Warn on unary methods with parameters

Fixes #9241

Author: nicolasstucki

compiler/src/dotty/tools/dotc/typer/RefChecks.scala

@@ -1047,6 +1047,30 @@ object RefChecks {
           report.error(i"private $sym cannot override ${other.showLocated}", sym.srcPos)
   end checkNoPrivateOverrides
 
+  /** Check that unary method definition do not receive parameters.
+   *  They can only receive inferred parameters such as type parameters and implicit parameters.
+   */
+  def checkUnaryMethods(sym: Symbol)(using Context): Unit =
+    def checkParameters(tpe: Type): Unit =
+      tpe match
+        case tpe: MethodType =>
+          if tpe.isImplicitMethod || tpe.isContextualMethod then
+            checkParameters(tpe.resType)
+          else
+            val what =
+              if tpe.paramNames.isEmpty then "empty parameter list.\n\nPossible fix: remove the `()` arguments."
+              else "parameters"
+            report.warning(s"Unary method cannot take $what", sym.sourcePos)
+        case tpe: PolyType =>
+          checkParameters(tpe.resType)
+        case _ =>
+          // ok
+
+    if sym.name.startsWith(nme.UNARY_PREFIX.toString) then
+      checkParameters(sym.info)
+
+  end checkUnaryMethods
+
   type LevelAndIndex = immutable.Map[Symbol, (LevelInfo, Int)]
 
   class OptLevelInfo {
@@ -1254,6 +1278,7 @@ class RefChecks extends MiniPhase { thisPhase =>
     checkExperimentalAnnots(tree.symbol)
     checkExperimentalSignature(tree.symbol, tree)
     checkImplicitNotFoundAnnotation.defDef(tree.symbol.denot)
+    checkUnaryMethods(tree.symbol)
     tree
   }
 

tests/neg-custom-args/fatal-warnings/i9241.scala

@@ -0,0 +1,15 @@
+class Foo {
+  def unary_~() : Foo = this // error
+  def unary_-(using Int)(): Foo = this // error
+  def unary_+()(implicit i: Int): Foo = this // error
+  def unary_![T](): Foo = this // error
+}
+
+
+class Bar {
+  def unary_~ : Bar = this
+  def unary_-(using Int): Bar = this
+  def unary_+(implicit i: Int): Bar = this
+  def unary_![T]: Bar = this
+}
+