Throw a better message attempting to adapt a platform type.

moshi/src/main/java/com/squareup/moshi/ClassJsonAdapter.java

@@ -35,7 +35,14 @@ final class ClassJsonAdapter<T> extends JsonAdapter<T> {
     @Override public JsonAdapter<?> create(
         Type type, Set<? extends Annotation> annotations, Moshi moshi) {
       Class<?> rawType = Types.getRawType(type);
-      if (rawType.isInterface() || rawType.isEnum() || isPlatformType(rawType)) return null;
+      if (rawType.isInterface() || rawType.isEnum()) return null;
+      if (isPlatformType(rawType)) {
+        throw new IllegalArgumentException("Platform "
+            + type
+            + " annotated "
+            + annotations
+            + " requires explicit JsonAdapter to be registered");
+      }
       if (!annotations.isEmpty()) return null;
 
       if (rawType.getEnclosingClass() != null && !Modifier.isStatic(rawType.getModifiers())) {

moshi/src/test/java/android/util/Pair.java

@@ -0,0 +1,4 @@
+package android.util;
+
+public final class Pair {
+}

moshi/src/test/java/com/squareup/moshi/ClassJsonAdapterTest.java

@@ -21,8 +21,6 @@ import java.util.Arrays;
 import java.util.Comparator;
 import java.util.List;
 import java.util.SimpleTimeZone;
-import java.util.UUID;
-import javax.crypto.KeyGenerator;
 import okio.Buffer;
 import org.junit.Test;
 
@@ -329,11 +327,6 @@ public final class ClassJsonAdapterTest {
     }
   }
 
-  @Test public void platformClassNotSupported() throws Exception {
-    assertThat(ClassJsonAdapter.FACTORY.create(UUID.class, NO_ANNOTATIONS, moshi)).isNull();
-    assertThat(ClassJsonAdapter.FACTORY.create(KeyGenerator.class, NO_ANNOTATIONS, moshi)).isNull();
-  }
-
   @Test public void anonymousClassNotSupported() throws Exception {
     Comparator<Object> c = new Comparator<Object>() {
       @Override public int compare(Object a, Object b) {

moshi/src/test/java/com/squareup/moshi/MoshiTest.java

@@ -15,6 +15,8 @@
  */
 package com.squareup.moshi;
 
+import android.util.Pair;
+import java.io.File;
 import java.io.IOException;
 import java.lang.annotation.Annotation;
 import java.lang.annotation.Retention;
@@ -27,6 +29,7 @@ import java.util.LinkedHashSet;
 import java.util.List;
 import java.util.Locale;
 import java.util.Set;
+import javax.crypto.KeyGenerator;
 import org.junit.Test;
 
 import static com.squareup.moshi.TestUtil.newReader;
@@ -707,6 +710,31 @@ public final class MoshiTest {
     }
   }
 
+  @Test public void platformTypeThrows() throws IOException {
+    Moshi moshi = new Moshi.Builder().build();
+    try {
+      moshi.adapter(File.class);
+      fail();
+    } catch (IllegalArgumentException e) {
+      assertThat(e).hasMessage(
+          "Platform class java.io.File annotated [] requires explicit JsonAdapter to be registered");
+    }
+    try {
+      moshi.adapter(KeyGenerator.class);
+      fail();
+    } catch (IllegalArgumentException e) {
+      assertThat(e).hasMessage(
+          "Platform class javax.crypto.KeyGenerator annotated [] requires explicit JsonAdapter to be registered");
+    }
+    try {
+      moshi.adapter(Pair.class);
+      fail();
+    } catch (IllegalArgumentException e) {
+      assertThat(e).hasMessage(
+          "Platform class android.util.Pair annotated [] requires explicit JsonAdapter to be registered");
+    }
+  }
+
   static class Pizza {
     final int diameter;
     final boolean extraCheese;