Confusing parsing error when defining a val named `_root_` with a type

[Linyxus]

Compiler version

main (and also in Scala 2)

Minimized code

def foo: Unit =
  val _root_: String = "abc"

Output

-- [E032] Syntax Error: issues/i17757.scala:2:21 --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
2 |  val _root_: String = "abc"
  |                     ^
  |                     pattern expected
  |
  | longer explanation available when compiling with `-explain`

Expectation

If you leave out the type, it will compile:

val _root_ = "abc"

So this one with a type annotation should compile as well?

Found together with @jan-pieter @nmcb when working on #18019

[odersky]

_root_ should not be used as a name of anything but a package. I guess we should enforce that. See #18050 for a related issue. Could be a good issue for a spree:

[scala-center-bot]

This issue was picked for the Issue Spree No. 33 of 11 July 2023 which takes place in 7 days. @dwijnand, @nmcb, @jan-pieter will be working on it. If you have any insight into the issue or guidance on how to fix it, please leave it here.

[dwijnand]

This isn't specific to _root_ or to colon, you get the same error with:

scala> val foo String = "abc"
-- [E032] Syntax Error: --------------------------------------------------------
1 |val foo String = "abc"
  |               ^
  |               pattern expected
  |
  | longer explanation available when compiling with `-explain`

But perhaps we can get the parser to realise that when it fails to find the infix op, too look if the first part (the LHS of the infix op), ends in a : and suggest a space.

Alternatively, it would be nice if val foo_: String = ... and ???: String just worked without the space...

[som-snytt]

Compare #16072