No unused value warning inside of `if` statement

[OndrejSpanel]

Compiler version

3.5.2, 3.6.2

Minimized code

object Main {
  def fun(): String = ""
  def main(args: Array[String]): Unit = {
    fun() // warning here
    if (true) {
      fun() // no warning
    }
  }
}

Compile with -Wnonunit-statement

Output

C:\Dev\Sandbox\src\main\scala\Main.scala:4:8
unused value of type String
    fun()

Expectation

The warning should be given both on lines 4 and 6, but the discarded value on line 6 is silently ignored.

[som-snytt]

I thought it was the other way around.

#20450

Oh that issue says -Wvalue-discard does warn at the line in question.

The two warnings are non "synonymous" because "value discard" is a discarded "expression", but "nonunit statement" checks for an expression in a syntactic statement.

In Scala 2, if (b) e is typechecked as if (b) e else (), which is a value that warns under "nonunit statement".

In Scala 3, e has expected type Unit, so it warns under value discard.

That much is "won't fix", but maybe the desired behavior is that the "nonunit statement" warning also enables "value discard", as it is the more expensive check and the user surely wants both.

[OndrejSpanel]

Thanks for the explanation. Truth is I was not aware of -Wvalue-discard existence, I did not pay much attention to documentation, I just saw -Wnonunit-statement and though this is what I want.

[Gedochao]

That much is "won't fix", but maybe the desired behavior is that the "nonunit statement" warning also enables "value discard", as it is the more expensive check and the user surely wants both.

Agreed, closing this as a won't fix.
We might want to consider unifying -Wvalue-discard with -Wnonunit-statement, but that's a separate thing.
Feel free to raise it as a separate issue.