Type inference forgets to process some type parameters #6472

scabug · 2012-10-03T22:21:09Z

Given an environment where T is not bound to some type, I can get this error message:

  Foo.scala:21: error: type mismatch;
   found   : Foo.Exp[Seq[T]]
   required: Foo.Exp[Seq[Nothing]]
    assertType[Exp[Seq[Nothing]]](v)
                                  ^

where the T in the output message is an uninferred type parameter!

The core of the offending code is:

  val empty = Const(Seq.empty)
  //...Implicit conversions to pimp map and head on Const[Seq[T]]...
  val v = empty.map(x => empty.head)
  //Not fine:
  assertType[Exp[Seq[Nothing]]](v)

See https://gist.github.com/3830228 for the complete code (I can attach it here if important).

As a consequence, in 2.10 that expression cannot be passed to this function:

def f[T: TypeTag](e: Exp[T]): T = throw new Exception()
f(empty.map(x => empty.head))
/*
Error:
Foo.scala:38: error: type parameter not specified
  f(empty.map(x => empty.head))
   ^

scabug · 2012-10-03T22:21:09Z

Imported From: https://issues.scala-lang.org/browse/SI-6472?orig=1
Reporter: @Blaisorblade
Affected Versions: 2.9.2, 2.10.0-M7, 2.10.0

scabug · 2012-10-04T06:57:54Z

@paulp said:
Considerably reduced. The difference between g0 and g1 below is only whether the implicit conversion is called explicitly.

object Bar {
  implicit def convert[T](xs: Seq[T]) = new Bippy(xs)
  class Bippy[T](xs: Seq[T]) { def bippy = xs }

  def g0 = identity(Seq().bippy)
  def g1 = identity(convert(Seq()).bippy)
  // def g0: Seq[T] = ...
  // def g1: Seq[Nothing] = ...
}

Blaisorblade · 2018-03-03T00:00:43Z

@milessabin You might want to triage this; I suspect "considering more variables" might be more approachable than "changing the inferred type for variables that are already inferred".

milessabin · 2018-03-03T14:15:37Z

I think this is most likely a consequence of a Nothing doing double duty as "unsolved". I definitely want to take a look at that problem at some point.

scabug added typer infer labels Apr 7, 2017

scabug added this to the Backlog milestone Apr 7, 2017

scabug mentioned this issue Apr 7, 2017

Strange requirement for getting type parameter of implicit conversion target inferred #6776

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Type inference forgets to process some type parameters #6472

Type inference forgets to process some type parameters #6472

scabug commented Oct 3, 2012

scabug commented Oct 3, 2012

scabug commented Oct 4, 2012

Blaisorblade commented Mar 3, 2018

milessabin commented Mar 3, 2018 •

edited

Type inference forgets to process some type parameters #6472

Type inference forgets to process some type parameters #6472

Comments

scabug commented Oct 3, 2012

scabug commented Oct 3, 2012

scabug commented Oct 4, 2012

Blaisorblade commented Mar 3, 2018

milessabin commented Mar 3, 2018 • edited

milessabin commented Mar 3, 2018 •

edited