Support covariant Map values, which are used by Kotlin

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

@@ -34,6 +34,7 @@ import javax.annotation.CheckReturnValue;
 import javax.annotation.Nullable;
 
 import static com.squareup.moshi.internal.Util.canonicalize;
+import static com.squareup.moshi.internal.Util.removeSubtypeWildcard;
 import static com.squareup.moshi.internal.Util.typeAnnotatedWithAnnotations;
 
 /**
@@ -112,7 +113,7 @@ public final class Moshi {
       throw new NullPointerException("annotations == null");
     }
 
-    type = canonicalize(type);
+    type = removeSubtypeWildcard(canonicalize(type));
 
     // If there's an equivalent adapter in the cache, we're done!
     Object cacheKey = cacheKey(type, annotations);
@@ -158,7 +159,7 @@ public final class Moshi {
       Set<? extends Annotation> annotations) {
     if (annotations == null) throw new NullPointerException("annotations == null");
 
-    type = canonicalize(type);
+    type = removeSubtypeWildcard(canonicalize(type));
 
     int skipPastIndex = factories.indexOf(skipPast);
     if (skipPastIndex == -1) {

moshi/src/main/java/com/squareup/moshi/internal/Util.java

@@ -139,6 +139,21 @@ public final class Util {
     }
   }
 
+  /**
+   * If type is a "? extends X" wildcard, returns X; otherwise returns type unchanged.
+   */
+  public static Type removeSubtypeWildcard(Type type) {
+    if (!(type instanceof WildcardType)) return type;
+
+    Type[] lowerBounds = ((WildcardType) type).getLowerBounds();
+    if (lowerBounds.length != 0) return type;
+
+    Type[] upperBounds = ((WildcardType) type).getUpperBounds();
+    if (upperBounds.length != 1) throw new IllegalArgumentException();
+
+    return upperBounds[0];
+  }
+
   public static Type resolve(Type context, Class<?> contextRawType, Type toResolve) {
     // This implementation is made a little more complicated in an attempt to avoid object-creation.
     while (true) {

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

@@ -86,6 +86,27 @@ public final class MapJsonAdapterTest {
     assertThat(jsonAdapter.fromJson(jsonReader)).isEqualTo(null);
   }
 
+  @Test public void covariantValue() throws Exception {
+    // Important for Kotlin maps, which are all Map<K, ? extends T>.
+    JsonAdapter<Map<String, Object>> jsonAdapter =
+        mapAdapter(String.class, Types.subtypeOf(Object.class));
+
+    Map<String, Object> map = new LinkedHashMap<>();
+    map.put("boolean", true);
+    map.put("float", 42.0);
+    map.put("String", "value");
+
+    String asJson = "{\"boolean\":true,\"float\":42.0,\"String\":\"value\"}";
+
+    Buffer buffer = new Buffer();
+    JsonWriter jsonWriter = JsonWriter.of(buffer);
+    jsonAdapter.toJson(jsonWriter, map);
+    assertThat(buffer.readUtf8()).isEqualTo(asJson);
+
+    JsonReader jsonReader = newReader(asJson);
+    assertThat(jsonAdapter.fromJson(jsonReader)).isEqualTo(map);
+  }
+
   @Test public void orderIsRetained() throws Exception {
     Map<String, Integer> map = new LinkedHashMap<>();
     map.put("c", 1);

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

@@ -537,15 +537,13 @@ public final class MoshiTest {
     assertThat(adapter.toJson(null)).isEqualTo("null");
   }
 
-  @Test public void upperBoundedWildcardsAreNotHandled() {
+  @Test public void upperBoundedWildcardsAreHandled() throws Exception {
     Moshi moshi = new Moshi.Builder().build();
-    try {
-      moshi.adapter(Types.subtypeOf(String.class));
-      fail();
-    } catch (IllegalArgumentException e) {
-      assertThat(e).hasMessage(
-          "No JsonAdapter for ? extends java.lang.String (with no annotations)");
-    }
+    JsonAdapter<Object> adapter = moshi.adapter(Types.subtypeOf(String.class));
+    assertThat(adapter.fromJson("\"a\"")).isEqualTo("a");
+    assertThat(adapter.toJson("b")).isEqualTo("\"b\"");
+    assertThat(adapter.fromJson("null")).isEqualTo(null);
+    assertThat(adapter.toJson(null)).isEqualTo("null");
   }
 
   @Test public void lowerBoundedWildcardsAreNotHandled() {