improve error message for a type mismatch with right-associative method #4701

scabug · 2011-06-16T00:20:37Z

Given the following class that emulates a particular aspect of prepending to an HList:

scala> class A {
  def ::[T](x: T) : Option[T] = Some(x)
}
defined class A

scala> val a = new A
a: A = A@62d5b9c1

and usage:

scala> val b: Option[String] = 3 :: a
<console>:9: error: type mismatch;
 found   : Int(3)
 required: String
       val b: Option[String] = 3 :: a
                                 ^

scala> val b: Option[String] = a.::(3)
<console>:9: error: type mismatch;
 found   : Int(3)
 required: String
       val b: Option[String] = a.::(3)
                                    ^

The direct call syntax in the second case produces a more useful error message than the first case.

scabug · 2011-06-16T00:20:37Z

Imported From: https://issues.scala-lang.org/browse/SI-4701?orig=1
Reporter: @harrah
See #4876

scabug · 2011-08-06T00:57:05Z

@soc said:
Weird. In 2.10.0.r25445-b20110805020350 the "(3)" is missing in the first case:

scala> val b: Option[String] = 3 :: a
<console>:9: error: type mismatch;
 found   : Int
 required: String
       val b: Option[String] = 3 :: a
                                 ^

scala> val b: Option[String] = a.::(3)
<console>:9: error: type mismatch;
 found   : Int(3)
 required: String
       val b: Option[String] = a.::(3)
                                    ^

To preserve evaluation order, `x :: y` is rewritten not to `y.::(x)` but to `{ val x$1 = x; y.::(x$1) }`. But, the `Ident(x$1)` tree is the target of any type-mismatch errors issued later on, so it should steal the position of `x` for the error message. The important effect of this important change: ``` % diff /code/scala/test/files/neg/t4701-neg.log /code/scala/test/files/neg/t4701.check--- t4701-neg.log +++ t4701.check @@ -4,3 +4,3 @@ hasType[HL[String]](nnn :: HN) // type mismatch error should have position at `nnn` - ^ + ^ ``` Fixes scala/bug#4701

scabug added enhancement quickfix help wanted usability labels Apr 7, 2017

SethTisue removed quickfix labels Feb 5, 2018

hrhino mentioned this issue Jan 1, 2020

Preserve position of argument of right-associative method scala/scala#8618

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improve error message for a type mismatch with right-associative method #4701

improve error message for a type mismatch with right-associative method #4701

scabug commented Jun 16, 2011

scabug commented Jun 16, 2011

scabug commented Aug 6, 2011

improve error message for a type mismatch with right-associative method #4701

improve error message for a type mismatch with right-associative method #4701

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scabug commented Jun 16, 2011

scabug commented Jun 16, 2011

scabug commented Aug 6, 2011