Better explanation for missing members that have extension methods

Author: odersky

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

@@ -270,7 +270,7 @@ trait TypeAssigner {
       def addendum =
         if (qualType.derivesFrom(defn.DynamicClass))
           "\npossible cause: maybe a wrong Dynamic method signature?"
-        else qual1.getAttachment(Typer.HiddenSearchFailure) match {
+        else qual1.getAttachment(Typer.HiddenSearchFailure) match
           case Some(failure) if !failure.reason.isInstanceOf[Implicits.NoMatchingImplicits] =>
             i""".
               |An extension method was tried, but could not be fully constructed:
@@ -283,10 +283,12 @@ trait TypeAssigner {
                  |If you do not want that, insert a `;` or empty line in front
                  |or drop any spaces behind the operator."""
             else
-              implicitSuggestionAddendum(
+              var add = implicitSuggestionAddendum(
                 ViewProto(qualType.widen,
                   SelectionProto(name, WildcardType, NoViewsAllowed, privateOK = false)))
-        }
+              if add.isEmpty then ""
+              else ", but could be made available as an extension method." ++ add
+      end addendum
       errorType(NotAMember(qualType, name, kind, addendum), tree.sourcePos)
     }
   }

tests/neg/missing-implicit.check

@@ -1,7 +1,7 @@
 -- [E008] Member Not Found Error: tests/neg/missing-implicit.scala:5:25 ------------------------------------------------
 5 |  case x :: xs1 if limit > 0 => consume(xs1, limit - x) // error // error
   |                   ^^^^^^^
-  |                   value > is not a member of T
+  |                   value > is not a member of T, but could be made available as an extension method.
   |
   |                   One of the following imports might fix the problem:
   |
@@ -11,12 +11,12 @@
 -- [E008] Member Not Found Error: tests/neg/missing-implicit.scala:5:51 ------------------------------------------------
 5 |  case x :: xs1 if limit > 0 => consume(xs1, limit - x) // error // error
   |                                             ^^^^^^^
-  |                                             value - is not a member of T
+  |                                 value - is not a member of T, but could be made available as an extension method.
   |
-  |                                             The following import might fix the problem:
+  |                                 The following import might fix the problem:
   |
-  |                                               import math.Numeric.Implicits.infixNumericOps
-  |                                                      
+  |                                   import math.Numeric.Implicits.infixNumericOps
+  |                                          
 -- Error: tests/neg/missing-implicit.scala:10:24 -----------------------------------------------------------------------
 10 |val f = Future[Unit] { } // error
    |                        ^
@@ -62,11 +62,11 @@
 -- [E008] Member Not Found Error: tests/neg/missing-implicit.scala:18:48 -----------------------------------------------
 18 |val d2: scala.concurrent.duration.Duration = 10.days // error
    |                                             ^^^^^^^
-   |                                             value days is not a member of Int
+   |                          value days is not a member of Int, but could be made available as an extension method.
    |
-   |                                             One of the following imports might fix the problem:
+   |                          One of the following imports might fix the problem:
    |
-   |                                               import concurrent.duration.DurationInt
-   |                                               import concurrent.duration.DurationLong
-   |                                               import concurrent.duration.DurationDouble
-   |                                                      
+   |                            import concurrent.duration.DurationInt
+   |                            import concurrent.duration.DurationLong
+   |                            import concurrent.duration.DurationDouble
+   |                                   

tests/neg/missing-implicit1.check

@@ -10,7 +10,7 @@
 -- [E008] Member Not Found Error: tests/neg/missing-implicit1.scala:16:16 ----------------------------------------------
 16 |  List(1, 2, 3).traverse(x => Option(x)) // error
    |  ^^^^^^^^^^^^^^^^^^^^^^
-   |  value traverse is not a member of List[Int] - did you mean List[Int].reverse?
+   |  value traverse is not a member of List[Int], but could be made available as an extension method.
    |
    |  The following import might make progress towards fixing the problem:
    |
@@ -37,7 +37,7 @@
 -- [E008] Member Not Found Error: tests/neg/missing-implicit1.scala:38:16 ----------------------------------------------
 38 |  List(1, 2, 3).traverse(x => Option(x)) // error
    |  ^^^^^^^^^^^^^^^^^^^^^^
-   |  value traverse is not a member of List[Int] - did you mean List[Int].reverse?
+   |  value traverse is not a member of List[Int], but could be made available as an extension method.
    |
    |  The following import might make progress towards fixing the problem:
    |