Better diagnostics for erroneous multi-line infix operations

Author: odersky

compiler/src/dotty/tools/dotc/ast/Desugar.scala

@@ -39,6 +39,11 @@ object desugar {
    */
   val CheckIrrefutable: Property.Key[MatchCheck] = Property.StickyKey()
 
+  /** A multi-line infix operation with the infix operator starting a new line.
+   *  Used for explaining potential errors.
+   */
+  val MultiLineInfix: Property.Key[Unit] = Property.StickyKey()
+
   /** What static check should be applied to a Match? */
   enum MatchCheck {
     case None, Exhaustive, IrrefutablePatDef, IrrefutableGenFrom
@@ -1194,7 +1199,10 @@ object desugar {
         case Tuple(args) => args.mapConserve(assignToNamedArg)
         case _ => arg :: Nil
       }
-      Apply(Select(fn, op.name).withSpan(selectPos), args)
+      val sel = Select(fn, op.name).withSpan(selectPos)
+      if (left.sourcePos.endLine < op.sourcePos.startLine)
+        sel.pushAttachment(MultiLineInfix, ())
+      Apply(sel, args)
     }
 
     if (isLeftAssoc(op.name))

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

@@ -271,7 +271,13 @@ trait TypeAssigner {
               |An extension method was tried, but could not be fully constructed:
               |
               |    ${failure.tree.show.replace("\n", "\n    ")}"""
-          case _ => ""
+          case _ =>
+            if (tree.hasAttachment(desugar.MultiLineInfix))
+              i""".
+                 |Note that `$name` is treated as an infix operator in Scala 3.
+                 |If you do not want that, insert a `;` or empty line in front
+                 |or drop any spaces behind the operator."""
+            else ""
         }
       errorType(NotAMember(qualType, name, kind, addendum), tree.sourcePos)
     }

tests/neg/multiLineOps.scala

@@ -0,0 +1,15 @@
+val x = 1
+  + 2
+  +3 // error: Expected a toplevel definition
+
+val b1 = {
+  22
+  * 22  // ok
+  */*one more*/22 // error: end of statement expected
+}                 // error: ';' expected, but '}' found
+
+val b2: Boolean = {
+  println(x)
+  ! "hello".isEmpty  // error: value ! is not a member of Unit
+}
+

tests/pos/multiLineOps.scala

@@ -0,0 +1,26 @@
+val x = 1
+  + 2
+  + 3
+
+class Channel {
+  def ! (msg: String): Channel = this
+  def send_! (msg: String): Channel = this
+}
+
+val c = Channel()
+
+def send() =
+  c ! "hello"
+    ! "world"
+    send_! "!"
+
+val b: Boolean =
+  "hello".isEmpty
+  && true &&
+  !"hello".isEmpty
+
+val b2: Boolean = {
+  println(x)
+  !"hello".isEmpty
+  ???
+}