Bring back explanations of implicit searches for extensions

Bring back explanations if an implicit search was tried to resolve an extension method.

Author: odersky

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

@@ -141,20 +141,26 @@ object ErrorReporting {
       if Feature.migrateTo3 then "\nThis patch can be inserted automatically under -rewrite."
       else ""
 
+    def whyFailedStr(fail: FailedExtension) =
+      i"""    failed with
+         |
+         |${fail.whyFailed.message.indented(8)}"""
+
     def selectErrorAddendum
       (tree: untpd.RefTree, qual1: Tree, qualType: Type, suggestImports: Type => String)
       (using Context): String =
 
-      val attempts = mutable.ListBuffer[(Tree, FailedExtension)]()
+      val attempts = mutable.ListBuffer[(Tree, String)]()
       val nested = mutable.ListBuffer[NestedFailure]()
       for
         failures <- qual1.getAttachment(Typer.HiddenSearchFailure)
         failure <- failures
       do
         failure.reason match
           case fail: NestedFailure => nested += fail
-          case fail: FailedExtension => attempts += ((failure.tree, fail))
-          case _ =>
+          case fail: FailedExtension => attempts += ((failure.tree, whyFailedStr(fail)))
+          case fail: Implicits.NoMatchingImplicits => // do nothing
+          case _ => attempts += ((failure.tree, ""))
       if qualType.derivesFrom(defn.DynamicClass) then
         "\npossible cause: maybe a wrong Dynamic method signature?"
       else if attempts.nonEmpty then
@@ -164,9 +170,7 @@ object ErrorReporting {
             .distinctBy(_._1)
             .map((treeStr, whyFailed) =>
               i"""
-                 |    $treeStr    failed with
-                 |
-                 |${whyFailed.whyFailed.message.indented(8)}""")
+                 |    $treeStr$whyFailed""")
         val extMethods =
           if attemptStrings.length > 1 then "Extension methods were"
           else "An extension method was"

tests/neg/i7056.check

@@ -0,0 +1,7 @@
+-- [E008] Not Found Error: tests/neg/i7056.scala:19:10 -----------------------------------------------------------------
+19 |val z = x.idnt1  // error
+   |        ^^^^^^^
+   |        value idnt1 is not a member of B.
+   |        An extension method was tried, but could not be fully constructed:
+   |
+   |            i7056$package.given_T1_T[T](given_PartialId_B).idnt1()

tests/neg/i7056.scala

@@ -0,0 +1,19 @@
+type A
+type B <: A
+
+type PartialId[X] = X match {
+  case B => X
+}
+
+trait T1[T] {
+  extension (t1: T) def idnt1: Any
+}
+
+given [T <: A](using PartialId[T]): T1[T] = new T1[T] {
+  extension (t1: T) def idnt1: Any = ???
+}
+
+given PartialId[B] = ???
+
+val x: B = ???
+val z = x.idnt1  // error