Fix BiTypeMap#inverse

Author: odersky

compiler/src/dotty/tools/dotc/cc/CaptureSet.scala

@@ -633,7 +633,7 @@ object CaptureSet:
      */
     override def computeApprox(origin: CaptureSet)(using Context): CaptureSet =
       val supApprox = super.computeApprox(this)
-      if source eq origin then supApprox.map(bimap.inverseTypeMap)
+      if source eq origin then supApprox.map(bimap.inverse)
       else source.upperApprox(this).map(bimap) ** supApprox
 
     override def toString = s"BiMapped$id($source, elems = $elems)"

compiler/src/dotty/tools/dotc/cc/CheckCaptures.scala

@@ -97,6 +97,7 @@ object CheckCaptures:
 
   final class SubstParamsBiMap(from: LambdaType, to: List[Type])(using Context)
   extends BiTypeMap:
+    thisMap =>
 
     def apply(tp: Type): Type = tp match
       case tp: ParamRef =>
@@ -109,20 +110,22 @@ object CheckCaptures:
       case _ =>
         mapOver(tp)
 
-    def inverse(tp: Type): Type = tp match
-      case tp: NamedType =>
-        var idx = 0
-        var to1 = to
-        while idx < to.length && (tp ne to(idx)) do
-          idx += 1
-          to1 = to1.tail
-        if idx < to.length then from.paramRefs(idx)
-        else if tp.prefix `eq` NoPrefix then tp
-        else tp.derivedSelect(apply(tp.prefix))
-      case _: ThisType =>
-        tp
-      case _ =>
-        mapOver(tp)
+    lazy val inverse = new BiTypeMap:
+      def apply(tp: Type): Type = tp match
+        case tp: NamedType =>
+          var idx = 0
+          var to1 = to
+          while idx < to.length && (tp ne to(idx)) do
+            idx += 1
+            to1 = to1.tail
+          if idx < to.length then from.paramRefs(idx)
+          else if tp.prefix `eq` NoPrefix then tp
+          else tp.derivedSelect(apply(tp.prefix))
+        case _: ThisType =>
+          tp
+        case _ =>
+          mapOver(tp)
+      def inverse = thisMap
   end SubstParamsBiMap
 
   /** Check that a @retains annotation only mentions references that can be tracked.

compiler/src/dotty/tools/dotc/cc/Setup.scala

@@ -238,6 +238,7 @@ extends tpd.TreeTraverser:
    */
   private class SubstParams(from: List[List[Symbol]], to: List[LambdaType])(using Context)
   extends DeepTypeMap, BiTypeMap:
+    thisMap =>
 
     def apply(t: Type): Type = t match
       case t: NamedType =>
@@ -253,15 +254,17 @@ extends tpd.TreeTraverser:
       case _ =>
         mapOver(t)
 
-    def inverse(t: Type): Type = t match
-      case t: ParamRef =>
-        def recur(from: List[LambdaType], to: List[List[Symbol]]): Type =
-          if from.isEmpty then t
-          else if t.binder eq from.head then to.head(t.paramNum).namedType
-          else recur(from.tail, to.tail)
-        recur(to, from)
-      case _ =>
-        mapOver(t)
+    lazy val inverse = new BiTypeMap:
+      def apply(t: Type): Type = t match
+        case t: ParamRef =>
+          def recur(from: List[LambdaType], to: List[List[Symbol]]): Type =
+            if from.isEmpty then t
+            else if t.binder eq from.head then to.head(t.paramNum).namedType
+            else recur(from.tail, to.tail)
+          recur(to, from)
+        case _ =>
+          mapOver(t)
+      def inverse = thisMap
   end SubstParams
 
   /** Update info of `sym` for CheckCaptures phase only */

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

@@ -165,7 +165,7 @@ object Substituters:
 
   final class SubstBindingMap(from: BindingType, to: BindingType)(using Context) extends DeepTypeMap, BiTypeMap {
     def apply(tp: Type): Type = subst(tp, from, to, this)(using mapCtx)
-    def inverse(tp: Type): Type = tp.subst(to, from)
+    def inverse = SubstBindingMap(to, from)
   }
 
   final class Subst1Map(from: Symbol, to: Type)(using Context) extends DeepTypeMap {
@@ -182,7 +182,7 @@ object Substituters:
 
   final class SubstSymMap(from: List[Symbol], to: List[Symbol])(using Context) extends DeepTypeMap, BiTypeMap {
     def apply(tp: Type): Type = substSym(tp, from, to, this)(using mapCtx)
-    def inverse(tp: Type) = tp.substSym(to, from) // implicitly requires that `to` contains no duplicates.
+    def inverse = SubstSymMap(to, from) // implicitly requires that `to` contains no duplicates.
   }
 
   final class SubstThisMap(from: ClassSymbol, to: Type)(using Context) extends DeepTypeMap {

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

@@ -5716,15 +5716,8 @@ object Types {
   trait BiTypeMap extends TypeMap:
     thisMap =>
 
-    /** The inverse of the type map as a function */
-    def inverse(tp: Type): Type
-
-    /** The inverse of the type map as a BiTypeMap map, which
-     *  has the original type map as its own inverse.
-     */
-    def inverseTypeMap(using Context) = new BiTypeMap:
-      def apply(tp: Type) = thisMap.inverse(tp)
-      def inverse(tp: Type) = thisMap.apply(tp)
+    /** The inverse of the type map */
+    def inverse: BiTypeMap
 
     /** A restriction of this map to a function on tracked CaptureRefs */
     def forward(ref: CaptureRef): CaptureRef = this(ref) match