Strange error message for failing an `Ordered[T]` type bound

[spamegg1]

Compiler version

3.1.0

Minimized code

def f[T <: Ordered[T]](t: T): T = t
f(1)

Output

Found:    (1 : Int)
Required: Ordered[
  Ordered[
    Ordered[
      Ordered[
        Ordered[
          Ordered[
            Ordered[
              Ordered[
                Ordered[
                  Ordered[Ordered[Ordered[Ordered[Ordered[Ordered[...[...]]]]]]]
                ]
              ]
            ]
          ]
        ]
      ]
    ]
  ]
]

One of the following imports might fix the problem:

  import math.BigDecimal.int2bigDecimal
  import math.BigInt.int2bigInt

Expectation

// this is also what Scala 2 gives:
inferred type arguments [Int] do not conform to method f's type parameter bounds [T <: Ordered[T]]

Info

Hello all, I accidentally came across this while reading Chapter 18 "Type Parameterization" of "Programming in Scala, 5th Edition". At the end of the chapter there is an example of

def orderedMergeSort[T <: Ordered[T]](xs: List[T]): List[T] = ...

then the book demonstrates why this cannot be used to sort a list of integers because Int is not a subtype of Ordered[Int]:

// this is what the book says should happen (also what Scala 2 gives):
scala> val wontCompile = orderedMergeSort(List(3, 2, 1))
<console>:5: error: inferred type arguments [Int] do
   not conform to method orderedMergeSort's type
     parameter bounds [T <: Ordered[T]]
       val wontCompile = orderedMergeSort(List(3, 2, 1))
                         ^

I thought the error message looked really strange, and obviously different from the book. So I minimized it.

Providing a type parameter gives a much better message:

scala> f[Int](1)
-- Error:
1 |f[Int](1)
  |  ^
  |  Type argument Int does not conform to upper bound Ordered[Int]

Also doing one of the mentioned imports fixes the problem too.

Wasn't sure if it is a bug. I asked in Discussions, @SethTisue encouraged me to ask in Scala Users, where @smarter encouraged me to report it.

[nicolasstucki]

Scala 2 shows this error

On line 2: error: inferred type arguments [Int] do not conform to method f's type parameter bounds [T <: Ordered[T]]
       f(1)
         ^
On line 2: error: type mismatch;
        found   : Int(1)
        required: T

[spamegg1]

Noticed a similar issue with other traits with type parameters as well:

 ➜ scala
Welcome to Scala 3.1.0 (11.0.13, Java OpenJDK 64-Bit Server VM).
Type in expressions for evaluation. Or try :help.
                                                                                                                                       
scala> trait Spam[T]
// defined trait Spam
                                                                                                                                       
scala> def f[T <: Spam[T]](t: T): T = t
def f[T <: Spam[T]](t: T): T
                                                                                                                                       
scala> f('a')
-- Error:
1 |f('a')
  |  ^^^
  |  Found:    ('a' : Char)
  |  Required: Spam[Spam[Spam[Spam[Spam[Spam[Spam[Spam[Spam[Spam[Spam[Spam[Spam[Spam[Spam[...[...]]]]]]]]]]]]]]]]
  |
  |  The following import might make progress towards fixing the problem:
  |
  |    import collection.Searching.search
  |