Disable WUnused for params of non-private defs

compiler/src/dotty/tools/dotc/transform/CheckUnused.scala

@@ -117,6 +117,10 @@ class CheckUnused extends MiniPhase:
 
   override def prepareForDefDef(tree: tpd.DefDef)(using Context): Context =
     unusedDataApply{ ud =>
+      if !tree.rawMods.is(Private) then
+        tree.termParamss.flatten.foreach { p =>
+          ud.addIgnoredParam(p.symbol)
+        }
       import ud.registerTrivial
       tree.registerTrivial
       traverseAnnotations(tree.symbol)
@@ -334,6 +338,8 @@ object CheckUnused:
     /** Trivial definitions, avoid registering params */
     private val trivialDefs = MutSet[Symbol]()
 
+    private val paramsToSkip = MutSet[Symbol]()
+
     /**
      * Push a new Scope of the given type, executes the given Unit and
      * pop it back to the original type.
@@ -369,6 +375,8 @@ object CheckUnused:
     def removeIgnoredUsage(sym: Symbol)(using Context): Unit =
       doNotRegister --= sym.everySymbol
 
+    def addIgnoredParam(sym: Symbol)(using Context): Unit =
+      paramsToSkip += sym
 
     /** Register an import */
     def registerImport(imp: tpd.Import)(using Context): Unit =
@@ -383,8 +391,9 @@ object CheckUnused:
       if memDef.isValidMemberDef then
         if memDef.isValidParam then
           if memDef.symbol.isOneOf(GivenOrImplicit) then
-            implicitParamInScope += memDef
-          else
+            if !paramsToSkip.contains(memDef.symbol) then
+              implicitParamInScope += memDef
+          else if !paramsToSkip.contains(memDef.symbol) then
             explicitParamInScope += memDef
         else if currScopeType.top == ScopeType.Local then
           localDefInScope += memDef

tests/neg-custom-args/fatal-warnings/i15503-scala2/scala2-t11681.scala

@@ -9,7 +9,7 @@ trait InterFace {
 }
 
 trait BadAPI extends InterFace {
-  def f(a: Int,
+  private def f(a: Int,
         b: String,               // error
         c: Double): Int = {
     println(c)
@@ -33,7 +33,7 @@ trait BadAPI extends InterFace {
 
   override def equals(other: Any): Boolean = true  // OK
 
-  def i(implicit s: String) = answer           // error
+  def i(implicit s: String) = answer           // ok
 
   /*
   def future(x: Int): Int = {
@@ -63,7 +63,7 @@ case class CaseyKasem(k: Int)        // OK
 case class CaseyAtTheBat(k: Int)(s: String)        // ok
 
 trait Ignorance {
-  def f(readResolve: Int) = answer           // error
+  def f(readResolve: Int) = answer           // ok
 }
 
 class Reusing(u: Int) extends Unusing(u)   // OK
@@ -78,28 +78,28 @@ trait Unimplementation {
 }
 
 trait DumbStuff {
-  def f(implicit dummy: DummyImplicit) = answer // todo // error
-  def g(dummy: DummyImplicit) = answer // error
+  def f(implicit dummy: DummyImplicit) = answer // ok
+  def g(dummy: DummyImplicit) = answer // ok
 }
 trait Proofs {
-  def f[A, B](implicit ev: A =:= B) = answer // todo // error
-  def g[A, B](implicit ev: A <:< B) = answer // todo // error
-  def f2[A, B](ev: A =:= B) = answer // error
-  def g2[A, B](ev: A <:< B) = answer // error
+  def f[A, B](implicit ev: A =:= B) = answer // ok
+  def g[A, B](implicit ev: A <:< B) = answer // ok
+  def f2[A, B](ev: A =:= B) = answer // ok
+  def g2[A, B](ev: A <:< B) = answer // ok
 }
 
 trait Anonymous {
-  def f = (i: Int) => answer      // error
+  def f = (i: Int) => answer      // ok
 
   def f1 = (_: Int) => answer     // OK
 
   def f2: Int => Int = _ + 1  // OK
 
-  def g = for (i <- List(1)) yield answer    // error
+  def g = for (i <- List(1)) yield answer    // ok
 }
 trait Context[A]
 trait Implicits {
-  def f[A](implicit ctx: Context[A]) = answer // error
+  def f[A](implicit ctx: Context[A]) = answer // ok
   def g[A: Context] = answer // OK
 }
 class Bound[A: Context] // OK

tests/neg-custom-args/fatal-warnings/i15503e.scala

@@ -1,14 +1,16 @@
 // scalac: -Wunused:explicits
 
-/* This goes around the "trivial method" detection */
-val default_val = 1
+object Foo {
+  /* This goes around the "trivial method" detection */
+  val default_val = 1
 
