Tighten condition when to emit a "unary method cannot have parameters" warning

Also, use a more precise term in the error message. Unary method generally means
something entirely different!

Generally, was it really worth it to go into so much trouble for a warning
for an edge case? The previous fix overshot, excluding things that
are OK. I still believe it would have been better to do nothing aboit #9142.
The time we have already spent on this and the number of code lines we use to
address the issue is in no relation to the benefit of the fix.

Author: odersky

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

@@ -1039,7 +1039,7 @@ object RefChecks {
             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)
+            report.warning(s"unary_<op> method cannot take $what", sym.sourcePos)
         case tpe: PolyType =>
           checkParameters(tpe.resType)
         case _ =>
@@ -1057,7 +1057,13 @@ object RefChecks {
         case tpe: PolyType =>
           checkExtensionParameters(tpe.resType)
 
-    if sym.name.startsWith(nme.UNARY_PREFIX.toString) then
+    def isUnaryPrefixName(name: Name) = name match
+      case name: SimpleName =>
+        name.startsWith("unary_") && nme.raw.isUnary(name.drop(6))
+      case _ =>
+        false
+
+    if isUnaryPrefixName(sym.name) then
       if sym.is(Extension) || sym.name.is(ExtMethName) then
         // if is method from `extension` or value class
         checkExtensionParameters(sym.info)

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

@@ -16,6 +16,7 @@ final class Baz private (val x: Int) extends AnyVal {
   def unary_- : Baz = ???
   def unary_+[T] : Baz = ???
   def unary_!() : Baz = ??? // error
+  def `unary_!_=`() : Baz = ??? // ok
   def unary_~(using Int) : Baz = ???
 }