refactor: improve Given search preference warning

**Problem**
It wasn't clear what action users was suppose to take to suppress
the new-from-3.6 Given search preference warning.

**Solution**
1. This refactors the code to give the warning an error code E205.
2. In case of warnings, tell the user to choose -source 3.5 vs 3.7,
   or use nowarn annotation.

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

@@ -218,6 +218,7 @@ enum ErrorMessageID(val isActive: Boolean = true) extends java.lang.Enum[ErrorMe
   case QuotedTypeMissingID // errorNumber: 202
   case DeprecatedAssignmentSyntaxID // errorNumber: 203
   case DeprecatedInfixNamedArgumentSyntaxID // errorNumber: 204
+  case GivenSearchPriorityID // errorNumber: 205
 
   def errorNumber = ordinal - 1
 

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

@@ -3361,3 +3361,41 @@ class DeprecatedInfixNamedArgumentSyntax()(using Context) extends SyntaxMsg(Depr
         + Message.rewriteNotice("This", version = SourceVersion.`3.6-migration`)
 
   def explain(using Context) = ""
+
+class GivenSearchPriorityWarning(
+    pt: Type,
+    cmp: Int,
+    prev: Int,
+    winner: TermRef,
+    loser: TermRef,
+    isLastOldVersion: Boolean
+)(using Context) extends Message(GivenSearchPriorityID):
+  def kind = MessageKind.PotentialIssue
+  def choice(nth: String, c: Int) =
+    if c == 0 then "none - it's ambiguous"
+    else s"the $nth alternative"
+  val (change, whichChoice) =
+    if isLastOldVersion
+    then ("will change in the future release", "Current choice ")
+    else ("has changed",                       "Previous choice")
+  def warningMessage: String =
+    i"""Given search preference for $pt between alternatives
+       |  ${loser}
+       |and
+       |  ${winner}
+       |$change.
+       |$whichChoice       : ${choice("first", prev)}
+       |Choice from Scala 3.7 : ${choice("second", cmp)}"""
+  def migrationHints: String =
+    i"""Suppress this warning by choosing -source 3.5, -source 3.7, or
+       |by using @annotation.nowarn("id=205")"""
+  def ambiguousNote: String =
+    i"""
+       |
+       |Note: $warningMessage"""
+  def msg(using Context) =
+    i"""$warningMessage
+       |
+       |$migrationHints"""
+
+  def explain(using Context) = ""

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

@@ -549,10 +549,10 @@ object Implicits:
   /** An ambiguous implicits failure */
   class AmbiguousImplicits(val alt1: SearchSuccess, val alt2: SearchSuccess, val expectedType: Type, val argument: Tree, val nested: Boolean = false) extends SearchFailureType:
 
-    private[Implicits] var priorityChangeWarnings: List[Message] = Nil
+    private[Implicits] var priorityChangeWarnings: List[GivenSearchPriorityWarning] = Nil
 
     def priorityChangeWarningNote(using Context): String =
-      priorityChangeWarnings.map(msg => s"\n\nNote: $msg").mkString
+      priorityChangeWarnings.map(_.ambiguousNote).mkString
 
     def msg(using Context): Message =
       var str1 = err.refStr(alt1.ref)
@@ -1312,7 +1312,7 @@ trait Implicits:
       // A map that associates a priority change warning (between -source 3.6 and 3.7)
       // with the candidate refs mentioned in the warning. We report the associated
       // message if one of the critical candidates is part of the result of the implicit search.
-      val priorityChangeWarnings = mutable.ListBuffer[(/*critical:*/ List[TermRef], Message)]()
+      val priorityChangeWarnings = mutable.ListBuffer[(/*critical:*/ List[TermRef], GivenSearchPriorityWarning)]()
 
       val sv = Feature.sourceVersion
       val isLastOldVersion = sv.stable == SourceVersion.`3.6`
@@ -1353,21 +1353,7 @@ trait Implicits:
                     cmp match
                       case 1 => (alt2, alt1)
                       case -1 => (alt1, alt2)
-              def choice(nth: String, c: Int) =
-                if c == 0 then  "none - it's ambiguous"
-                else s"the $nth alternative"
-              val (change, whichChoice) =
-                if isLastOldVersion
-                then ("will change", "Current choice ")
-                else ("has changed", "Previous choice")
-              val msg =
-                em"""Given search preference for $pt between alternatives
-                    |  ${loser.ref}
-                    |and
-                    |  ${winner.ref}
-                    |$change.
-                    |$whichChoice          : ${choice("first", prev)}
-                    |New choice from Scala 3.7: ${choice("second", cmp)}"""
+              val msg = GivenSearchPriorityWarning(pt, cmp, prev, winner.ref, loser.ref, isLastOldVersion)
               val critical = alt1.ref :: alt2.ref :: Nil
               priorityChangeWarnings += ((critical, msg))
               if isLastOldVersion then prev else cmp

tests/neg/given-triangle.check

@@ -7,6 +7,6 @@
    |  (given_B : B)
    |and
    |  (given_A : A)
-   |will change.
-   |Current choice           : the first alternative
-   |New choice from Scala 3.7: the second alternative
+   |will change in the future release.
+   |Current choice        : the first alternative
+   |Choice from Scala 3.7 : the second alternative

tests/warn/i21036a.check

@@ -1,10 +1,13 @@
--- Warning: tests/warn/i21036a.scala:7:17 ------------------------------------------------------------------------------
+-- [E205] Potential Issue Warning: tests/warn/i21036a.scala:7:17 -------------------------------------------------------
 7 |val y = summon[A] // warn
   |                 ^
   |                 Given search preference for A between alternatives
   |                   (b : B)
   |                 and
   |                   (a : A)
-  |                 will change.
-  |                 Current choice           : the first alternative
-  |                 New choice from Scala 3.7: the second alternative
+  |                 will change in the future release.
+  |                 Current choice        : the first alternative
+  |                 Choice from Scala 3.7 : the second alternative
+  |
+  |                 Suppress this warning by choosing -source 3.5, -source 3.7, or
+  |                 by using @annotation.nowarn("id=205")

tests/warn/i21036b.check

@@ -1,10 +1,13 @@
--- Warning: tests/warn/i21036b.scala:7:17 ------------------------------------------------------------------------------
+-- [E205] Potential Issue Warning: tests/warn/i21036b.scala:7:17 -------------------------------------------------------
 7 |val y = summon[A] // warn
   |                 ^
   |                 Given search preference for A between alternatives
   |                   (b : B)
   |                 and
   |                   (a : A)
   |                 has changed.
-  |                 Previous choice          : the first alternative
-  |                 New choice from Scala 3.7: the second alternative
+  |                 Previous choice       : the first alternative
+  |                 Choice from Scala 3.7 : the second alternative
+  |
+  |                 Suppress this warning by choosing -source 3.5, -source 3.7, or
+  |                 by using @annotation.nowarn("id=205")

tests/warn/i21036c.scala

@@ -0,0 +1,7 @@
+trait A
+trait B extends A
+given b: B = ???
+given a: A = ???
+
+@annotation.nowarn("id=205")
+val y = summon[A] // don't warn