3d5e476a7177af46a8
diff --git a/src/main/java/com/android/tools/r8/horizontalclassmerging/policies/NoDifferentApiReferenceLevel.java b/src/main/java/com/android/tools/r8/horizontalclassmerging/policies/NoDifferentApiReferenceLevel.java
index a021a28..0024b64 100644
--- a/src/main/java/com/android/tools/r8/horizontalclassmerging/policies/NoDifferentApiReferenceLevel.java
+++ b/src/main/java/com/android/tools/r8/horizontalclassmerging/policies/NoDifferentApiReferenceLevel.java
@@ -38,6 +38,11 @@
   @Override
   public ComputedApiLevel getMergeKey(DexProgramClass clazz) {
     assert enableApiCallerIdentification;
-    return getApiReferenceLevelForMerging(appView, apiLevelCompute, clazz);
+    ComputedApiLevel apiReferenceLevelForMerging =
+        getApiReferenceLevelForMerging(appView, apiLevelCompute, clazz);
+    if (apiReferenceLevelForMerging.isUnknownApiLevel()) {
+      return ineligibleForClassMerging();
+    }
+    return apiReferenceLevelForMerging;
   }
 }
diff --git a/src/test/java/com/android/tools/r8/apimodel/ApiModelNoUnknownMergeTest.java b/src/test/java/com/android/tools/r8/apimodel/ApiModelNoUnknownMergeTest.java
index 23d7a6d..67082dc 100644
--- a/src/test/java/com/android/tools/r8/apimodel/ApiModelNoUnknownMergeTest.java
+++ b/src/test/java/com/android/tools/r8/apimodel/ApiModelNoUnknownMergeTest.java
@@ -13,6 +13,7 @@
 import com.android.tools.r8.TestParametersCollection;
 import com.android.tools.r8.references.ClassReference;
 import com.android.tools.r8.references.Reference;
+import com.android.tools.r8.utils.codeinspector.HorizontallyMergedClassesInspector;
 import java.util.HashSet;
 import java.util.Set;
 import org.junit.Test;
@@ -57,9 +58,7 @@
                   };
             })
         .addHorizontallyMergedClassesInspector(
-            inspector ->
-                inspector.assertClassesMerged(
-                    LibraryClassFooCaller.class, LibraryClassBarCaller.class))
+            HorizontallyMergedClassesInspector::assertNoClassesMerged)
         .compile()
         .addBootClasspathClasses(LibraryClass.class)
         .run(parameters.getRuntime(), Main.class)