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Description
In the following code, the 2nd call to f at the end fails to find the implicit value of type GetResult[(String, String)]. The version with the explicit getTuple call succeeds.
object TupledEvidenceTest {
abstract class TupledEvidence[M[_], T] { type T }
implicit def witnessTuple2[M[_], T1, T2](implicit ev1: M[T1], ev2: M[T2]):
TupledEvidence[M, (T1, T2)] { type T = (T1, T2) } = sys.error("")
class GetResult[T]
implicit val getString = new GetResult[String]
implicit def getTuple[T](implicit w: TupledEvidence[GetResult, T]): GetResult[w.T] = sys.error("")
def f[T : GetResult] = ""
f[(String,String)](getTuple[(String, String)])
f[(String,String)]
}Both calls work if you use the following version of getTuple without the path-dependent return type.
implicit def getTuple[T](implicit w: TupledEvidence[GetResult, T]): GetResult[T] = sys.error("")