Merge pull request #15164 from szymon-rd/fix/15128

Add error reporting for mirror synthesis

Author: bishabosha

compiler/src/dotty/tools/dotc/transform/SymUtils.scala

@@ -146,7 +146,7 @@ object SymUtils:
       if (!self.is(Sealed))
         s"it is not a sealed ${self.kindString}"
       else if (!self.isOneOf(AbstractOrTrait))
-        s"it is not an abstract class"
+        "it is not an abstract class"
       else {
         val children = self.children
         val companionMirror = self.useCompanionAsSumMirror

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

@@ -257,24 +257,26 @@ class ImplicitSearchError(
       ++ ErrorReporting.matchReductionAddendum(pt)
   }
 
-  private def formatMsg(shortForm: String)(headline: String = shortForm) = arg match {
+  private def formatMsg(shortForm: String)(headline: String = shortForm) = arg match 
     case arg: Trees.SearchFailureIdent[?] =>
-      shortForm
+      arg.tpe match
+        case _: NoMatchingImplicits => headline
+        case tpe: SearchFailureType =>
+          i"$headline. ${tpe.explanation}"
+        case _ => headline
     case _ =>
-      arg.tpe match {
+      arg.tpe match
         case tpe: SearchFailureType =>
           val original = arg match
             case Inlined(call, _, _) => call
             case _ => arg
-
           i"""$headline.
             |I found:
             |
             |    ${original.show.replace("\n", "\n    ")}
             |
             |But ${tpe.explanation}."""
-      }
-  }
+        case _ => headline
 
   /** Format `raw` implicitNotFound or implicitAmbiguous argument, replacing
    *  all occurrences of `${X}` where `X` is in `paramNames` with the
@@ -302,12 +304,10 @@ class ImplicitSearchError(
   private def location(preposition: String) = if (where.isEmpty) "" else s" $preposition $where"
 
   private def defaultAmbiguousImplicitMsg(ambi: AmbiguousImplicits) =
-    formatMsg(s"ambiguous given instances: ${ambi.explanation}${location("of")}")(
-      s"ambiguous given instances of type ${pt.show} found${location("for")}"
-    )
+    s"Ambiguous given instances: ${ambi.explanation}${location("of")}"
 
   private def defaultImplicitNotFoundMessage =
-    ex"no given instance of type $pt was found${location("for")}"
+    ex"No given instance of type $pt was found${location("for")}"
 
   /** Construct a custom error message given an ambiguous implicit
    *  candidate `alt` and a user defined message `raw`.

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

@@ -549,6 +549,18 @@ object Implicits:
     override def msg(using Context) = _msg
     def explanation(using Context) = msg.toString
 
+  /** A search failure type for failed synthesis of terms for special types */ 
+  class SynthesisFailure(reasons: List[String], val expectedType: Type) extends SearchFailureType:
+    def argument = EmptyTree
+
+    private def formatReasons = 
+      if reasons.length > 1 then 
+        reasons.mkString("\n\t* ", "\n\t* ", "") 
+      else 
+        reasons.mkString
+      
+    def explanation(using Context) = em"Failed to synthesize an instance of type ${clarify(expectedType)}: ${formatReasons}"
+
 end Implicits
 
 import Implicits._
@@ -854,7 +866,12 @@ trait Implicits:
         if fail.isAmbiguous then failed
         else
           if synthesizer == null then synthesizer = Synthesizer(this)
-          synthesizer.uncheckedNN.tryAll(formal, span).orElse(failed)
+          val (tree, errors) = synthesizer.uncheckedNN.tryAll(formal, span)
+          if errors.nonEmpty then
+            SearchFailure(new SynthesisFailure(errors, formal), span).tree
+          else
+            tree.orElse(failed)
+          
 
   /** Search an implicit argument and report error if not found */
   def implicitArgTree(formal: Type, span: Span)(using Context): Tree = {