Heal member-select on opaque reference

When the prefix of an opaque isn't the .this reference of the module
class, then its RHS isn't visible.  TypeComparer uses ctx.owner to
"heal" or "lift" this type such that it is.  We reuse that logic for
member selection.

Author: dwijnand

Diff for: compiler/src/dotty/tools/dotc/core/TypeComparer.scala

@@ -1575,7 +1575,7 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
    *  Note: It would be legal to do the lifting also if M does not contain opaque types,
    *  but in this case the retries in tryLiftedToThis would be redundant.
    */
-  private def liftToThis(tp: Type): Type = {
+  def liftToThis(tp: Type): Type = {
 
     def findEnclosingThis(moduleClass: Symbol, from: Symbol): Type =
       if ((from.owner eq moduleClass) && from.isPackageObject && from.is(Opaque)) from.thisType

Diff for: compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -688,72 +688,92 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
     val tree = cpy.Select(tree0)(qual, selName)
     val superAccess = qual.isInstanceOf[Super]
     val rawType = selectionType(tree, qual)
-    val checkedType = accessibleType(rawType, superAccess)
-
-    def finish(tree: untpd.Select, qual: Tree, checkedType: Type): Tree =
-      val select = toNotNullTermRef(assignType(tree, checkedType), pt)
-      if selName.isTypeName then checkStable(qual.tpe, qual.srcPos, "type prefix")
-      checkLegalValue(select, pt)
-      ConstFold(select)
-
-    if checkedType.exists then
-      finish(tree, qual, checkedType)
-    else if selName == nme.apply && qual.tpe.widen.isInstanceOf[MethodType] then
-      // Simplify `m.apply(...)` to `m(...)`
-      qual
-    else if couldInstantiateTypeVar(qual.tpe.widen) then
-       // there's a simply visible type variable in the result; try again with a more defined qualifier type
-       // There's a second trial where we try to instantiate all type variables in `qual.tpe.widen`,
-       // but that is done only after we search for extension methods or conversions.
-      typedSelect(tree, pt, qual)
-    else if qual.tpe.isSmallGenericTuple then
-      val elems = qual.tpe.widenTermRefExpr.tupleElementTypes.getOrElse(Nil)
-      typedSelect(tree, pt, qual.cast(defn.tupleType(elems)))
-    else
-      val tree1 = tryExtensionOrConversion(
-          tree, pt, IgnoredProto(pt), qual, ctx.typerState.ownedVars, this, inSelect = true)
-        .orElse {
-          if ctx.gadt.isNarrowing then
-            // try GADT approximation if we're trying to select a member
-            // Member lookup cannot take GADTs into account b/c of cache, so we
-            // approximate types based on GADT constraints instead. For an example,
-            // see MemberHealing in gadt-approximation-interaction.scala.
-            val wtp = qual.tpe.widen
-            gadts.println(i"Trying to heal member selection by GADT-approximating $wtp")
-            val gadtApprox = Inferencing.approximateGADT(wtp)
-            gadts.println(i"GADT-approximated $wtp ~~ $gadtApprox")
-            val qual1 = qual.cast(gadtApprox)
-            val tree1 = cpy.Select(tree0)(qual1, selName)
-            val checkedType1 = accessibleType(selectionType(tree1, qual1), superAccess = false)
-            if checkedType1.exists then
-              gadts.println(i"Member selection healed by GADT approximation")
-              finish(tree1, qual1, checkedType1)
-            else if qual1.tpe.isSmallGenericTuple then
-              gadts.println(i"Tuple member selection healed by GADT approximation")
-              typedSelect(tree, pt, qual1)
-            else
-              tryExtensionOrConversion(tree1, pt, IgnoredProto(pt), qual1, ctx.typerState.ownedVars, this, inSelect = true)
-          else EmptyTree
-        }
-      if !tree1.isEmpty then
-        tree1
-      else if canDefineFurther(qual.tpe.widen) then
-        typedSelect(tree, pt, qual)
-      else if qual.tpe.derivesFrom(defn.DynamicClass)
-        && selName.isTermName && !isDynamicExpansion(tree)
-      then
-        val tree2 = cpy.Select(tree0)(untpd.TypedSplice(qual), selName)
-        if pt.isInstanceOf[FunOrPolyProto] || pt == LhsProto then
-          assignType(tree2, TryDynamicCallType)
+
+    def tryType(tree: untpd.Select, qual: Tree, rawType: Type) =
+      val checkedType = accessibleType(rawType, superAccess)
+      if checkedType.exists then
+        val select = toNotNullTermRef(assignType(tree, checkedType), pt)
+        if selName.isTypeName then checkStable(qual.tpe, qual.srcPos, "type prefix")
+        checkLegalValue(select, pt)
+        ConstFold(select)
+      else EmptyTree
+
+    def trySmallGenericTuple(tree: untpd.Select, qual: Tree, withCast: Boolean) =
+      if qual.tpe.isSmallGenericTuple then
+        if withCast then
+          val elems = qual.tpe.widenTermRefExpr.tupleElementTypes.getOrElse(Nil)
+          typedSelect(tree, pt, qual.cast(defn.tupleType(elems)))
         else
-          typedDynamicSelect(tree2, Nil, pt)
-      else
-        assignType(tree,
-          rawType match
-            case rawType: NamedType =>
-              inaccessibleErrorType(rawType, superAccess, tree.srcPos)
-            case _ =>
-              notAMemberErrorType(tree, qual, pt))
+          typedSelect(tree, pt, qual)
+      else EmptyTree
+
+    def tryExt(tree: untpd.Select, qual: Tree) =
+      tryExtensionOrConversion(
+        tree, pt, IgnoredProto(pt), qual, ctx.typerState.ownedVars, this, inSelect = true)
+
+    def tryGadt(tree: untpd.Select) =
+      if ctx.gadt.isNarrowing then
+        // try GADT approximation if we're trying to select a member
+        // Member lookup cannot take GADTs into account b/c of cache, so we
+        // approximate types based on GADT constraints instead. For an example,
+        // see MemberHealing in gadt-approximation-interaction.scala.
+        val wtp = qual.tpe.widen
+        gadts.println(i"Trying to heal member selection by GADT-approximating $wtp")
+        val gadtApprox = Inferencing.approximateGADT(wtp)
+        gadts.println(i"GADT-approximated $wtp ~~ $gadtApprox")
+        val qual1 = qual.cast(gadtApprox)
+        val tree1 = cpy.Select(tree0)(qual1, selName)
+        tryType(tree1, qual1, selectionType(tree1, qual1))
+          .orElse(trySmallGenericTuple(tree, qual1, withCast = false))
+          .orElse(tryExt(tree1, qual1))
+      else EmptyTree
+
+    tryType(tree, qual, rawType)
+      .orElse {
+        if selName == nme.apply && qual.tpe.widen.isInstanceOf[MethodType] then
+          // Simplify `m.apply(...)` to `m(...)`
+          qual
+        else EmptyTree
+      }
+      .orElse {
+        if couldInstantiateTypeVar(qual.tpe.widen) then
+          // there's a simply visible type variable in the result; try again with a more defined qualifier type
+          // There's a second trial where we try to instantiate all type variables in `qual.tpe.widen`,
+          // but that is done only after we search for extension methods or conversions.
+          typedSelect(tree, pt, qual)
+        else EmptyTree
+      }
+      .orElse {
+        val wtp = qual.tpe.widen
+        val liftedTp = comparing(_.liftToThis(wtp))
+        if liftedTp ne wtp then
+          val qual1 = qual.cast(liftedTp)
+          val tree1 = cpy.Select(tree0)(qual1, selName)
+          val rawType1 = selectionType(tree1, qual1)
+          tryType(tree1, qual1, rawType1)
+        else EmptyTree
+      }
+      .orElse(trySmallGenericTuple(tree, qual, withCast = true))
+      .orElse(tryExt(tree, qual))
+      .orElse(tryGadt(tree))
+      .orElse:
+        if canDefineFurther(qual.tpe.widen) then
+          typedSelect(tree, pt, qual)
+        else if qual.tpe.derivesFrom(defn.DynamicClass)
+          && selName.isTermName && !isDynamicExpansion(tree)
+        then
+          val tree2 = cpy.Select(tree0)(untpd.TypedSplice(qual), selName)
+          if pt.isInstanceOf[FunOrPolyProto] || pt == LhsProto then
+            assignType(tree2, TryDynamicCallType)
+          else
+            typedDynamicSelect(tree2, Nil, pt)
+        else
+          assignType(tree,
+            rawType match
+              case rawType: NamedType =>
+                inaccessibleErrorType(rawType, superAccess, tree.srcPos)
+              case _ =>
+                notAMemberErrorType(tree, qual, pt))
   end typedSelect
 
