Missing Unused Expression Warning

[jamesward]

reproduction steps

using Scala 2.13.2,

val isAThing = {
  "blah" == "asdf" &&
  "foo" == "bar"
  "baz" == "baz"
}

With these compiler options:

  "-unchecked",
  "-deprecation",
  "-explaintypes",
  "-feature",
  "-Wconf:any:error",
  "-Wdead-code",
  "-Wunused",
  "-Wvalue-discard",

problem

The compiler does not catch that the first two expressions are unused.

[som-snytt]

This isn't under a flag. It doesn't do anything with Object#equals.

scala> def f = { 1 + 1 ; 2 }
                   ^
       warning: a pure expression does nothing in statement position; multiline expressions might require enclosing parentheses
def f: Int

scala> def f = { 1 == 1 ; 2 }
                   ^
       warning: a pure expression does nothing in statement position; multiline expressions might require enclosing parentheses
def f: Int

scala> def f = { "" == "" ; 2 }
def f: Int

Here, "pure" means "looks like a constant", IIRC. It could special case equals for String, or string equality for constants could be constant-folded, etc.

The heuristic could be improved, and maybe add a strict option that would be more like detecting value discard.

-Wunused only means a symbol that was introduced but not referenced. (For imports, a selector was never used to look up a symbol.)

[SethTisue]

shouldn't -Wvalue-discard be catching it though?

[som-snytt]

-Wvalue-discard warns that value discard conversion was applied. That's not in the middle of a block, in "statement position", but only at the end, in "result position", when Unit is expected.

The heuristic could be improved, and maybe add a strict option that would be more like detecting value discard.

[SethTisue]

PR scala/scala#9893

[som-snytt]

-Wnonunit-statement in the linked PR