Compiler is overly secretive when it fails to infer the type of parameters

[Sporarum]

Compiler version

Scala 3.3.0

Minimized code & Output

def foo(x: Int) = 0

foo(x => x) // error:
// Missing parameter type
// I could not infer the type of the parameter x.

https://scastie.scala-lang.org/Te6OUPEKTnmMn304qWsPew

Expectation

The fact the inference fails hides a more essential issue:

There is no type such that this succeeds.

(Except if there are implicit conversions, with the following, inferring Int would work:

given Conversion[Int => Int, Int] = f => f(42)

)

Even considering this point, a better error would be something like:

Found: T1 => T1
Expected: Int

Where T1 could not be inferred

It displays more information, naturally leading the user to realize x => x would not work in any case

[Sporarum]

I might tackle this myself, if not, it might be a good candidate for a Spree, as it leads through multiple parts of the compiler with a clear goal in mind ?
(At least for the first exploratory phase)

[Sporarum]

Thinking more about this, I believe this would be helpful even in cases where the type to be inferred is underdetermined (as opposed to overdetermined like in my example)

(Which is why I renamed this issue)

[Sporarum]

This also has the upside of making the solution much simpler: No need to wonder about this question of under- vs over-determination, just always be precise

[som-snytt]

This is a famous tpolecat request. I can't find a Scala 2 ticket, however. I have a memory of reading it recently, because it said that it happens more often than you might think.

[odersky]

This looks very hard, and it's not clear at all the wins are large enough to warrant any complication in the compiler's code base. For me, Missing parameter type is perfectly fine as an error.

[Sporarum]

There are actually two things being proposed, which are different in terms of scope:

1. Detect and warn when a type is over-contrained, and inform the user appropriately (Very hard, possibly impossible)
2. Add no checks and just include more information in the error message, similar to what is done with "an extension method was tried, but" (Hopefully easy)

From this moment on, this issue is about 2.
(I will not open another issue for 1. but anyone is free to do so)

[scala-center-bot]

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