Output "no best given instance" for ambiguous implicit error messages

Author: mbovel

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

@@ -2938,11 +2938,20 @@ class MissingImplicitArgument(
 
     def location(preposition: String) = if (where.isEmpty) "" else s" $preposition $where"
 
+    /** Default error message for non-nested ambiguous implicits. */
     def defaultAmbiguousImplicitMsg(ambi: AmbiguousImplicits) =
       s"Ambiguous given instances: ${ambi.explanation}${location("of")}"
 
+    /** Default error messages for non-ambiguous implicits, or nested ambiguous
+     *  implicits.
+     *
+     *  The default message is shown for ambiguous implicits only if they have
+     *  the `nested` flag set. In this case, we output "no best given instance"
+     *  instead of "no given instance".
+     */
     def defaultImplicitNotFoundMessage =
-      i"No given instance of type $pt was found${location("for")}"
+      val bestStr = if arg.tpe.isInstanceOf[AmbiguousImplicits] then " best" else ""
+      i"No$bestStr 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`.

tests/neg/19414-desugared.check

@@ -1,7 +1,7 @@
 -- [E172] Type Error: tests/neg/19414-desugared.scala:22:34 ------------------------------------------------------------
 22 |  summon[BodySerializer[JsObject]] // error: Ambiguous given instances
    |                                  ^
-   |No given instance of type BodySerializer[JsObject] was found for parameter x of method summon in object Predef.
+   |No best given instance of type BodySerializer[JsObject] was found for parameter x of method summon in object Predef.
    |I found:
    |
    |    given_BodySerializer_B[B](

tests/neg/19414.check

@@ -1,7 +1,7 @@
 -- [E172] Type Error: tests/neg/19414.scala:15:34 ----------------------------------------------------------------------
 15 |  summon[BodySerializer[JsObject]] // error: Ambiguous given instances
    |                                  ^
-   |No given instance of type BodySerializer[JsObject] was found for parameter x of method summon in object Predef.
+   |No best given instance of type BodySerializer[JsObject] was found for parameter x of method summon in object Predef.
    |I found:
    |
    |    given_BodySerializer_B[B](

tests/neg/given-ambiguous-1.check

@@ -1,7 +1,7 @@
 -- [E172] Type Error: tests/neg/given-ambiguous-1.scala:12:23 ----------------------------------------------------------
 12 |def f: Unit = summon[B] // error: Ambiguous given instances
    |                       ^
-   |                No given instance of type B was found for parameter x of method summon in object Predef.
+   |                No best given instance of type B was found for parameter x of method summon in object Predef.
    |                I found:
    |
    |                    given_B(/* ambiguous: both given instance a1 and given instance a2 match type A */summon[A])

tests/neg/given-ambiguous-default-1.check

@@ -1,7 +1,7 @@
 -- [E172] Type Error: tests/neg/given-ambiguous-default-1.scala:18:23 --------------------------------------------------
 18 |def f: Unit = summon[B] // error: Ambiguous given instances
    |                       ^
-   |            No given instance of type B was found for parameter x of method summon in object Predef.
+   |            No best given instance of type B was found for parameter x of method summon in object Predef.
    |            I found:
    |
    |                given_B(a = /* ambiguous: both given instance a1 and given instance a2 match type A */summon[A])

tests/neg/given-ambiguous-default-2.check

@@ -1,7 +1,7 @@
 -- [E172] Type Error: tests/neg/given-ambiguous-default-2.scala:18:23 --------------------------------------------------
 18 |def f: Unit = summon[C] // error: Ambiguous given instances
    |                       ^
-   |No given instance of type C was found for parameter x of method summon in object Predef.
+   |No best given instance of type C was found for parameter x of method summon in object Predef.
    |I found:
    |
    |    given_C(a = /* ambiguous: both given instance a1 and given instance a2 match type A */summon[A], this.given_C$default$2)