Regression test for 19417

Author: odersky

tests/pos/i19417/defs_1.scala

@@ -0,0 +1,5 @@
+trait QueryParamDecoder[E]:
+  def emap[T](fn: E => Either[Throwable, T]): QueryParamDecoder[T]
+object QueryParamDecoder:
+  def apply[T](implicit ev: QueryParamDecoder[T]): QueryParamDecoder[T] = ev
+  implicit lazy val stringQueryParamDecoder: QueryParamDecoder[String] = ???

tests/pos/i19417/usage_2.scala

@@ -0,0 +1,2 @@
+given[E](using e: EnumOf[E]): QueryParamDecoder[E] = QueryParamDecoder[String].emap(_ => Right(???))
+trait EnumOf[E]