   def typedSelect(tree: untpd.Select, pt: Type)(using Context): Tree = {

Diff for: tests/pos/i19609.orig.scala

@@ -0,0 +1,12 @@
+object o {
+  opaque type T = String
+
+  summon[o.T =:= T]      // OK
+  summon[o.T =:= String] // OK
+
+  def test1(t: T): Int =
+    t.length // OK
+
+  def test2(t: o.T): Int =
+    t.length // Error: value length is not a member of Playground.o.T
+}

Diff for: tests/pos/i19609.scala

@@ -0,0 +1,24 @@
+object o { u =>
+  opaque type T = String
+
+  def st = summon[String =:= T]
+  def su = summon[String =:= u.T]
+  def so = summon[String =:= o.T]
+
+  def ts = summon[T =:= String]
+  def tu = summon[T =:= u.T]
+  def to = summon[T =:= o.T]
+
+  def us = summon[u.T =:= String]
+  def ut = summon[u.T =:= T]
+  def uo = summon[u.T =:= o.T]
+
+  def os = summon[o.T =:= String]
+  def ot = summon[o.T =:= T]
+  def ou = summon[o.T =:= u.T]
+
+  def ms(x: String): Int = x.length // ok
+  def mt(x:      T): Int = x.length // ok
+  def mu(x:    u.T): Int = x.length // ok
+  def mo(x:    o.T): Int = x.length // was: error: value length is not a member of o.T
+}