Merge pull request #6852 from dotty-staging/fix-#6847

Fix #6847: Add regression test

Author: liufengyun

tests/pos/i6847.scala

@@ -0,0 +1,16 @@
+trait Syntax[F[_]] {
+  def (a: A) ret[A]: F[A]
+}
+
+trait Instance[A]
+
+implicit val instanceSyntax: Syntax[Instance] = new Syntax[Instance] {
+  def (a: A) ret[A]: Instance[A] = new Instance[A] {}
+}
+
+object Instance {
+  def defer[A](a: => A): Instance[A] = {
+    ().ret
+    new Instance[A] {}
+  }
+}