Backport "Avoid bidirectional GADT typebounds from fullBounds" (#16945)

Backports #15683 and #16602

Author: Kordyjan

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

@@ -71,6 +71,9 @@ abstract class Constraint extends Showable {
    */
   def nonParamBounds(param: TypeParamRef)(using Context): TypeBounds
 
+  /** The current bounds of type parameter `param` */
+  def bounds(param: TypeParamRef)(using Context): TypeBounds
+
   /** A new constraint which is derived from this constraint by adding
    *  entries for all type parameters of `poly`.
    *  @param tvars   A list of type variables associated with the params,

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

@@ -749,16 +749,7 @@ trait ConstraintHandling {
     }
 
   /** The current bounds of type parameter `param` */
-  def bounds(param: TypeParamRef)(using Context): TypeBounds = {
-    val e = constraint.entry(param)
-    if (e.exists) e.bounds
-    else {
-      // TODO: should we change the type of paramInfos to nullable?
-      val pinfos: List[param.binder.PInfo] | Null = param.binder.paramInfos
-      if (pinfos != null) pinfos(param.paramNum) // pinfos == null happens in pos/i536.scala
-      else TypeBounds.empty
-    }
-  }
+  def bounds(param: TypeParamRef)(using Context): TypeBounds = constraint.bounds(param)
 
   /** Add type lambda `tl`, possibly with type variables `tvars`, to current constraint
    *  and propagate all bounds.

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

@@ -141,7 +141,8 @@ object Contexts {
     def tree: Tree[?]
     def scope: Scope
     def typerState: TyperState
-    def gadt: GadtConstraint
+    def gadt: GadtConstraint = gadtState.gadt
+    def gadtState: GadtState
     def searchHistory: SearchHistory
     def source: SourceFile
 
@@ -410,7 +411,7 @@ object Contexts {
       val constrCtx = outersIterator.dropWhile(_.outer.owner == owner).next()
       superOrThisCallContext(owner, constrCtx.scope)
         .setTyperState(typerState)
-        .setGadt(gadt)
+        .setGadtState(gadtState)
         .fresh
         .setScope(this.scope)
     }
@@ -541,8 +542,8 @@ object Contexts {
     private var _typerState: TyperState = uninitialized
     final def typerState: TyperState = _typerState
 
-    private var _gadt: GadtConstraint = uninitialized
-    final def gadt: GadtConstraint = _gadt
+    private var _gadtState: GadtState = uninitialized
+    final def gadtState: GadtState = _gadtState
 
     private var _searchHistory: SearchHistory = uninitialized
     final def searchHistory: SearchHistory = _searchHistory
@@ -567,7 +568,7 @@ object Contexts {
       _owner = origin.owner
       _tree = origin.tree
       _scope = origin.scope
-      _gadt = origin.gadt
+      _gadtState = origin.gadtState
       _searchHistory = origin.searchHistory
       _source = origin.source
       _moreProperties = origin.moreProperties
@@ -624,12 +625,12 @@ object Contexts {
       this._scope = typer.scope
       setTypeAssigner(typer)
 
-    def setGadt(gadt: GadtConstraint): this.type =
-      util.Stats.record("Context.setGadt")
-      this._gadt = gadt
+    def setGadtState(gadtState: GadtState): this.type =
+      util.Stats.record("Context.setGadtState")
+      this._gadtState = gadtState
       this
     def setFreshGADTBounds: this.type =
-      setGadt(gadt.fresh)
+      setGadtState(gadtState.fresh)
 
     def setSearchHistory(searchHistory: SearchHistory): this.type =
       util.Stats.record("Context.setSearchHistory")
@@ -721,7 +722,7 @@ object Contexts {
           .updated(notNullInfosLoc, Nil)
           .updated(compilationUnitLoc, NoCompilationUnit)
       c._searchHistory = new SearchRoot
-      c._gadt = GadtConstraint.empty
+      c._gadtState = GadtState(GadtConstraint.empty)
       c
   end FreshContext