by-name parameter does not work with function literal syntax

[scabug]

scala> val v = (headVal: Int, currentResultStream: => Int) => head
:1: error: identifier expected but '=>' found.
val v = (headVal: Int, currentResultStream: => Int) => head
^

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-9313?orig=1
Reporter: Saurabh Wadhawan (saurabh18e)
Affected Versions: 2.10.4

[scabug]

Saurabh Wadhawan (saurabh18e) said:
by-name parameter syntax should work with function literal syntax as well

[scabug]

@retronym said:
This is supported:

scala> val f: (Boolean, => Int) => Int = (b, i) => if (b) i else 0
f: (Boolean, => Int) => Int = <function2>

scala> f(false, ???)
res0: Int = 0

scala> f(true, ???)
scala.NotImplementedError: an implementation is missing
  at scala.Predef$.$qmark$qmark$qmark(Predef.scala:225)

[scabug]

Saurabh Wadhawan (saurabh18e) said:
val f1 = (i: Int) => 1
above works

but below doesn't work
val f2 = (i: => Int) => 1

the by-name parameter symbol should be specified in the definition on RHS rather than in the type on LHS

[scabug]

Saurabh Wadhawan (saurabh18e) said:
scala> val f2 = (i: => Int) => 1
:1: error: identifier expected but '=>' found.
val f2 = (i: => Int) => 1
^

scala> val f2: (=> Int) => Int = (i) => 1
f2: (=> Int) => Int =

[Jasper-M]

The syntax specification says:

Binding  ::=  (id | ‘_’) [‘:’ Type]

Is there a reason I'm not seeing why it can't be this?

Binding  ::=  (id | ‘_’) [‘:’ ParamType]

ParamType is a Type that can also be by-name (=> Type) or varargs (Type*). At first glance that would make the syntax more regular, but perhaps I'm missing something.

[Ichoran]

Just a note that this is still an issue; someone ran into this today and asked about it on gitter.

[SethTisue]

not fixed in Scala 3. this came up again on Gitter today. https://gitter.im/scala/scala?at=611c09487bc44d0a472893e0

scala> val x: (=> Int) => Int = (x: =>Int) => x                                                                         
1 |val x: (=> Int) => Int = (x: =>Int) => x
  |                             ^^
  |                             an identifier expected, but '=>' found

scala> val x: (=> Int) => Int = (x: (=>Int)) => x                                                                       
1 |val x: (=> Int) => Int = (x: (=>Int)) => x
  |                              ^^^^^
  |                           By-name parameter type => Int not allowed here.

scala> val x: (=> Int) => Int = (x: Int) => x                                                                           
1 |val x: (=> Int) => Int = (x: Int) => x
  |                         ^^^^^^^^^^^^^
  |                         Found:    Int => Int
  |                         Required: (=> Int) => Int

scala> val x: (=> Int) => Int = x => x
val x: (=> Int) => Int = Lambda$1335/1897050650@3a17acd4

that's Scala 3, but Scala 2.13.6 gives similar errors.