Better reporting of nested implicit failures

Error messages of nested implicit failures are now
reported with the top-level message if -explaintypes is set.

3 files changed, 47 insertions, 15 deletions

diff --git a/compiler/src/dotty/tools/dotc/reporting/Reporter.scala b/compiler/src/dotty/tools/dotc/reporting/Reporter.scala
index 8477cfe28..26c1e5ebc 100644
--- a/compiler/src/dotty/tools/dotc/reporting/Reporter.scala
+++ b/compiler/src/dotty/tools/dotc/reporting/Reporter.scala
@@ -286,11 +286,16 @@ abstract class Reporter extends interfaces.ReporterResult {
   }
 
   /** Should this diagnostic not be reported at all? */
-  def isHidden(m: MessageContainer)(implicit ctx: Context): Boolean = ctx.mode.is(Mode.Printing)
+  def isHidden(m: MessageContainer)(implicit ctx: Context): Boolean =
+    ctx.mode.is(Mode.Printing)
 
   /** Does this reporter contain not yet reported errors or warnings? */
   def hasPending: Boolean = false
 
+  /** If this reporter buffers messages, remove and return all buffered messages. */
+  def removeBufferedMessages(implicit ctx: Context): List[MessageContainer] = Nil
+
   /** Issue all error messages in this reporter to next outer one, or make sure they are written. */
-  def flush()(implicit ctx: Context): Unit = {}
+  def flush()(implicit ctx: Context): Unit =
+    removeBufferedMessages.foreach(ctx.reporter.report)
 }
diff --git a/compiler/src/dotty/tools/dotc/reporting/StoreReporter.scala b/compiler/src/dotty/tools/dotc/reporting/StoreReporter.scala
index 586273c2e..34b109882 100644
--- a/compiler/src/dotty/tools/dotc/reporting/StoreReporter.scala
+++ b/compiler/src/dotty/tools/dotc/reporting/StoreReporter.scala
@@ -36,11 +36,9 @@ class StoreReporter(outer: Reporter) extends Reporter {
     }
   }
 
-  override def flush()(implicit ctx: Context) =
-    if (infos != null) {
-      infos.foreach(ctx.reporter.report(_))
-      infos = null
-    }
+  override def removeBufferedMessages(implicit ctx: Context): List[MessageContainer] =
+    if (infos != null) try infos.toList finally infos = null
+    else Nil
 
   override def errorsReported = hasErrors || outer.errorsReported
 }
diff --git a/compiler/src/dotty/tools/dotc/typer/Implicits.scala b/compiler/src/dotty/tools/dotc/typer/Implicits.scala
index f3dceea71..9bd6e6c45 100644
--- a/compiler/src/dotty/tools/dotc/typer/Implicits.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Implicits.scala
@@ -24,6 +24,7 @@ import Constants._
 import Applications._
 import ProtoTypes._
 import ErrorReporting._
+import reporting.diagnostic.MessageContainer
 import Inferencing.fullyDefinedType
 import Trees._
 import Hashable._
@@ -212,6 +213,8 @@ object Implicits {
   /** A "no matching implicit found" failure */
   case object NoImplicitMatches extends SearchFailure
 
+  case object DivergingImplicit extends SearchFailure
+
   /** A search failure that can show information about the cause */
   abstract class ExplainedSearchFailure extends SearchFailure {
     protected def pt: Type
@@ -233,9 +236,35 @@ object Implicits {
       "\n " + explanation
   }
 
-  class NonMatchingImplicit(ref: TermRef, val pt: Type, val argument: tpd.Tree) extends ExplainedSearchFailure {
-    def explanation(implicit ctx: Context): String =
-      em"${err.refStr(ref)} does not $qualify"
+  class NonMatchingImplicit(ref: TermRef,
+                            val pt: Type,
+                            val argument: tpd.Tree,
+                            trail: List[MessageContainer]) extends ExplainedSearchFailure {
+    private val separator = "\n**** because ****\n"
+
+    /** Replace repeated parts beginning with `separator` by ... */
+    private def elideRepeated(str: String): String = {
+      val startIdx = str.indexOfSlice(separator)
+      val nextIdx = str.indexOfSlice(separator, startIdx + separator.length)
+      if (nextIdx < 0) str
+      else {
+        val prefix = str.take(startIdx)
+        val first = str.slice(startIdx, nextIdx)
+        var rest = str.drop(nextIdx)
+        if (rest.startsWith(first)) {
+          rest = rest.drop(first.length)
+          val dots = "\n\n     ...\n"
+          if (!rest.startsWith(dots)) rest = dots ++ rest
+        }
+        prefix ++ first ++ rest
+      }
+    }
+    
+    def explanation(implicit ctx: Context): String = {
+      val headMsg = em"${err.refStr(ref)} does not $qualify"
+      val trailMsg = trail.map(mc => i"$separator  ${mc.message}").mkString
+      elideRepeated(headMsg ++ trailMsg)
+    }
   }
 
   class ShadowedImplicit(ref: TermRef, shadowing: Type, val pt: Type, val argument: tpd.Tree) extends ExplainedSearchFailure {
@@ -587,7 +616,7 @@ trait Implicits { self: Typer =>
     val wildProto = implicitProto(pt, wildApprox(_))
 
     /** Search failures; overridden in ExplainedImplicitSearch */
-    protected def nonMatchingImplicit(ref: TermRef): SearchFailure = NoImplicitMatches
+    protected def nonMatchingImplicit(ref: TermRef, trail: List[MessageContainer]): SearchFailure = NoImplicitMatches
     protected def divergingImplicit(ref: TermRef): SearchFailure = NoImplicitMatches
     protected def shadowedImplicit(ref: TermRef, shadowing: Type): SearchFailure = NoImplicitMatches
     protected def failedSearch: SearchFailure = NoImplicitMatches
@@ -628,7 +657,7 @@ trait Implicits { self: Typer =>
             { implicits.println(i"invalid eqAny[$tp1, $tp2]"); false }
         }
         if (ctx.reporter.hasErrors)
-          nonMatchingImplicit(ref)
+          nonMatchingImplicit(ref, ctx.reporter.removeBufferedMessages)
         else if (contextual && !ctx.mode.is(Mode.ImplicitShadowing) &&
                  !shadowing.tpe.isError && !refMatches(shadowing)) {
           implicits.println(i"SHADOWING $ref in ${ref.termSymbol.owner} is shadowed by $shadowing in ${shadowing.symbol.owner}")
@@ -637,7 +666,7 @@ trait Implicits { self: Typer =>
         else generated1 match {
           case TypeApply(fn, targs @ (arg1 :: arg2 :: Nil))
           if fn.symbol == defn.Predef_eqAny && !validEqAnyArgs(arg1.tpe, arg2.tpe) =>
-            nonMatchingImplicit(ref)
+            nonMatchingImplicit(ref, Nil)
           case _ =>
             SearchSuccess(generated1, ref, ctx.typerState)
         }
@@ -743,8 +772,8 @@ trait Implicits { self: Typer =>
       fail
     }
     def failures = myFailures.toList
-    override def nonMatchingImplicit(ref: TermRef) =
-      record(new NonMatchingImplicit(ref, pt, argument))
+    override def nonMatchingImplicit(ref: TermRef, trail: List[MessageContainer]) =
+      record(new NonMatchingImplicit(ref, pt, argument, trail))
     override def divergingImplicit(ref: TermRef) =
       record(new DivergingImplicit(ref, pt, argument))
     override def shadowedImplicit(ref: TermRef, shadowing: Type): SearchFailure =