diff --git a/compiler/src/dotty/tools/dotc/typer/ErrorReporting.scala b/compiler/src/dotty/tools/dotc/typer/ErrorReporting.scala
index f83159c69b6d..8ff7e0c26697 100644
--- a/compiler/src/dotty/tools/dotc/typer/ErrorReporting.scala
+++ b/compiler/src/dotty/tools/dotc/typer/ErrorReporting.scala
@@ -167,6 +167,7 @@ object ErrorReporting {
      */
     def userDefinedErrorString(raw: String, paramNames: List[String], args: List[Type]): String = {
       def translate(name: String): Option[String] = {
+        assert(paramNames.length == args.length)
         val idx = paramNames.indexOf(name)
         if (idx >= 0) Some(quoteReplacement(ex"${args(idx)}")) else None
       }
diff --git a/compiler/src/dotty/tools/dotc/typer/Implicits.scala b/compiler/src/dotty/tools/dotc/typer/Implicits.scala
index ff83645aa476..329d5e97d2f3 100644
--- a/compiler/src/dotty/tools/dotc/typer/Implicits.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Implicits.scala
@@ -1217,7 +1217,7 @@ trait Implicits { self: Typer =>
           err.userDefinedErrorString(
             raw,
             pt.typeSymbol.typeParams.map(_.name.unexpandedName.toString),
-            pt.widenExpr.argInfos))
+            pt.widenExpr.dropDependentRefinement.argInfos))
 
         def hiddenImplicitsAddendum: String =
 
diff --git a/tests/neg/i6503.scala b/tests/neg/i6503.scala
new file mode 100644
index 000000000000..a670f2cee421
--- /dev/null
+++ b/tests/neg/i6503.scala
@@ -0,0 +1,37 @@
+trait MapImpl {
+  type Key
+  type Value
+  type Map
+  val lookup: Map => Key => Value
+}
+import scala.collection.immutable.HashMap
+
+class HashMapImpl[K, V] extends MapImpl {
+  type Key = K
+  type Value = V
+  type Map = HashMap[K, V]
+  val lookup: Map => Key => Value = m => k => m(k)
+}
+
+object Foo {
+  val Server0:
+    (mImpl: MapImpl) => mImpl.Map => mImpl.Key => mImpl.Value
+    = mImpl => mImpl.lookup
+  val Client:
+    (server: (mImpl: MapImpl { type Key = String; type Value = Int}) => mImpl.Map => String => Int) => Int =
+    server => server(HashMapImpl[String, Int])(HashMap())("test lookup key")
+  val Client1:
+    (server: (mImpl: MapImpl { type Key = String; type Value = Int}) => mImpl.Map => String => Int) => Int =
+    server => server(??? : (HashMapImpl[String, Int]))(???)("test lookup key")
+  val Client2:
+  (server: (mImpl: MapImpl { type Key = String; type Value = Int}) => mImpl.Map => String => Int) => Int =
+    server => server(???)(???)("test lookup key")
+  val Server1: (mImpl: MapImpl {type Key = String; type Value = Int}) => mImpl.Map => String => Int =
+    mImpl => Server0(mImpl)
+  implicitly[(mImpl: MapImpl {type Key = String; type Value = Int}) => mImpl.Map => String => Int <:< (mImpl: MapImpl) => mImpl.Map => mImpl.Key => mImpl.Value] // error
+  implicitly[(mImpl: MapImpl {type Key = String; type Value = Int}) => mImpl.Map => mImpl.Key => mImpl.Value <:< (mImpl: MapImpl) => mImpl.Map => mImpl.Key => mImpl.Value] // error
+
+  val Result1: Int = Client(Server0) // error
+  val Result2: Int = Client(Server1)
+  val Result3: Int = Client(mImpl => Server0(mImpl))
+}