Show match type reduction failures for other errors

Also show match type reduction failures for implicit search errors and member not found errors.

Author: odersky

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

@@ -16,7 +16,7 @@ import printing.Formatting
 import ErrorMessageID._
 import ast.Trees
 import config.{Feature, ScalaVersion}
-import typer.ErrorReporting.err
+import typer.ErrorReporting.{err, matchReductionAddendum}
 import typer.ProtoTypes.ViewProto
 import scala.util.control.NonFatal
 import StdNames.nme
@@ -45,22 +45,15 @@ import transform.SymUtils._
   abstract class TypeMsg(errorId: ErrorMessageID) extends Message(errorId):
     def kind = "Type"
 
-  abstract class TypeMismatchMsg(found: Type, expected: Type)(errorId: ErrorMessageID)(using Context) extends Message(errorId):
+  trait ShowMatchTrace(tps: Type*)(using Context) extends Message:
+    override def msgSuffix: String = matchReductionAddendum(tps*)
+
+  abstract class TypeMismatchMsg(found: Type, expected: Type)(errorId: ErrorMessageID)(using Context)
+  extends Message(errorId), ShowMatchTrace(found, expected):
     def kind = "Type Mismatch"
     def explain = err.whyNoMatchStr(found, expected)
     override def canExplain = true
 
-    override def msgSuffix: String =
-      val collectMatchTrace = new TypeAccumulator[String]:
-        def apply(s: String, tp: Type): String =
-          if s.nonEmpty then s
-          else tp match
-            case tp: AppliedType if tp.isMatchAlias => MatchTypeTrace.record(tp.tryNormalize)
-            case tp: MatchType => MatchTypeTrace.record(tp.tryNormalize)
-            case _ => foldOver(s, tp)
-      collectMatchTrace(collectMatchTrace("", found), expected)
-  end TypeMismatchMsg
-
   abstract class NamingMsg(errorId: ErrorMessageID) extends Message(errorId):
     def kind = "Naming"
 
@@ -292,7 +285,7 @@ import transform.SymUtils._
   end TypeMismatch
 
   class NotAMember(site: Type, val name: Name, selected: String, addendum: => String = "")(using Context)
-  extends NotFoundMsg(NotAMemberID) {
+  extends NotFoundMsg(NotAMemberID), ShowMatchTrace(site) {
     //println(i"site = $site, decls = ${site.decls}, source = ${site.typeSymbol.sourceFile}") //DEBUG
 
     def msg = {

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

@@ -51,6 +51,16 @@ object ErrorReporting {
       case _ =>
         report.error(em"missing arguments for $meth", tree.srcPos)
 
+  def matchReductionAddendum(tps: Type*)(using Context): String =
+    val collectMatchTrace = new TypeAccumulator[String]:
+      def apply(s: String, tp: Type): String =
+        if s.nonEmpty then s
+        else tp match
+          case tp: AppliedType if tp.isMatchAlias => MatchTypeTrace.record(tp.tryNormalize)
+          case tp: MatchType => MatchTypeTrace.record(tp.tryNormalize)
+          case _ => foldOver(s, tp)
+    tps.foldLeft("")(collectMatchTrace)
+
   class Errors(using Context) {
 
     /** An explanatory note to be added to error messages
@@ -253,7 +263,9 @@ class ImplicitSearchError(
       val shortMessage = userDefinedImplicitNotFoundParamMessage
         .orElse(userDefinedImplicitNotFoundTypeMessage)
         .getOrElse(defaultImplicitNotFoundMessage)
-      formatMsg(shortMessage)() ++ hiddenImplicitsAddendum
+      formatMsg(shortMessage)()
+      ++ hiddenImplicitsAddendum
+      ++ ErrorReporting.matchReductionAddendum(pt)
   }
 
   private def formatMsg(shortForm: String)(headline: String = shortForm) = arg match {

tests/neg/i12049.scala

@@ -20,3 +20,10 @@ type Reverse[X <: Tuple] = X match
 val z1: (B, A) = ??? : Reverse[(A, B)]
 val z2: (B, A, B, A) = ??? : Reverse[(A, B, A, B)]
 val z3: (A, B, A) = ??? : Reverse[(A, B, A)] // error
+
+val _ = summon[M[B]]
+val _ = summon[String =:= Last[Int *: Int *: Boolean *: String *: EmptyTuple]]
+val _ = summon[(A, B, A) =:= Reverse[(A, B, A)]]
+
+val _ = (??? : M[B]).length
+