Add a -3.6-migration warning for opaque select changes

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

@@ -767,7 +767,22 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
         val qual1 = qual.cast(liftedTp)
         val tree1 = cpy.Select(tree0)(qual1, selName)
         val rawType1 = selectionType(tree1, qual1)
-        tryType(tree1, qual1, rawType1)
+        val adapted = tryType(tree1, qual1, rawType1)
+        if !adapted.isEmpty && sourceVersion == `3.6-migration` then
+          val adaptedOld = tryExt(tree, qual)
+          if !adaptedOld.isEmpty then
+            val symOld = adaptedOld.symbol
+            val underlying = liftedTp match
+              case tp: TypeProxy => i" ${tp.translucentSuperType}"
+              case _ => ""
+            report.migrationWarning(
+              em"""Previously this selected the extension ${symOld}${symOld.showExtendedLocation}
+                  |Now it selects $selName on the opaque type's underlying type$underlying
+                  |
+                  |You can change this back by selecting $adaptedOld
+                  |Or by defining the extension method outside of the opaque type's scope.
+                  |""", tree0)
+        adapted
       else EmptyTree
 
     // Otherwise, try to expand a named tuple selection

tests/warn/i21239.Frac.check

@@ -0,0 +1,8 @@
+-- Migration Warning: tests/warn/i21239.Frac.scala:14:8 ----------------------------------------------------------------
+14 |      f + Frac.wrap(((-g.numerator).toLong << 32) | (g.unwrap & 0xFFFFFFFFL)) // warn
+   |      ^^^
+   |      Previously this selected the extension method + in object Frac
+   |      Now it selects + on the opaque type's underlying type Long
+   |
+   |      You can change this back by selecting kse.maths.Frac.+(f)
+   |      Or by defining the extension method outside of the opaque type's scope.

tests/warn/i21239.Frac.scala

@@ -0,0 +1,15 @@
+package kse.maths
+
+import scala.language.`3.6-migration`
+
+opaque type Frac = Long
+object Frac {
+  inline def wrap(f: Long): kse.maths.Frac = f
+  extension (f: Frac)
+    inline def unwrap: Long = f
+    inline def numerator: Int = ((f: Long) >>> 32).toInt
+  extension (f: kse.maths.Frac)
+    def +(g: Frac): kse.maths.Frac = f // eliding domain-specific addition logic
+    def -(g: Frac): kse.maths.Frac =
+      f + Frac.wrap(((-g.numerator).toLong << 32) | (g.unwrap & 0xFFFFFFFFL)) // warn
+}