Type ascribe trees that require opaque type usage

compiler/src/dotty/tools/dotc/core/TypeComparer.scala

@@ -46,6 +46,7 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
     state = c.typerState
     monitored = false
     GADTused = false
+    opaquesUsed = false
     recCount = 0
     needsGc = false
     if Config.checkTypeComparerReset then checkReset()
@@ -61,6 +62,9 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
   /** Indicates whether the subtype check used GADT bounds */
   private var GADTused: Boolean = false
 
+  /** Indicates whether the subtype check used opaque types */
+  private var opaquesUsed: Boolean = false
+
   private var myInstance: TypeComparer = this
   def currentInstance: TypeComparer = myInstance
 
@@ -142,8 +146,10 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
 
   def testSubType(tp1: Type, tp2: Type): CompareResult =
     GADTused = false
+    opaquesUsed = false
     if !topLevelSubType(tp1, tp2) then CompareResult.Fail
     else if GADTused then CompareResult.OKwithGADTUsed
+    else if opaquesUsed then CompareResult.OKwithOpaquesUsed // we cast on GADTused, so handles if both are used
     else CompareResult.OK
 
   /** The current approximation state. See `ApproxState`. */
@@ -1483,12 +1489,12 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
 
     def tryLiftedToThis1: Boolean = {
       val tp1a = liftToThis(tp1)
-      (tp1a ne tp1) && recur(tp1a, tp2)
+      (tp1a ne tp1) && recur(tp1a, tp2) && { opaquesUsed = true; true }
     }
 
     def tryLiftedToThis2: Boolean = {
       val tp2a = liftToThis(tp2)
-      (tp2a ne tp2) && recur(tp1, tp2a)
+      (tp2a ne tp2) && recur(tp1, tp2a) && { opaquesUsed = true; true }
     }
 
     // begin recur
@@ -2935,7 +2941,7 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
 object TypeComparer {
 
   enum CompareResult:
-    case OK, Fail, OKwithGADTUsed
+    case OK, Fail, OKwithGADTUsed, OKwithOpaquesUsed
 
   /** Class for unification variables used in `natValue`. */
   private class AnyConstantType extends UncachedGroundType with ValueType {

compiler/src/dotty/tools/dotc/transform/TreeChecker.scala

@@ -1,4 +1,5 @@
-package dotty.tools.dotc
+package dotty.tools
+package dotc
 package transform
 
 import core.Names.Name

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

@@ -4063,6 +4063,9 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
                res
               } =>
             insertGadtCast(tree, wtp, pt)
+          case CompareResult.OKwithOpaquesUsed if !tree.tpe.frozen_<:<(pt)(using ctx.withOwner(defn.RootClass)) =>
+            // guard to avoid extra Typed trees, eg. from testSubType(O.T, O.T) which returns OKwithOpaquesUsed
+            Typed(tree, TypeTree(pt))
           case _ =>
             //typr.println(i"OK ${tree.tpe}\n${TypeComparer.explained(_.isSubType(tree.tpe, pt))}") // uncomment for unexpected successes
             tree

tests/pos/i17948.all.scala

@@ -0,0 +1,44 @@
+object O:
+  opaque type T = Int
+
+  inline def get0: Int = Do.get              // no proxy needed
+  inline def get1: Int = Do.get: O.T         // no proxy needed
+  inline def get2: Int = Do.get:   T         // proxied
+
+  inline def set0: Unit = Do.set(0)          // was: broken
+  inline def set1: Unit = Do.set(1: O.T)     // no proxy needed
+  inline def set2: Unit = Do.set(2:   T)     // proxied
+
+  inline def mod0: Int = Do.mod(0)           // was: broken
+  inline def mod1: Int = Do.mod(1): O.T      // was: broken
+  inline def mod2: Int = Do.mod(2):   T      // was: broken
+  inline def mod3: Int = Do.mod(3: O.T)      // no proxy needed
+  inline def mod4: Int = Do.mod(4: O.T): O.T // no proxy needed
+  inline def mod5: Int = Do.mod(5: O.T):   T // proxied
+  inline def mod6: Int = Do.mod(6:   T)      // proxied
+  inline def mod7: Int = Do.mod(7:   T): O.T // proxied
+  inline def mod8: Int = Do.mod(8:   T):   T // proxied
+
+class Test:
+  def testGet0: Int = O.get0
+  def testGet1: Int = O.get1
+  def testGet2: Int = O.get2
+
+  def testSet0: Unit = O.set0
+  def testSet1: Unit = O.set1
+  def testSet2: Unit = O.set2
+
+  def testMod0: Int = O.mod0
+  def testMod1: Int = O.mod1
+  def testMod2: Int = O.mod2
+  def testMod3: Int = O.mod3
+  def testMod4: Int = O.mod4
+  def testMod5: Int = O.mod5
+  def testMod6: Int = O.mod6
+  def testMod7: Int = O.mod7
+  def testMod8: Int = O.mod8
+
+object Do:
+  def get: O.T          = ???
+  def set(x: O.T): Unit = ()
+  def mod(x: O.T): O.T  = x

tests/pos/i17948.scala

@@ -0,0 +1,12 @@
+object O:
+  opaque type T = Int
+  inline def x: Int = P.id(2)
+
+object P:
+  def id(x: O.T): O.T = x
+
+object Test {
+  def main(args: Array[String]): Unit = println(foo())
+
+  def foo(): Int = O.x
+}