Understanding type inference. Why do I have to specify the types explicitly ?

A.scala

scala> import scala.reflect._
import scala.reflect._

scala> def f[K: ClassTag, V: ClassTag, T<:Product2[K, V]](names: T, values: T) = {
    val x = values;
    println(classTag[K]);
    names.productIterator.toList.zip(x.productIterator.map(_.toString).toSeq).map(kv => kv._1+"="+kv._2)
}
     |      |      |      | f: [K, V, T <: Product2[K,V]](names: T, values: T)(implicit evidence$1: scala.reflect.ClassTag[K], implicit evidence$2: scala.reflect.ClassTag[V])List[String]

scala>  f(("one", "two"), ("1", "2")) // Why can't these types be inferred ?
<console>:13: error: inferred type arguments [Nothing,Nothing,(String, String)] do not conform to method f's type parameter bounds [K,V,T <: Product2[K,V]]
               f(("one", "two"), ("1", "2"))
               ^
<console>:13: error: type mismatch;
 found   : (String, String)
 required: T
               f(("one", "two"), ("1", "2"))
                 ^
<console>:13: error: type mismatch;
 found   : (String, String)
 required: T
               f(("one", "two"), ("1", "2"))
                                 ^

scala>  f[String, String](("one", "two"), ("1", "2"))
<console>:13: error: wrong number of type parameters for method f: [K, V, T <: Product2[K,V]](names: T, values: T)(implicit evidence$1: scala.reflect.ClassTag[K], implicit evidence$2: scala.reflect.ClassTag[V])List[String]
               f[String, String](("one", "two"), ("1", "2"))
                ^

scala>  f[String, String, (String, String)](("one", "two"), ("1", "2"))
java.lang.String
res16: List[String] = List(one=1, two=2)

scala>  f[String, Int, (String, String)](("one", "two"), ("1", "2"))
<console>:13: error: type arguments [String,Int,(String, String)] do not conform to method f's type parameter bounds [K,V,T <: Product2[K,V]]
               f[String, Int, (String, String)](("one", "two"), ("1", "2"))
                ^

scala> 

I have run into almost the same thing just yesterday: http://stackoverflow.com/questions/23275767/fixing-type-inference-for-higher-kinded-type

Apparently, your other type parameters K and V must be visible inside the argument list. Don't ask me why, but then you can make it work:

def g[K: ClassTag, V: ClassTag, T <: Product2[K, V]](names: T with Product2[K, V], 
  values: T with Product2[K, V]) = "ok"

g(("one", "two"), ("1", "2"))