"is not a class type" error for given for intersection type

[user753]

Compiler version

3.1.0

Minimized example

trait Foo:
  def foo: Int

trait Bar:
  def bar: Int  

given (Foo & Bar) with
  def foo = 1
  def bar = 2

Output

compilation error

Foo & Bar is not a class type

Expectation

I think something like

Intersection type cannot be used with given

would be clearer

[som-snytt]

Or, say, "Did you mean...?" and tell us what to write.

One imagines a session like the following.

As a footnote, error recovery on missing with is a bit wonky.

scala> def f(using x: AutoCloseable & Runnable) = try x.run() finally x.close()
def f(using x: AutoCloseable & Runnable): Unit

scala> f
-- Error:
1 |f
  | ^
  | no implicit argument of type AutoCloseable & Runnable was found for parameter x of method f

scala> given AutoCloseable & Runnable with { def close() = () }
-- Error:
1 |given AutoCloseable & Runnable with { def close() = () }
  |                    ^
  |                    'with' expected, but identifier found
-- Error:
1 |given AutoCloseable & Runnable with { def close() = () }
  |                                                        ^
  |                                                        '{' expected, but eof found

scala> given (AutoCloseable & Runnable) with { def close() = () }
-- Error:
1 |given (AutoCloseable & Runnable) with { def close() = () }
  |      ^^^^^^^^^^^^^^^^^^^^^^^^^^
  |      AutoCloseable & Runnable is not a class type

scala> given AutoCloseable with Runnable with { def close() = () ; def run() = () }
// defined object given_AutoCloseable_Runnable

scala> given AutoCloseable { def close() = () }
-- Error:
1 |given AutoCloseable { def close() = () }
  |                    ^
  |                    'with' expected, but '{' found
-- Error:
1 |given AutoCloseable { def close() = () }
  |                                        ^
  |                                        '{' expected, but eof found

[user753]

So It's possible to do this

given Foo with Bar with 
  def foo = 1
  def bar = 2

So I am not sure what is the problem with given Foo & Bar?

[som-snytt]

Probably because you have to construct an instance. A & B is not the same as A with B because of linearization, where order matters.

scala> trait T { def f: Int = 42 }
// defined trait T

scala> trait U { def f: Int = 27 }
// defined trait U

scala> class C extends T, U { override def f = super.f } ; C().f
// defined class C
val res0: Int = 27

scala> class C extends U, T { override def f = super.f } ; C().f
// defined class C
val res1: Int = 42

That reminds me, I don't remember where I noticed withless syntax.

[user753]

A & B is not the same as A with B because of linearization, where order matters.

Yes, but given Foo with Bar can be used for using Foo & Bar

given Foo with Bar with 
  def foo = 1
  def bar = 2

def fooBar(using fb: Foo & Bar) = fb.foo + fb.bar  

println(fooBar) // 3

[som-snytt]

I meant to say that & is commutative but with is not. Foo with Bar is both Foo & Bar and Bar & Foo, but not the same as Bar with Foo; that is my guess why with is required here.

I forgot to add that comma is not a synonym for with in the type of a given:

scala> given AutoCloseable, Runnable with { def close() = () ; def run() = () }
-- Error:
1 |given AutoCloseable, Runnable with { def close() = () ; def run() = () }
  |                   ^
  |                   'with' expected, but ',' found
-- Error:
1 |given AutoCloseable, Runnable with { def close() = () ; def run() = () }
  |                                                                        ^
  |                                                                        '{' expected, but eof found

While browsing the repo, I saw an old comment about disallowing with in anonymous classes, so you could only have one parent, IIRC. So I don't know what their intentions are.

[bishabosha]

The difference here is that intersection types can only be the types of return values, or parameters - a given <type> with ... is similar to a class declaration, so can not use an intersection type. If you would like an intersection type then you need an alias given : given <type> = ...

[nicolasstucki]

We sould probably add a -explain to NotClassType in messages.scala.

[scala-center-bot]

This issue was picked for the Issue Spree No. 42 of February 6th, 2024. @SethTisue, @spavikevik, @iusildra, @aherlihy will be working on it. If you have any insight into the issue or guidance on how to fix it, please leave it here.