Fix #5978: Add regression test

Author: odersky

tests/pos/i5978.scala

@@ -0,0 +1,30 @@
+import scala.language.implicitConversions
+
+opaque type Position[Buffer] = Int
+
+trait TokenParser[Token, R]
+
+object TextParser {
+  implied TP for TokenParser[Char, Position[CharSequence]] {}
+
+  implied FromCharToken
+    given (T: TokenParser[Char, Position[CharSequence]]) for Conversion[Char, Position[CharSequence]] = ???
+}
+
+
+object Testcase {
+  def main(args: Array[String]): Unit = {
+    import TextParser._
+    import implied TextParser._
+
+    val tp_v: TokenParser[Char, Position[CharSequence]] = TextParser.TP
+    val tp_i = the[TokenParser[Char, Position[CharSequence]]]
+    val co_i = the[Conversion[Char, Position[CharSequence]]]
+    val co_x : Position[CharSequence] = 'x'
+
+    {
+      implied XXX for Conversion[Char, Position[CharSequence]] = co_i
+      val co_y : Position[CharSequence] = 'x'
+    }
+  }
+}