Improve missing parameter type error message

Author: odersky

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

@@ -195,21 +195,16 @@ object messages {
         else
           ""
 
+      val inferred =
+        if (pt == WildcardType) ""
+        else i"\nWhat I could infer was: $pt"
+
       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"""
+         |I could not infer the type of the parameter ${param.name}$ofFun.$inferred"""
     }
 
-    val explanation: String =
-      hl"""|Anonymous functions must define a type. For example, if you define a function like this one:
-           |
-           |${"val f = { case x: Int => x + 1 }"}
-           |
-           |Make sure you give it a type of what you expect to match and help the type inference system:
-           |
-           |${"val f: Any => Int = { case x: Int => x + 1 }"} """
+    val explanation: String = ""
   }
 
   case class WildcardOnTypeArgumentNotAllowedOnNew()(implicit ctx: Context)

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

@@ -940,7 +940,7 @@ class Typer extends Namer
           }
         case _ =>
       }
-      errorType(AnonymousFunctionMissingParamType(param, params, tree, pt), param.sourcePos)
+      errorType(AnonymousFunctionMissingParamType(param, params, tree, formal), param.sourcePos)
     }
 
     def protoFormal(i: Int): Type =