diff --git a/src/main/java/com/android/tools/r8/naming/InterfaceMethodNameMinifier.java b/src/main/java/com/android/tools/r8/naming/InterfaceMethodNameMinifier.java
index 86dac13..a8bd96c 100644
--- a/src/main/java/com/android/tools/r8/naming/InterfaceMethodNameMinifier.java
+++ b/src/main/java/com/android/tools/r8/naming/InterfaceMethodNameMinifier.java
@@ -104,7 +104,7 @@
     // Used for iterating the sub trees that has this node as root.
     final Set<DexType> children = new HashSet<>();
     // Collection of the frontier reservation types and the interface type itself.
-    final Set<DexType> reservationTypes = new HashSet<>();
+    private final Set<DexType> reservationTypes = new HashSet<>();
 
     InterfaceReservationState(DexClass iface) {
       this.iface = iface;
@@ -139,6 +139,10 @@
       return isReserved == null ? null : method.getName();
     }
 
+    void addReservationType(DexType type) {
+      this.reservationTypes.add(type);
+    }
+
     void reserveName(DexString reservedName, DexEncodedMethod method) {
       forAll(
           s -> {
@@ -162,7 +166,7 @@
                 }
                 return null;
               });
-      return result == null ? true : result;
+      return result == null || result;
     }
 
     void addRenaming(DexString newName, DexEncodedMethod method) {
@@ -304,7 +308,7 @@
                 }
                 return null;
               });
-      return result == null ? true : result;
+      return result == null || result;
     }
 
     void addRenaming(DexString newName, MethodNameMinifier.State minifierState) {
@@ -398,7 +402,7 @@
       assert iface.isInterface();
       minifierState.allocateReservationStateAndReserve(iface.type, iface.type);
       InterfaceReservationState iFaceState = new InterfaceReservationState(iface);
-      iFaceState.reservationTypes.add(iface.type);
+      iFaceState.addReservationType(iface.type);
       interfaceStateMap.put(iface.type, iFaceState);
     }
   }
@@ -644,8 +648,12 @@
                   InterfaceReservationState iState = interfaceStateMap.get(directlyImplemented);
                   if (iState != null) {
                     DexType frontierType = minifierState.getFrontier(clazz.type);
-                    assert minifierState.getReservationState(frontierType) != null;
-                    iState.reservationTypes.add(frontierType);
+                    iState.addReservationType(frontierType);
+                    // The reservation state should already be added, but if a class is extending
+                    // an interface, we will not visit the class during the sub-type traversel
+                    if (minifierState.getReservationState(clazz.type) == null) {
+                      minifierState.allocateReservationStateAndReserve(clazz.type, frontierType);
+                    }
                   }
                 }
               }
diff --git a/src/main/java/com/android/tools/r8/naming/Minifier.java b/src/main/java/com/android/tools/r8/naming/Minifier.java
index 9893fb7..4bc638a 100644
--- a/src/main/java/com/android/tools/r8/naming/Minifier.java
+++ b/src/main/java/com/android/tools/r8/naming/Minifier.java
@@ -26,6 +26,7 @@
 import com.android.tools.r8.utils.SymbolGenerationUtils;
 import com.android.tools.r8.utils.SymbolGenerationUtils.MixedCasing;
 import com.android.tools.r8.utils.Timing;
+import java.util.Comparator;
 import java.util.HashSet;
 import java.util.List;
 import java.util.Set;
@@ -47,7 +48,7 @@
     assert appView.options().isMinifying();
     SubtypingInfo subtypingInfo = appView.appInfo().computeSubtypingInfo();
     timing.begin("ComputeInterfaces");
-    Set<DexClass> interfaces = new TreeSet<>((a, b) -> a.type.compareTo(b.type));
+    Set<DexClass> interfaces = new TreeSet<>(Comparator.comparing(a -> a.type));
     interfaces.addAll(appView.appInfo().computeReachableInterfaces());
     timing.end();
     timing.begin("MinifyClasses");
diff --git a/src/main/java/com/android/tools/r8/shaking/Enqueuer.java b/src/main/java/com/android/tools/r8/shaking/Enqueuer.java
index 8e65d5c..32cd619 100644
--- a/src/main/java/com/android/tools/r8/shaking/Enqueuer.java
+++ b/src/main/java/com/android/tools/r8/shaking/Enqueuer.java
@@ -649,6 +649,33 @@
     }
   }
 
+  private void warnIfClassExtendsInterfaceOrImplementsClass(DexProgramClass clazz) {
+    if (clazz.superType != null) {
+      DexClass superClass = definitionFor(clazz.superType);
+      if (superClass != null && superClass.isInterface()) {
+        options.reporter.warning(
+            new StringDiagnostic(
+                "Class "
+                    + clazz.toSourceString()
+                    + " extends "
+                    + superClass.toSourceString()
+                    + " which is an interface"));
+      }
+    }
+    for (DexType iface : clazz.interfaces.values) {
+      DexClass ifaceClass = definitionFor(iface);
+      if (ifaceClass != null && !ifaceClass.isInterface()) {
+        options.reporter.warning(
+            new StringDiagnostic(
+                "Class "
+                    + clazz.toSourceString()
+                    + " implements "
+                    + ifaceClass.toSourceString()
+                    + " which is not an interface"));
+      }
+    }
+  }
+
   private void enqueueRootItems(Map<DexReference, Set<ProguardKeepRuleBase>> items) {
     items.entrySet().forEach(this::enqueueRootItem);
   }
@@ -1681,6 +1708,9 @@
       markTypeAsLive(holder.superType, reason);
     }
 
