Make message more obvious in the myopic case

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

@@ -217,6 +217,7 @@ enum ErrorMessageID(val isActive: Boolean = true) extends java.lang.Enum[ErrorMe
   case NonNamedArgumentInJavaAnnotationID // errorNumber: 201
   case QuotedTypeMissingID // errorNumber: 202
   case AmbiguousNamedTupleAssignmentID // errorNumber: 203
+  case DeprecatedNamedInfixArgID // errorNumber: 204 - used ONLY in LTS
 
   def errorNumber = ordinal - 1
 

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

@@ -110,7 +110,13 @@ object ErrorReporting {
             case tp => i" and expected result type $tp"
           }
           i"(${tp.typedArgs().tpes}%, %)$result"
-        s"arguments ${argStr(tp)}"
+        def hasNames = tp.args.exists:
+          case tree: untpd.Tuple => tree.trees.exists:
+            case NamedArg(_, _) => true
+            case _ => false
+          case _ => false
+        val addendum = if hasNames then " (a named tuple)" else ""
+        s"arguments ${argStr(tp)}$addendum"
       case _ =>
         i"expected type $tp"
     }

docs/_docs/reference/other-new-features/named-tuples.md

@@ -17,8 +17,9 @@ val persons: List[Person] = ...
 val minors = persons.filter: p =>
   p.age < 18
 ```
-Named bindings in tuples are similar to function parameters and arguments. We use `name: Type` for element types and `name = value` for element values. It is illegal to mix named and unnamed elements in a tuple, or to use the same same
-name for two different elements.
+Named bindings in tuples are similar to function parameters and arguments.
+We use `name: Type` for element types and `name = value` for element values.
+It is illegal to mix named and unnamed elements in a tuple, or to use the same name for two different elements.
 
 Fields of named tuples can be selected by their name, as in the line `p.age < 18` above.
 

tests/neg/infix-named-args.check

@@ -0,0 +1,13 @@
+-- [E134] Type Error: tests/neg/infix-named-args.scala:2:13 ------------------------------------------------------------
+2 |  def f = 42 + (x = 1) // error // a named tuple!
+  |          ^^^^
+  |          None of the overloaded alternatives of method + in class Int with types
+  |           (x: Double): Double
+  |           (x: Float): Float
+  |           (x: Long): Long
+  |           (x: Int): Int
+  |           (x: Char): Int
+  |           (x: Short): Int
+  |           (x: Byte): Int
+  |           (x: String): String
+  |          match arguments ((x : Int)) (a named tuple)

tests/neg/infix-named-args.scala

@@ -0,0 +1,7 @@
+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
+}