Fix #2495: Improve error message

Author: nicolasstucki

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

@@ -189,12 +189,15 @@ object messages {
     val msg = {
       val ofFun =
         if (MethodType.syntheticParamNames(args.length + 1) contains param.name)
-          i" of expanded function $tree"
+          i" of expanded function:\n$tree"
         else
           ""
 
-      i"""missing parameter type for parameter ${param.name}$ofFun, expected = $pt
-         |The argument types of an anonymous function must be fully known. (SLS 8.5)"""
+      i"""missing parameter type
+         |
+         |The argument types of an anonymous function must be fully known. (SLS 8.5)
+         |Expected type: $pt
+         |Missing type for parameter ${param.name}$ofFun"""
     }
 
     val explanation =

compiler/test-resources/repl/errorThenValid

@@ -1,7 +1,10 @@
 scala> val xs = scala.collection.mutable.ListBuffer[Int]
 1 | val xs = scala.collection.mutable.ListBuffer[Int]
   |          ^
-  |missing parameter type for parameter elems, expected = ?
+  |missing parameter type
+  |
   |The argument types of an anonymous function must be fully known. (SLS 8.5)
+  |Expected type: ?
+  |Missing type for parameter elems
 scala> val xs = scala.collection.mutable.ListBuffer[Int]()
 val xs: scala.collection.mutable.ListBuffer[Int] = ListBuffer()

compiler/test-resources/repl/i2495

@@ -0,0 +1,16 @@
+scala> object TypeParam { sealed trait F[A]; case object FI extends F[Int]; case object FS extends F[String]; trait FFn { def apply[A](f: F[A]): A }; val test4: FFn = { case FI => 1; case FS => "" } }
+1 | object TypeParam { sealed trait F[A]; case object FI extends F[Int]; case object FS extends F[String]; trait FFn { def apply[A](f: F[A]): A }; val test4: FFn = { case FI => 1; case FS => "" } }
+  |                                                                                                                                                                   ^
+  |missing parameter type
+  |
+  |The argument types of an anonymous function must be fully known. (SLS 8.5)
+  |Expected type: TypeParam.FFn
+  |Missing type for parameter x$1 of expanded function:
+  |x$1 =>
+  |  x$1 match
+  |    {
+  |      case FI =>
+  |        1
+  |      case FS =>
+  |        ""
+  |    }