-def f1(a: Int) = a // OK
-def f2(a: Int) = default_val // error
-def f3(a: Int)(using Int) = a // OK
-def f4(a: Int)(using Int) = default_val // error
-def f6(a: Int)(using Int) = summon[Int] // error
-def f7(a: Int)(using Int) = summon[Int] + a // OK
+  private def f1(a: Int) = a // OK
+  private def f2(a: Int) = default_val // error
+  private def f3(a: Int)(using Int) = a // OK
+  private def f4(a: Int)(using Int) = default_val // error
+  private def f6(a: Int)(using Int) = summon[Int] // error
+  private def f7(a: Int)(using Int) = summon[Int] + a // OK
+}
 
 package scala2main.unused.args:
   object happyBirthday {

tests/neg-custom-args/fatal-warnings/i15503f.scala

@@ -3,11 +3,12 @@
 /* This goes around the "trivial method" detection */
 val default_int = 1
 
-def f1(a: Int) = a // OK
-def f2(a: Int) = 1 // OK
-def f3(a: Int)(using Int) = a // OK
-def f4(a: Int)(using Int) = default_int // OK
-def f6(a: Int)(using Int) = summon[Int] // OK
-def f7(a: Int)(using Int) = summon[Int] + a // OK
-def f8(a: Int)(using foo: Int) = a // error
-
+object Xd {
+  private def f1(a: Int) = a // OK
+  private def f2(a: Int) = 1 // OK
+  private def f3(a: Int)(using Int) = a // OK
+  private def f4(a: Int)(using Int) = default_int // OK
+  private def f6(a: Int)(using Int) = summon[Int] // OK
+  private def f7(a: Int)(using Int) = summon[Int] + a // OK
+  private def f8(a: Int)(using foo: Int) = a // error
+}

tests/neg-custom-args/fatal-warnings/i15503g.scala

@@ -1,18 +1,19 @@
 // scalac: -Wunused:params
 
 /* This goes around the "trivial method" detection */
-val default_int = 1
+object Foo {
+  val default_int = 1
 
-def f1(a: Int) = a // OK
-def f2(a: Int) = default_int // error
-def f3(a: Int)(using Int) = a // OK
-def f4(a: Int)(using Int) = default_int // error
-def f6(a: Int)(using Int) = summon[Int] // error
-def f7(a: Int)(using Int) = summon[Int] + a // OK
-
-/* --- Trivial method check --- */
-def g1(x: Int) = 1 // OK
-def g2(x: Int) = ??? // OK
+  private def f1(a: Int) = a // OK
+  private def f2(a: Int) = default_int // error
+  private def f3(a: Int)(using Int) = a // OK
+  private def f4(a: Int)(using Int) = default_int // error
+  private def f6(a: Int)(using Int) = summon[Int] // error
+  private def f7(a: Int)(using Int) = summon[Int] + a // OK
+  /* --- Trivial method check --- */
+  private def g1(x: Int) = 1 // OK
+  private def g2(x: Int) = ??? // OK
+}
 
 package foo.test.i17101:
   type Test[A] = A

tests/neg-custom-args/fatal-warnings/i15503h.scala

@@ -7,7 +7,7 @@ class A {
   val b = 2 // OK
 
   private def c = 2 // error
-  def d(using x:Int): Int = b // error
+  def d(using x:Int): Int = b // ok
   def e(x: Int) = 1 // OK
   def f =
     val x = 1 // error

tests/neg-custom-args/fatal-warnings/i15503i.scala

@@ -17,10 +17,10 @@ class A {
   private def c2 = 2 // OK
   def c3 = c2
 
-  def d1(using x:Int): Int = default_int // error
+  def d1(using x:Int): Int = default_int // ok
   def d2(using x:Int): Int = x // OK
 
-  def e1(x: Int) = default_int // error
+  def e1(x: Int) = default_int // ok
   def e2(x: Int) = x // OK
   def f =
     val x = 1 // error
@@ -44,7 +44,11 @@ package foo.test.scala.annotation:
   val default_int = 12
 
   def a1(a: Int) = a // OK
-  def a2(a: Int) = default_int // error
+  def a2(a: Int) = default_int // ok
+
+  private def a2_p(a: Int) = default_int // error
+  def a2_p_used = a2_p(3)
+
   def a3(@unused a: Int) = default_int //OK
 
   def b1 =