Type named `$` is replaced by empty string in error messages

[TomasMikula]

Compiler version

3.1.3-RC2

Minimized code

case class $[A](value: A)

def f(x: Int): Unit = {}

f(T(3))

Output

Found:   [Int]
Required: Int

Notice the missing $.

Expectation

Found:    $[Int]
Required: Int

[nicolasstucki]

Names with $ are reserved for compiler internal use by the language specification, their usage in user code has undefined behavior. Therefore this is not a bug.

[nicolasstucki]

We could still try to print the $ anyway

[Florian3k]

I've tried to debug this. Trailing $ are being removed by RefinedPrinter.nameString:

override def nameString(name: Name): String =
  def strippedName = if printDebug then name else name.stripModuleClassSuffix
  ...

I understand that this is desired behaviour, because we want do display compiler-generated names without trailing dollars. I don't see any easy way to distinguish between user provided names and those generated by compiler.

Potentially, we could add field to Name indicating whether it's coming from source code or not, but I suspect that this will require significant changes. Since using $ inside names is undefined behaviour, I don't think making such changes is desirable.

[TomasMikula]

Why then allow $ in source names at all? There should be some consistency.

[som-snytt]

stripModuleClassSuffix should not strip the bare $, a suffix which is also a prefix. Clearly an empty name is wrong.

$ is not forbidden because we all need to do some Java interop on occasion; also some Scala interop. One might refer to A$B in tooling.

[Florian3k]

This problem exists with any name ending with $, example:

case class T$[A](value: A)

def f(x: Int): Unit = {}

@main def main = f(T$(3))

Output:

5 |@main def main = f(T$(3))
  |                   ^^^^^
  |                   Found:    T[Int]
  |                   Required: Int
  |
  | longer explanation available when compiling with `-explain`

[som-snytt]

There are perverse usages of $, which must run afoul of internal conventions, and possibly reasonable usages, such as a regex library that wants to name something $. Or a monetary unit called $.

It might be useful to add a FAQ about these edges.