Merge pull request #10692 from som-snytt/issue/12935-no-warn-empty-ev…

…idence

Don't warn unused evidence of empty interface

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

@@ -762,15 +762,18 @@ trait TypeDiagnostics extends splain.SplainDiagnostics {
             opc.iterator.exists(_.low == m)
           }
         }
+        def isEmptyMarker(p: Symbol): Boolean = p.info.members.reverseIterator.forall(isUniversalMember(_)) // nonTrivialMembers(p).isEmpty
         def isConvention(p: Symbol): Boolean = (
             p.name.decoded == "args" && p.owner.isMethod && p.owner.name.decoded == "main"
           ||
             p.isImplicit && cond(p.tpe.typeSymbol) { case SameTypeClass | SubTypeClass | DummyImplicitClass => true }
         )
         def warningIsOnFor(s: Symbol) =
           if (!s.isImplicit) settings.warnUnusedExplicits
-          else if (!s.isSynthetic) settings.warnUnusedImplicits
-          else settings.warnUnusedSynthetics
+          else {
+            if (!s.isSynthetic) settings.warnUnusedImplicits
+            else settings.warnUnusedSynthetics
+          } && !isEmptyMarker(s)
         def warnable(s: Symbol) = (
           warningIsOnFor(s)
             && !isImplementation(s.owner)

test/files/neg/t12591.scala

@@ -1,5 +1,5 @@
 // scalac: -Werror -Wunused:synthetics
-trait Context[A]
+trait Context[A] { def m(a: A): A = a } // Context has a member, so warn if unused
 
 object Example {
   def g[A: Context] = f

test/files/neg/warn-unused-implicits.check

@@ -4,6 +4,12 @@ warn-unused-implicits.scala:13: warning: parameter s in method f is never used
 warn-unused-implicits.scala:33: warning: parameter s in method i is never used
   def i(implicit s: String, t: Int) = t           // yes, warn
                  ^
+warn-unused-implicits.scala:52: warning: parameter ev in method ==> is never used
+    def ==>[B](b: B)(implicit ev: BadCanEqual[A, B]): Boolean = a == b // warn, ev.run
+                              ^
+warn-unused-implicits.scala:60: warning: parameter m in method f is never used
+  def f[A](implicit m: MembersOnly[A]) = toString.nonEmpty  // warn implicit trait with private member
+                    ^
 error: No warnings can be incurred under -Werror.
-2 warnings
+4 warnings
 1 error

test/files/neg/warn-unused-implicits.scala

@@ -40,3 +40,22 @@ trait U { _: T =>
   override def f()(implicit i: Int): Int = g()  // no warn, required by baseclass, scala/bug#12876
   def g(): Int
 }
+
+trait CanEqual[A, B]
+trait BadCanEqual[A, B] extends Runnable
+
+class EqTest {
+  implicit class ArrowAssert[A](a: A) {
+    def ==>[B](b: B)(implicit ev: CanEqual[A, B]): Boolean = a == b // no warn, no non-trivial members
+  }
+  implicit class BadArrowAssert[A](a: A) {
+    def ==>[B](b: B)(implicit ev: BadCanEqual[A, B]): Boolean = a == b // warn, ev.run
+  }
+}
+
+trait MembersOnly[A] {
+  private def member() = 42 // don't care whether member is accessible; maybe they must pass it elsewhere
+}
+class Privately {
+  def f[A](implicit m: MembersOnly[A]) = toString.nonEmpty  // warn implicit trait with private member
+}

test/files/neg/warn-unused-params.check

@@ -43,6 +43,9 @@ warn-unused-params.scala:111: warning: parameter b in method f is never used
 warn-unused-params.scala:134: warning: parameter s in method i is never used
   def i(implicit s: String) = answer           // yes, warn
                  ^
+warn-unused-params.scala:142: warning: parameter s in method f is never used
+  def f(s: Serializable) = toString.nonEmpty  // warn explicit param of marker trait
+        ^
 error: No warnings can be incurred under -Werror.
-15 warnings
+16 warnings
 1 error

test/files/neg/warn-unused-params.scala

@@ -96,7 +96,7 @@ trait Anonymous {
 
   def g = for (i <- List(1)) yield answer    // warn map.(i => 42)
 }
-trait Context[A]
+trait Context[A] { def m(a: A): A = a }
 trait Implicits {
   def f[A](implicit ctx: Context[A]) = answer
   def g[A: Context] = answer
@@ -137,3 +137,7 @@ trait BadMix { _: InterFace =>
 class Unequal {
   override def equals(other: Any) = toString.nonEmpty   // no warn non-trivial RHS, required by universal method
 }
+
+class Seriously {
+  def f(s: Serializable) = toString.nonEmpty  // warn explicit param of marker trait
+}