+    // Warn if the class extends an interface or implements a class
+    warnIfLibraryExtendsInterfaceOrImplementsClass(holder);
+
     // If this is an interface that has just become live, then report previously seen but unreported
     // implemented-by edges.
     transitionUnusedInterfaceToLive(holder);
diff --git a/src/test/java/com/android/tools/r8/ExternalR8TestCompileResult.java b/src/test/java/com/android/tools/r8/ExternalR8TestCompileResult.java
index ea6b2f7..a7c0baf 100644
--- a/src/test/java/com/android/tools/r8/ExternalR8TestCompileResult.java
+++ b/src/test/java/com/android/tools/r8/ExternalR8TestCompileResult.java
@@ -5,7 +5,6 @@
 package com.android.tools.r8;
 
 import com.android.tools.r8.ToolHelper.ProcessResult;
-import com.android.tools.r8.errors.Unimplemented;
 import com.android.tools.r8.utils.AndroidApp;
 import com.android.tools.r8.utils.codeinspector.CodeInspector;
 import java.io.IOException;
@@ -41,14 +40,6 @@
     return proguardMap;
   }
 
-  public String stdout() {
-    return processResult.stdout;
-  }
-
-  public String stderr() {
-    return processResult.stdout;
-  }
-
   @Override
   public ExternalR8TestCompileResult self() {
     return this;
@@ -61,12 +52,12 @@
 
   @Override
   public String getStdout() {
-    throw new Unimplemented("Unexpected attempt to access stdout from external R8");
+    return processResult.stdout;
   }
 
   @Override
   public String getStderr() {
-    throw new Unimplemented("Unexpected attempt to access stderr from external R8");
+    return processResult.stderr;
   }
 
   @Override
diff --git a/src/test/java/com/android/tools/r8/cf/bootstrap/BootstrapCurrentEqualityTest.java b/src/test/java/com/android/tools/r8/cf/bootstrap/BootstrapCurrentEqualityTest.java
index 070d6c8..f2660ef 100644
--- a/src/test/java/com/android/tools/r8/cf/bootstrap/BootstrapCurrentEqualityTest.java
+++ b/src/test/java/com/android/tools/r8/cf/bootstrap/BootstrapCurrentEqualityTest.java
@@ -257,8 +257,8 @@
             .setMode(mode)
             .compile();
     // Check that the process outputs (exit code, stdout, stderr) are the same.
-    assertEquals(result.stdout(), runR8R8.stdout());
-    assertEquals(result.stderr(), runR8R8.stderr());
+    assertEquals(result.getStdout(), runR8R8.getStdout());
+    assertEquals(result.getStderr(), runR8R8.getStderr());
     // Check that the output jars are the same.
     assertProgramsEqual(result.outputJar(), runR8R8.outputJar());
   }
diff --git a/src/test/java/com/android/tools/r8/naming/b173184123/ClassExtendsInterfaceNamingTest.java b/src/test/java/com/android/tools/r8/naming/b173184123/ClassExtendsInterfaceNamingTest.java
index daf872a..fccc7b2 100644
--- a/src/test/java/com/android/tools/r8/naming/b173184123/ClassExtendsInterfaceNamingTest.java
+++ b/src/test/java/com/android/tools/r8/naming/b173184123/ClassExtendsInterfaceNamingTest.java
@@ -5,8 +5,6 @@
 package com.android.tools.r8.naming.b173184123;
 
 import static org.hamcrest.CoreMatchers.containsString;
-import static org.hamcrest.MatcherAssert.assertThat;
-import static org.junit.Assert.assertThrows;
 
 import com.android.tools.r8.NeverClassInline;
 import com.android.tools.r8.NeverInline;
@@ -52,28 +50,28 @@
                 .setSuper(DescriptorUtils.javaTypeToDescriptor(Interface.class.getTypeName()))
                 .transform())
         .build();
-    // TODO(b/173184123): Should not result in an error.
-    AssertionError assertionError =
-        assertThrows(
-            AssertionError.class,
-            () -> {
-              testForExternalR8(
-                      parameters.getBackend(),
-                      parameters.isCfRuntime()
-                          ? parameters.getRuntime()
-                          : TestRuntime.getCheckedInJdk11())
-                  .addProgramFiles(classFiles)
-                  .enableAssertions(false)
-                  .setMinApi(parameters.getApiLevel())
-                  .addKeepMainRule(Main.class)
-                  .addKeepClassAndMembersRules(Interface.class)
-                  .addKeepRules("-neverclassinline @com.android.tools.r8.NeverClassInline class *")
-                  .addKeepRules("-neverinline class * { @**.NeverInline *; }")
-                  .allowTestProguardOptions(true)
-                  .compile();
-            });
-    assertThat(
-        assertionError.getMessage(), containsString("Error: java.lang.NullPointerException"));
+    testForExternalR8(
+            parameters.getBackend(),
+            parameters.isCfRuntime() ? parameters.getRuntime() : TestRuntime.getCheckedInJdk11())
+        .addProgramFiles(classFiles)
+        .enableAssertions(false)
+        .useR8WithRelocatedDeps()
+        .setMinApi(parameters.getApiLevel())
+        .addKeepMainRule(Main.class)
+        .addKeepClassAndMembersRules(Interface.class)
+        .addKeepRules("-neverclassinline @com.android.tools.r8.NeverClassInline class *")
+        .addKeepRules("-neverinline class * { @**.NeverInline *; }")
+        .allowTestProguardOptions(true)
+        .compile()
+        .assertStderrThatMatches(
+            containsString(
+                "Class "
+                    + ConcreteClass.class.getTypeName()
+                    + " extends "
+                    + Interface.class.getTypeName()
+                    + " which is an interface"))
+        .run(parameters.getRuntime(), Main.class)
+        .assertFailureWithErrorThatThrows(IncompatibleClassChangeError.class);
   }
 
   public interface Interface {