Regression in parser for refined givens

[WojciechMazur]

Regression found during work on fix for other parser regression in #21957

Compiler version

Last good release: 3.5.0-RC1-bin-20240506-1cdf99f-NIGHTLY
First bad release: 3.5.0-RC1-bin-20240508-b10d64e-NIGHTLY

Bisect points it could have been introduced by either ea3c688 or 5189e68

Minimized code

trait Foo[T]:
  def foo(v: T): Unit

given myFooOfUnit: Foo[Unit]{ type X = Int } = new Foo[Unit] {
    type X = Int
    def foo(v: Unit): Unit = ???
  }

@main def Test = 
  summon[Foo[Unit]{type X = Int}]

Output

[error] ./tmp.scala:5:29
[error] 'with' expected, but '{' found
[error] given myFooOfUnit: Foo[Unit]{ type X = Int } = new Foo[Unit] {
[error]                             ^
[error] ./tmp.scala:11:34
[error] '}' expected, but eof found
[error]   summon[Foo[Unit]{type X = Int}]
[error]                     

Expectation

Should compile

[Gedochao]

cc @odersky @KacperFKorban

[KacperFKorban]

This is a similar issue to what I discussed with @EugeneFlesselle recently (but I haven't filed a discussion for it yet).
In the following code snippet, the { type X = String } part is considered the implementation of the given, not the refinement.

trait Bar:
  type Self
  type X

given w: Int is Bar { type X = String }

But this is probably impossible to disambiguate when parsing in this example.

[odersky]

The syntax always specified that in an alias given the implemented type must be an AnnotType.

For instance, in the LTS syntax.md at https://github.com/scala/scala3/blob/lts-3.3/docs/_docs/reference/syntax.md:

GivenDef          ::=  [GivenSig] (AnnotType [‘=’ Expr] | StructuralInstance)

So I believe this was a mistake that got fixed in 3.5. The workaround is simple: Put the refined type in parens.

given myFooOfUnit: (Foo[Unit]{ type X = Int }) = ...

I think we can leave this as is.