Check if param was used in default arg getter

[Cherry-picked 0cbe43b]

Author: som-snytt

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

@@ -568,7 +568,7 @@ object CheckUnused:
         // A class param is unused if its param accessor is unused.
         // (The class param is not assigned to a field until constructors.)
         // A local param accessor warns as a param; a private accessor as a private member.
-        // Avoid warning for case class elements because they are aliased via unapply.
+        // Avoid warning for case class elements because they are aliased via unapply (i.e. may be extracted).
         if m.isPrimaryConstructor then
           val alias = m.owner.info.member(sym.name)
           if alias.exists then
@@ -589,6 +589,7 @@ object CheckUnused:
           )
           && !sym.name.isInstanceOf[DerivedName]
           && !ctx.platform.isMainMethod(m)
+          && !usedByDefaultGetter(sym, m)
         then
           warnAt(pos)(UnusedSymbol.explicitParams(sym))
       end checkExplicit
@@ -600,6 +601,16 @@ object CheckUnused:
         checkExplicit()
     end checkParam
 
+    // does the param have an alias in a default arg method that is used?
+    def usedByDefaultGetter(param: Symbol, meth: Symbol): Boolean =
+      val cls = meth.enclosingClass
+      val MethName = meth.name
+      cls.info.decls.exists: d =>
+        d.name match
+        case DefaultGetterName(MethName, _) =>
+          d.paramSymss.exists(_.exists(p => p.name == param.name && infos.refs(p)))
+        case _ => false
+
     def checkImplicit(sym: Symbol, pos: SrcPos) =
       val m = sym.owner
       def allowed =
@@ -631,7 +642,7 @@ object CheckUnused:
               || aliasSym.isAllOf(Protected | ParamAccessor, butNot = CaseAccessor) && m.owner.is(Given)
             if checking && !infos.refs(alias.symbol) then
               warnAt(pos)(UnusedSymbol.implicitParams(aliasSym))
-        else
+        else if !usedByDefaultGetter(sym, m) then
           warnAt(pos)(UnusedSymbol.implicitParams(sym))
 
     def checkLocal(sym: Symbol, pos: SrcPos) =

tests/warn/i15503e.scala

@@ -92,4 +92,4 @@ object UnwrapTyped:
     error("Compiler bug: `codeOf` was not evaluated by the compiler")
 
 object `default usage`:
-  def f(i: Int)(j: Int = i * 2) = j // warn I guess
+  def f(i: Int)(j: Int = i * 2) = j // ~warn~ I guess (see tests/warn/i23349.scala)

tests/warn/i23349.scala

@@ -1,4 +1,4 @@
-//> using options -Wunused:explicits
+//> using options -Wunused:explicits,implicits
 
 // An external class that doesn't get its own `copy` method.
 class Foo(val a: String, val b: Int)
@@ -12,4 +12,9 @@ extension (self: Foo)
 //
 // Example 2: implement `copyFoo` with parameter groups.
 //
-def copyFoo(foo: Foo)(a: String = foo.a, b: Int = foo.b): Foo = Foo(a, b) // warn
+def copyFoo(foo: Foo)(a: String = foo.a, b: Int = foo.b): Foo = Foo(a, b)
+
+class C:
+  def copyFoo(foo: Foo, bar: String)(a: String = foo.a, b: Int = foo.b)(c: String = bar): Foo = Foo(a, b) // warn c
+  def copyUsing(using foo: Foo, bar: String)(a: String = foo.a, b: Int = foo.b)(c: String = bar): Foo = // warn c
+    Foo(a, b)