Final versioning scheme

Author: odersky

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

@@ -1749,12 +1749,13 @@ trait Applications extends Compatibility {
         val tp1p = prepare(tp1)
         val tp2p = prepare(tp2)
 
-        if Feature.sourceVersion.isAtMost(SourceVersion.`3.3`) // !!! change to 3.4
+        if Feature.sourceVersion.isAtMost(SourceVersion.`3.4`)
             || oldResolution
             || !compareGivens
         then
           // Intermediate rules: better means specialize, but map all type arguments downwards
-          // These are enabled for 3.0-3.4, and in 3.5-migration when we compare with previous rules
+          // These are enabled for 3.0-3.4, and for all comparisons between old-style implicits,
+          // and in 3.5-migration when we compare with previous rules.
           val flip = new TypeMap:
             def apply(t: Type) = t match
               case t @ AppliedType(tycon, args) =>

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

@@ -1290,27 +1290,22 @@ trait Implicits:
        */
       def compareAlternatives(alt1: RefAndLevel, alt2: RefAndLevel): Int =
         def comp(using Context) = explore(compare(alt1.ref, alt2.ref, preferGeneral = true))
-        def oldCompare() = comp(using searchContext().addMode(Mode.OldOverloadingResolution))
-        def newCompare() = comp(using searchContext())
-
         if alt1.ref eq alt2.ref then 0
         else if alt1.level != alt2.level then alt1.level - alt2.level
-        else if Feature.sourceVersion.isAtMost(SourceVersion.`3.3`) then // !!! change to 3.4
-          oldCompare()
-        else if Feature.sourceVersion == SourceVersion.`3.5-migration` || true then // !!! drop
-          val was = oldCompare()
-          val now = newCompare()
-          if was != now then
-            def choice(cmp: Int) = cmp match
-              case -1 => "the second alternative"
-              case  1 => "the first alternative"
-              case _  => "none - it's ambiguous"
-            report.warning(
-              em"""Change in given search preference for $pt between alternatives ${alt1.ref} and ${alt2.ref}
-                  |Previous choice: ${choice(was)}
-                  |New choice     : ${choice(now)}""", srcPos)
-          now
-        else newCompare()
+        else
+          val cmp = comp(using searchContext())
+          if Feature.sourceVersion == SourceVersion.`3.5-migration` then
+            val prev = comp(using searchContext().addMode(Mode.OldOverloadingResolution))
+            if cmp != prev then
+              def choice(c: Int) = c match
+                case -1 => "the second alternative"
+                case  1 => "the first alternative"
+                case _  => "none - it's ambiguous"
+              report.warning(
+                em"""Change in given search preference for $pt between alternatives ${alt1.ref} and ${alt2.ref}
+                    |Previous choice: ${choice(prev)}
+                    |New choice     : ${choice(cmp)}""", srcPos)
+          cmp
       end compareAlternatives
 
       /** If `alt1` is also a search success, try to disambiguate as follows:

docs/_docs/reference/changed-features/implicit-resolution.md

@@ -163,8 +163,20 @@ The new rules are as follows: An implicit `a` defined in `A` is more specific th
 
 Condition (*) is new. It is necessary to ensure that the defined relation is transitive.
 
+[//]: # todo: expand with precise rules
 
 
+**9.** Given disambiguation has changed. When comparing two givens that both match an expected type, we used to pick the most specific one, in alignment with
+overloading resolution. From Scala 3.5 on, we pick the most general one instead. Compiling with Scala 3.5-migration will print a warning in all cases where the preference has changed. Example:
+```scala
+class A
+class B extends A
+class C extends A
 
+given A = A()
+given B = B()
+given C = C()
+
+summon[A]  // was ambiguous, will now return `given_A`
+```
 
-[//]: # todo: expand with precise rules

tests/neg/i15264.scala

@@ -1,3 +1,4 @@
+import language.`3.5`
 object priority:
     // lower number = higher priority
     class Prio0 extends Prio1

tests/neg/implicit-arg.scala

@@ -0,0 +1,7 @@
+class C
+
+def foo(using C) = ()
+
+def Test =
+  val c = C()
+  foo(c)

tests/run/given-triangle.scala

@@ -1,3 +1,5 @@
+import language.`3.5`
+
 class A
 class B extends A
 class C extends A

tests/run/implicit-specifity.scala

@@ -1,3 +1,5 @@
+import language.`3.5`
+
 case class Show[T](val i: Int)
 object Show {
   def apply[T](implicit st: Show[T]): Int = st.i

tests/run/implied-priority.scala

@@ -1,5 +1,6 @@
 /* These tests show various mechanisms available for implicit prioritization.
  */
+import language.`3.5`
 
 class E[T](val str: String)  // The type for which we infer terms below