Lint if args adapted to Unit and discarded

src/compiler/scala/tools/nsc/settings/Warnings.scala

@@ -213,6 +213,7 @@ trait Warnings {
     val MultiargInfix          = LintWarning("multiarg-infix",            "Infix operator was defined or used with multiarg operand.")
     val ImplicitRecursion      = LintWarning("implicit-recursion",        "Implicit resolves to an enclosing definition.")
     val UniversalMethods       = LintWarning("universal-methods",         "Require arg to is/asInstanceOf.")
+    val ArgDiscard             = LintWarning("arg-discard",               "-Wvalue-discard for adapted arguments.")
 
     def allLintWarnings = values.toSeq.asInstanceOf[Seq[LintWarning]]
   }
@@ -246,6 +247,7 @@ trait Warnings {
   def multiargInfix              = lint contains MultiargInfix
   def lintImplicitRecursion      = lint.contains(ImplicitRecursion) || (warnSelfImplicit.value: @nowarn("cat=deprecation"))
   def lintUniversalMethods       = lint.contains(UniversalMethods)
+  def lintArgDiscard             = lint.contains(ArgDiscard)
 
   // The Xlint warning group.
   val lint = MultiChoiceSetting(

src/compiler/scala/tools/nsc/typechecker/Adaptations.scala

@@ -77,20 +77,28 @@ trait Adaptations {
         )
       }
       @inline def msg(what: String): String = s"adaptation of an empty argument list by inserting () $what"
-      @inline def noAdaptation = {
+      @inline def noAdaptation: false = {
         context.error(t.pos, adaptWarningMessage(msg("has been removed"), showAdaptation = false))
         false // drop adaptation
       }
-      @inline def deprecatedAdaptation = {
+      @inline def deprecatedAdaptation: true = {
         val twist =
           if (isLeakyTarget) "leaky (Object-receiving) target makes this especially dangerous"
           else "this is unlikely to be what you want"
         val text = s"${msg("is deprecated")}: ${twist}"
         context.deprecationWarning(t.pos, t.symbol, adaptWarningMessage(text), "2.11.0")
         true // keep adaptation
       }
-      @inline def warnAdaptation = {
-        if (settings.warnAdaptedArgs && !isInfix) context.warning(t.pos, adaptWarningMessage(
+      @inline def warnAdaptation: true = {
+        def discardedArgs = t match {
+          case Apply(_, stat @ Block(Apply(TypeApply(Select(adapter, _), _), adapted) :: Nil, expr) :: Nil) =>
+            isTupleSymbol(adapter.symbol.companion) && expr.tpe == UnitTpe && adapted == args
+          case _ => false
+        }
+        if (settings.lintArgDiscard && discardedArgs) context.warning(t.pos, adaptWarningMessage(
+          s"adapted the argument list to expected Unit type: arguments will be discarded"),
+          WarningCategory.LintAdaptedArgs)
+        else if (settings.warnAdaptedArgs && !isInfix) context.warning(t.pos, adaptWarningMessage(
           s"adapted the argument list to the expected ${args.size}-tuple: add additional parens instead"),
           WarningCategory.LintAdaptedArgs)
         true // keep adaptation

test/files/neg/t12495.check

@@ -0,0 +1,12 @@
+t12495.scala:4: warning: discarded non-Unit value of type (Int, Int)
+  def g = f(42, 27)
+           ^
+t12495.scala:4: warning: adapted the argument list to expected Unit type: arguments will be discarded
+        signature: Function1.apply(v1: T1): R
+  given arguments: 42, 27
+ after adaptation: Function1((42, 27): Unit)
+  def g = f(42, 27)
+           ^
+error: No warnings can be incurred under -Werror.
+2 warnings
+1 error

test/files/neg/t12495.scala

@@ -0,0 +1,5 @@
+// scalac: -Werror -Xlint:arg-discard,adapted-args -Wvalue-discard
+class C {
+  val f = (u: Unit) => println(s"[$u]")
+  def g = f(42, 27)
+}