Fix named arg deprecation

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -1108,7 +1108,7 @@ object Parsers {
                 def deprecateInfixNamedArg(t: Tree): Unit = t match
                   case Tuple(ts) => ts.foreach(deprecateInfixNamedArg)
                   case Parens(t) => deprecateInfixNamedArg(t)
-                  case t: Assign => report.deprecationWarning(em"named argument is deprecated for infix syntax", t.srcPos)
+                  case t: NamedArg => report.deprecationWarning(InfixNamedArgDeprecation(), t.srcPos)
                   case _ =>
                 deprecateInfixNamedArg(top)
                 InfixOp(opInfo.operand, opInfo.operator, top)

compiler/src/dotty/tools/dotc/reporting/messages.scala

@@ -3352,3 +3352,10 @@ final class AmbiguousNamedTupleAssignment(key: Name, value: untpd.Tree)(using Co
       |To assign a value, use curly braces: `{${key} = ${value}}`."""
 
   override protected def explain(using Context): String = ""
+
+class InfixNamedArgDeprecation()(using Context) extends SyntaxMsg(DeprecatedNamedInfixArgID):
+  def msg(using Context) = "Named argument syntax is deprecated for infix application"
+  def explain(using Context) =
+    i"""The argument will be parsed as a named tuple in future.
+        |
+        |To avoid this warning, either remove the argument names or use dotted selection."""

tests/neg/infix-named-args.check

@@ -11,3 +11,11 @@
   |           (x: Byte): Int
   |           (x: String): String
   |          match arguments ((x : Int)) (a named tuple)
+-- [E007] Type Mismatch Error: tests/neg/infix-named-args.scala:13:18 --------------------------------------------------
+13 |  def g = this ** 2 // error
+   |                  ^
+   |                  Found:    (2 : Int)
+   |                  Required: X
+   |
+   | longer explanation available when compiling with `-explain`
+there were 6 deprecation warnings; re-run with -deprecation for details

tests/neg/infix-named-args.scala

@@ -1,7 +1,13 @@
-class C {
+class C:
   def f = 42 + (x = 1) // error // a named tuple!
   def multi(x: Int, y: Int): Int = x + y
   def **(x: Int, y: Int): Int = x + y
   def g = new C() `multi` (x = 42, y = 27) // werror // werror // not actually a tuple! appearances to the contrary
   def h = new C() ** (x = 42, y = 27) // werror // werror
-}
+
+type X = (x: Int)
+
+class D(d: Int):
+  def **(x: X): Int = d * x.x
+  def f = this ** (x = 2)
+  def g = this ** 2 // error

tests/warn/infix-named-args.scala

@@ -0,0 +1,7 @@
+//> using options -deprecation
+type X = (x: Int)
+
+class E(e: Int):
+  def **(x: Int): Int = e * x
+  def **(x: X): Int = e * x.x
+  def f = this ** (x = 2) // warn