Awesome warning for named arg clash with named tuple

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

@@ -1054,14 +1054,17 @@ object Parsers {
         else {
           val opInfo = opStack.head
           val opPrec = precedence(opInfo.operator.name)
-          if (prec < opPrec || leftAssoc && prec == opPrec) {
+          if prec < opPrec || leftAssoc && prec == opPrec then
             opStack = opStack.tail
-            recur {
-              atSpan(opInfo.operator.span union opInfo.operand.span union top.span) {
+            recur:
+              atSpan(opInfo.operator.span union opInfo.operand.span union top.span):
+                def deprecateInfixNamedArg(t: Tree): Unit = t match
+                  case Tuple(ts) => ts.foreach(deprecateInfixNamedArg)
+                  case Parens(t) => deprecateInfixNamedArg(t)
+                  case t: Assign => report.deprecationWarning(InfixNamedArgDeprecation(), t.srcPos)
+                  case _ =>
+                deprecateInfixNamedArg(top)
                 InfixOp(opInfo.operand, opInfo.operator, top)
-              }
-            }
-          }
           else top
         }
       recur(top)

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

@@ -214,6 +214,10 @@ enum ErrorMessageID(val isActive: Boolean = true) extends java.lang.Enum[ErrorMe
   case UnusedSymbolID // errorNumber: 198
   case TailrecNestedCallID //errorNumber: 199 - unused in LTS
   case FinalLocalDefID // errorNumber: 200
+  case NonNamedArgumentInJavaAnnotationID // errorNumber: 201 - unused in LTS
+  case QuotedTypeMissingID // errorNumber: 202 - unused in LTS
+  case AmbiguousNamedTupleAssignmentID // errorNumber: 203 - unused in LTS
+  case DeprecatedNamedInfixArgID // errorNumber: 204 - used ONLY in LTS
 
   def errorNumber = ordinal - 1
 

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

@@ -3150,3 +3150,12 @@ object UnusedSymbol {
     def privateMembers(using Context): UnusedSymbol = new UnusedSymbol(i"unused private member")
     def patVars(using Context): UnusedSymbol = new UnusedSymbol(i"unused pattern variable")
 }
+
+class InfixNamedArgDeprecation()(using Context)
+  extends Message(DeprecatedNamedInfixArgID):
+    def kind = MessageKind.PotentialIssue
+    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/warn/infix-named-args.scala

@@ -0,0 +1,7 @@
+//> using options -deprecation
+
+class C {
+  def f = 42 + (x = 1) // warn
+  def multi(x: Int, y: Int): Int = x + y
+  def g = new C() `multi` (x = 42, y = 27) // warn // warn
+}