Deprecate infix named args

Author: som-snytt

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

@@ -1104,9 +1104,14 @@ object Parsers {
           if (prec < opPrec || leftAssoc && prec == opPrec) {
             opStack = opStack.tail
             recur {
-              atSpan(opInfo.operator.span union opInfo.operand.span union top.span) {
+              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(em"named argument is deprecated for infix syntax", t.srcPos)
+                  case _ =>
+                deprecateInfixNamedArg(top)
                 InfixOp(opInfo.operand, opInfo.operator, top)
-              }
             }
           }
           else top

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
+}