Type errors should evaluate type matches with known arguments

[belamenso]

Compiler version

3.6.2

Minimized example

import scala.Tuple

class Container[Rs <: Tuple] {
  def guard(a: Tuple.Tail[Rs]): Unit = ()
}

@main def main(): Unit =
  Container[(1,2,3)].guard((5, 6, 8))

Output Error message

[error] -- [E007] Type Mismatch Error: Main.scala:8:27 
[error] 8 |  Container[(1,2,3)].guard((5, 6, 8))
[error]   |                           ^^^^^^^^^
[error]   |                   Found:    (Int, Int, Int)
[error]   |                   Required: Tuple.Tail[((1 : Int), (2 : Int), (3 : Int))]
[error]   |
[error]   | longer explanation available when compiling with `-explain`
[error] one error found

Why this Error/Warning was not helpful

The message was unhelpful because it contains unevaluated type matches (Tuple.Tail) that depend on entirely known types.

Suggested improvement

It could look like

[error] 8 |  Container[(1,2,3)].guard((5, 6, 8))
[error]   |                           ^^^^^^^^^
[error]   |                   Found:    (Int, Int, Int)
[error]   |                   Required: ((2 : Int), (3 : Int))

[belamenso]

@hamzaremmal thanks for quick action. I have another closely related bug, but I do not know if it is the same as this one or not. It could be the same cause (where it just does not evaluate the type in the error message) or it could be something else.

Single indirection makes implicitNotFound print the argument name not its value

Dotty version 3.6.2

type IntLike[T] <: Boolean = T match
  case Int => true
@annotation.implicitNotFound("Comparing ${T} to Int")
type IsIntLike[T] = IntLike[T] =:= true
type Indirection[A] = IsIntLike[A] & (true =:= true) // something after & is needed for reproduction
def f[Z](using Indirection[Z]): Unit = ()
@main def main(): Unit = f[Double]

This is supposed to fail to compile, the problem is in the error message. Current error:

[error] -- [E172] Type Error: Main.scala:7:34 
[error] 7 |@main def main(): Unit = f[Double]
[error]   |                                  ^
[error]   |                                  Comparing A to Int
[error] one error found

The custom error message prints A which is the parameter name from Indirection instead of its value Double.

[hamzaremmal]

@belamenso Can you please open another issue for this second issue.

[belamenso]

Ok, moved to #22348 .