Give full info for nested ambiguities

Author: odersky

compiler/src/dotty/tools/dotc/printing/RefinedPrinter.scala

@@ -351,7 +351,8 @@ class RefinedPrinter(_ctx: Context) extends PlainPrinter(_ctx) {
       case id: Trees.SearchFailureIdent[?] =>
         tree.typeOpt match {
           case reason: Implicits.SearchFailureType =>
-            toText(id.name) ~ "summon[" ~ toText(reason.clarify(reason.expectedType)) ~ "]"
+            toText(id.name)
+            ~ ("summon[" ~ toText(reason.clarify(reason.expectedType)) ~ "]").close
           case _ =>
             toText(id.name)
         }

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

@@ -379,10 +379,10 @@ object Implicits {
   }
 
   object SearchFailure {
-    def apply(tpe: SearchFailureType)(implicit src: SourceFile): SearchFailure = {
-      val id =
-        if (tpe.isInstanceOf[AmbiguousImplicits]) "/* ambiguous */"
-        else "/* missing */"
+    def apply(tpe: SearchFailureType)(using Context): SearchFailure = {
+      val id = tpe match
+        case tpe: AmbiguousImplicits => i"/* ambiguous: ${tpe.explanation} */"
+        case _ => "/* missing */"
       SearchFailure(untpd.SearchFailureIdent(id.toTermName).withTypeUnchecked(tpe))
     }
   }
@@ -451,7 +451,7 @@ object Implicits {
   @sharable object NoMatchingImplicits extends NoMatchingImplicits(NoType, EmptyTree, OrderingConstraint.empty)
 
   @sharable val NoMatchingImplicitsFailure: SearchFailure =
-    SearchFailure(NoMatchingImplicits)(NoSource)
+    SearchFailure(NoMatchingImplicits)(using NoContext)
 
   /** An ambiguous implicits failure */
   class AmbiguousImplicits(val alt1: SearchSuccess, val alt2: SearchSuccess, val expectedType: Type, val argument: Tree) extends SearchFailureType {

tests/neg/i9185.check

@@ -1,10 +1,12 @@
 -- [E008] Not Found Error: tests/neg/i9185.scala:7:21 ------------------------------------------------------------------
 7 |  val value2 = "ola".pure  // error
   |               ^^^^^^^^^^
-  |               value pure is not a member of String.
-  |               An extension method was tried, but could not be fully constructed:
+  |value pure is not a member of String.
+  |An extension method was tried, but could not be fully constructed:
   |
-  |                   M.pure[A, F]("ola")(/* ambiguous */summon[M[F]])
+  |    M.pure[A, F]("ola")(
+  |      /* ambiguous: both object listMonad in object M and object optionMonad in object M match type M[F] */summon[M[F]]
+  |    )
 -- Error: tests/neg/i9185.scala:8:26 -----------------------------------------------------------------------------------
 8 |  val value3 = pure("ola") // error
   |                          ^