Tentative fix for #2422 (to be verified)

Author: cowtowncoder

release-notes/VERSION-2.x

@@ -18,6 +18,8 @@ Project: jackson-databind
   to `handleUnknownVanilla()`
  (proposed by Vladimir T, follow up to #822)
 #2416: Optimize `ValueInstantiator` construction for default `Collection`, `Map` types
+#2422: `scala.collection.immutable.ListMap` fails to serialize since 2.9.3
+ (reported by dejanlokar1@github)
 #2425: Add global config override setting for `@JsonFormat.lenient()`
 
 2.10.0.pr1 (19-Jul-2019)

src/main/java/com/fasterxml/jackson/databind/type/TypeFactory.java

@@ -487,8 +487,11 @@ private String _resolveTypePlaceholders(JavaType sourceType, JavaType actualType
                 // 19-Apr-2018, tatu: Hack for [databind#1964] -- allow type demotion
                 //    for `java.util.Map` key type if (and only if) target type is
                 //    `java.lang.Object`
+                // 19-Aug-2019, tatu: Further, allow for all Map-like types, with assumption
+                //    first argument would be key; initially just because Scala Maps have
+                //    some issues (see [databind#2422])
                 if (i == 0) {
-                    if (sourceType.hasRawClass(Map.class)
+                    if (sourceType.isMapLikeType()
                             && act.hasRawClass(Object.class)) {
                         continue;
                     }