Add priority change warnings to ambiguous implicits error messages

Previously warnings were produced but not shown since at the same position we already have an ambiguity
error. We now add the note to the error message.

Author: odersky

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

@@ -2988,7 +2988,7 @@ class MissingImplicitArgument(
 
     /** Default error message for non-nested ambiguous implicits. */
     def defaultAmbiguousImplicitMsg(ambi: AmbiguousImplicits) =
-      s"Ambiguous given instances: ${ambi.explanation}${location("of")}"
+      s"Ambiguous given instances: ${ambi.explanation}${location("of")}${ambi.priorityChangeWarningNote}"
 
     /** Default error messages for non-ambiguous implicits, or nested ambiguous
      *  implicits.

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

@@ -549,6 +549,12 @@ 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 priorityChangeWarning: Message | Null = null
+
+    def priorityChangeWarningNote(using Context): String =
+      if priorityChangeWarning != null then s"\n\nNote: $priorityChangeWarning"
+      else ""
+
     def msg(using Context): Message =
       var str1 = err.refStr(alt1.ref)
       var str2 = err.refStr(alt2.ref)
@@ -1348,13 +1354,21 @@ trait Implicits:
                 case _  => "none - it's ambiguous"
               if sv.stable == SourceVersion.`3.5` then
                 warn(
-                  em"""Given search preference for $pt between alternatives ${alt1.ref} and ${alt2.ref} will change
+                  em"""Given search preference for $pt between alternatives
+                      |  ${alt1.ref}
+                      |and
+                      |  ${alt2.ref}
+                      |will change.
                       |Current choice           : ${choice(prev)}
                       |New choice from Scala 3.6: ${choice(cmp)}""")
                 prev
               else
                 warn(
-                  em"""Change in given search preference for $pt between alternatives ${alt1.ref} and ${alt2.ref}
+                  em"""Given search preference for $pt between alternatives
+                      |  ${alt1.ref}
+                      |and
+                      |  ${alt2.ref}
+                      |has changed.
                       |Previous choice          : ${choice(prev)}
                       |New choice from Scala 3.6: ${choice(cmp)}""")
                 cmp
@@ -1615,6 +1629,12 @@ trait Implicits:
       val result = rank(sort(eligible), NoMatchingImplicitsFailure, Nil)
       for (critical, msg) <- priorityChangeWarnings do
         if result.found.exists(critical.contains(_)) then
+          result match
+            case result: SearchFailure =>
+              result.reason match
+                case ambi: AmbiguousImplicits => ambi.priorityChangeWarning = msg
+                case _ =>
+            case _ =>
           report.warning(msg, srcPos)
       result
     end searchImplicit

tests/neg/given-triangle.check

@@ -2,3 +2,11 @@
 15 |@main def Test = f  // error
    |                  ^
    |Ambiguous given instances: both given instance given_B and given instance given_C match type A of parameter a of method f
+   |
+   |Note: Given search preference for A between alternatives
+   |  (given_A : A)
+   |and
+   |  (given_B : B)
+   |will change.
+   |Current choice           : the second alternative
+   |New choice from Scala 3.6: the first alternative

tests/neg/i21303/JavaEnum.java

@@ -0,0 +1 @@
+public enum JavaEnum { ABC, DEF, GHI }

tests/neg/i21303/Test.scala

@@ -0,0 +1,33 @@
+//> using options -source 3.6-migration
+import scala.deriving.Mirror
+import scala.compiletime.*
+import scala.reflect.ClassTag
+import scala.annotation.implicitNotFound
+
+
+trait TSType[T]
+object TSType extends DefaultTSTypes with TSTypeMacros
+
+trait TSNamedType[T] extends TSType[T]
+
+trait DefaultTSTypes extends JavaTSTypes
+trait JavaTSTypes {
+  given javaEnumTSType[E <: java.lang.Enum[E]: ClassTag]: TSNamedType[E] = ???
+}
+object DefaultTSTypes extends DefaultTSTypes
+trait TSTypeMacros {
+  inline given [T: Mirror.Of]: TSType[T] = derived[T]
+  inline def derived[T](using m: Mirror.Of[T]): TSType[T] = {
+    val elemInstances = summonAll[m.MirroredElemTypes]
+    ???
+  }
+
+  private inline def summonAll[T <: Tuple]: List[TSType[_]] = {
+    inline erasedValue[T] match {
+      case _: EmptyTuple => Nil
+      case _: (t *: ts)  => summonInline[TSType[t]] :: summonAll[ts]
+    }
+  }
+}
+
+@main def Test = summon[TSType[JavaEnum]] // error

tests/pos/i21303/JavaEnum.java

@@ -0,0 +1 @@
+public enum JavaEnum { ABC, DEF, GHI }

tests/pos/i21303/Test.scala

@@ -0,0 +1,32 @@
+import scala.deriving.Mirror
+import scala.compiletime.*
+import scala.reflect.ClassTag
+import scala.annotation.implicitNotFound
+
+
+trait TSType[T]
+object TSType extends DefaultTSTypes with TSTypeMacros
+
+trait TSNamedType[T] extends TSType[T]
+
+trait DefaultTSTypes extends JavaTSTypes
+trait JavaTSTypes {
+  given javaEnumTSType[E <: java.lang.Enum[E]: ClassTag]: TSType[E] = ???
+}
+object DefaultTSTypes extends DefaultTSTypes
+trait TSTypeMacros {
+  inline given [T: Mirror.Of]: TSType[T] = derived[T]
+  inline def derived[T](using m: Mirror.Of[T]): TSType[T] = {
+    val elemInstances = summonAll[m.MirroredElemTypes]
+    ???
+  }
+
+  private inline def summonAll[T <: Tuple]: List[TSType[_]] = {
+    inline erasedValue[T] match {
+      case _: EmptyTuple => Nil
+      case _: (t *: ts)  => summonInline[TSType[t]] :: summonAll[ts]
+    }
+  }
+}
+
+@main def Test = summon[TSType[JavaEnum]]

tests/warn/i21036a.check

@@ -1,6 +1,10 @@
 -- 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
+  |                 Given search preference for A between alternatives
+  |                   (b : B)
+  |                 and
+  |                   (a : A)
+  |                 will change.
   |                 Current choice           : the first alternative
   |                 New choice from Scala 3.6: the second alternative

tests/warn/i21036b.check

@@ -1,6 +1,10 @@
 -- Warning: tests/warn/i21036b.scala:7:17 ------------------------------------------------------------------------------
 7 |val y = summon[A] // warn
   |                 ^
-  |                 Change in given search preference for A between alternatives (b : B) and (a : A)
+  |                 Given search preference for A between alternatives
+  |                   (b : B)
+  |                 and
+  |                   (a : A)
+  |                 has changed.
   |                 Previous choice          : the first alternative
   |                 New choice from Scala 3.6: the second alternative