Capture the kse3 issue in test cases and close it

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/neg/i21239.check

@@ -0,0 +1,7 @@
+-- [E007] Type Mismatch Error: tests/neg/i21239.scala:14:18 ------------------------------------------------------------
+14 |    def get2: V = get // error
+   |                  ^^^
+   |                  Found:    AnyRef
+   |                  Required: V
+   |
+   | longer explanation available when compiling with `-explain`

tests/neg/i21239.orig.check

@@ -0,0 +1,7 @@
+-- [E007] Type Mismatch Error: tests/neg/i21239.orig.scala:32:8 --------------------------------------------------------
+32 |        get // error
+   |        ^^^
+   |        Found:    AnyRef
+   |        Required: V
+   |
+   | longer explanation available when compiling with `-explain`

tests/neg/i21239.orig.scala

@@ -0,0 +1,33 @@
+// 1
+// A re-minimisated reproduction of the original issue in kse3
+// The one in the issue removes the usage of the package
+// in the second extension bundle, which is crucial to
+// why my change broke this code
+package kse.flow
+
+import java.util.concurrent.atomic.AtomicReference
+
+opaque type Worm[V] = AtomicReference[AnyRef]
+object Worm:
+  val notSetSentinel: AnyRef = new AnyRef {}
+
+  extension [V](worm: Worm[V])
+    inline def wormAsAtomic: AtomicReference[AnyRef] = worm
+
+  extension [V](worm: kse.flow.Worm[V])
+
+    inline def setIfEmpty(v: => V): Boolean =
+      var old = worm.wormAsAtomic.get()
+      if old eq Worm.notSetSentinel then
+        worm.wormAsAtomic.compareAndSet(old, v.asInstanceOf[AnyRef])
+      else false
+
+    inline def get: V = worm.wormAsAtomic.get() match
+      case x if x eq Worm.notSetSentinel => throw new java.lang.IllegalStateException("Retrieved value before being set")
+      case x                             => x.asInstanceOf[V]
+
+    inline def getOrSet(v: => V): V = worm.wormAsAtomic.get() match
+      case x if x eq Worm.notSetSentinel =>
+        setIfEmpty(v)
+        get // error
+      case x => x.asInstanceOf[V]

tests/neg/i21239.scala

@@ -0,0 +1,14 @@
+// 2
+// A more minimised reproduction
+package lib
+
+import java.util.concurrent.atomic.AtomicReference
+
+opaque type Worm[V] = AtomicReference[AnyRef]
+object Worm:
+  extension [V](worm: Worm[V])
+    inline def wormAsAtomic: AtomicReference[AnyRef] = worm
+
+  extension [V](worm: lib.Worm[V])
+    def get: V  = worm.wormAsAtomic.get().asInstanceOf[V]
+    def get2: V = get // error

tests/pos/i21239.alt.scala

@@ -0,0 +1,28 @@
+// 4
+// An alternative way to fix it,
+// defining the extension method externally,
+// in a scope that doesn't see through
+// the opaque type definition.
+// The setup here also makes sure those extension
+// are on the opaque type's companion object
+// (via class extension), meaning that they continue
+// to be in implicit scope (as enforced by the usage test)
+import java.util.concurrent.atomic.AtomicReference
+
+package lib:
+  object Worms:
+    opaque type Worm[V] = AtomicReference[AnyRef]
+    object Worm extends WormOps:
+      extension [V](worm: Worm[V])
+        inline def wormAsAtomic: AtomicReference[AnyRef] = worm
+
+  import Worms.Worm
+  trait WormOps:
+    extension [V](worm: Worm[V])
+      def get: V  = worm.wormAsAtomic.get().asInstanceOf[V]
+      def get2: V = get
+
+package test:
+  import lib.Worms.Worm
+  object Test:
+    def usage(worm: Worm[String]): String = worm.get2

tests/pos/i21239.orig.scala

@@ -0,0 +1,34 @@
+// 5
+// Finally, an alternative way to fix the original issue,
+// by reimplementing `getOrSet` to not even need
+// our `get` extension.
+import java.util.concurrent.atomic.AtomicReference
+
+opaque type Worm[V] = AtomicReference[AnyRef]
+object Worm:
+  val notSetSentinel: AnyRef = new AnyRef {}
+
+  extension [V](worm: Worm[V])
+    inline def wormAsAtomic: AtomicReference[AnyRef] = worm // deprecate?
+
+    inline def setIfEmpty(v: => V): Boolean =
+      val x = worm.get()
+      if x eq notSetSentinel then
+        val value = v
+        worm.set(value.asInstanceOf[AnyRef])
+        true
+      else false
+
+    inline def get: V =
+      val x = worm.get()
+      if x eq notSetSentinel then
+        throw IllegalStateException("Retrieved value before being set")
+      else x.asInstanceOf[V]
+
+    inline def getOrSet(v: => V): V =
+      val x = worm.get()
+      if x eq notSetSentinel then
+        val value = v
+        worm.set(value.asInstanceOf[AnyRef])
+        value
+      else x.asInstanceOf[V]

tests/pos/i21239.scala

@@ -0,0 +1,18 @@
+// 3
+// One way to fix the issue, using the
+// "universal function call syntax"
+// (to borrow from what Rust calls the syntax to
+// disambiguate which trait's method is intended.)
+import java.util.concurrent.atomic.AtomicReference
+
+package lib:
+  opaque type Worm[V] = AtomicReference[AnyRef]
+  object Worm:
+    extension [V](worm: Worm[V])
+      def get: V  = worm.get().asInstanceOf[V]
+      def get2: V = Worm.get(worm)
+
+package test:
+  import lib.Worm
+  object Test:
+    def usage(worm: Worm[String]): String = worm.get2

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
+}