Better implicit reporting under -explain-implicits

Author: odersky

compiler/src/dotty/tools/dotc/config/ScalaSettings.scala

@@ -24,7 +24,8 @@ class ScalaSettings extends Settings.SettingGroup {
   val deprecation = BooleanSetting("-deprecation", "Emit warning and location for usages of deprecated APIs.")
   val migration = BooleanSetting("-migration", "Emit warning and location for migration issues from Scala 2.")
   val encoding = StringSetting("-encoding", "encoding", "Specify character encoding used by source files.", Properties.sourceEncoding)
-  val explaintypes = BooleanSetting("-explaintypes", "Explain type errors in more detail.")
+  val explainTypes = BooleanSetting("-explain-types", "Explain type errors in more detail.")
+  val explainImplicits = BooleanSetting("-explain-implicits", "Explain implicit search errors in more detail.")
   val explain = BooleanSetting("-explain", "Explain errors in more detail.")
   val feature = BooleanSetting("-feature", "Emit warning and location for usages of features that should be imported explicitly.")
   val help = BooleanSetting("-help", "Print a synopsis of standard options")

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

@@ -114,7 +114,7 @@ object ErrorReporting {
       val expected1 = dropJavaMethod(expected)
       if ((found1 eq found) != (expected eq expected1) && (found1 <:< expected1))
         "\n (Note that Scala's and Java's representation of this type differs)"
-      else if (ctx.settings.explaintypes.value)
+      else if (ctx.settings.explainTypes.value)
         "\n" + ctx.typerState.show + "\n" + TypeComparer.explained((found <:< expected)(_))
       else
         ""

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

@@ -334,11 +334,14 @@ object Implicits {
   class FailedImplicit(failures: List[ExplainedSearchFailure], val pt: Type, val argument: tpd.Tree) extends ExplainedSearchFailure {
     def explanation(implicit ctx: Context): String =
       if (failures.isEmpty) s"  No implicit candidates were found that $qualify"
-      else "  " + (failures map (_.explanation) mkString "\n  ")
-    override def postscript(implicit ctx: Context): String =
+      else failures.map(_.explanation).mkString("\n").replace("\n", "\n  ")
+    override def postscript(implicit ctx: Context): String = {
+      val what =
+        if (argument.isEmpty) i"value of type $pt"
+        else i"conversion from ${argument.tpe.widen} to $pt"
       i"""
-         |Implicit search failure summary:
          |$explanation"""
+    }
   }
 }
 
@@ -671,7 +674,7 @@ trait Implicits { self: Typer =>
     ctx.traceIndented(s"search implicit ${pt.show}, arg = ${argument.show}: ${argument.tpe.show}", implicits, show = true) {
       assert(!pt.isInstanceOf[ExprType])
       val isearch =
-        if (ctx.settings.explaintypes.value) new ExplainedImplicitSearch(pt, argument, pos)
+        if (ctx.settings.explainImplicits.value) new ExplainedImplicitSearch(pt, argument, pos)
         else new ImplicitSearch(pt, argument, pos)
       val result = isearch.bestImplicit
       result match {