Split out immutable GadtConstraint (#16602)

This change eagerly allocates a GadtConstraint whenever its state is
changed, rather than simply changing the reference(s) in, what's now
called, `GadtConstraintHandling`, such as `constraint`. Previous to a
recent change, in order to restore the GADT constraints, a fresh copy
was eagerly created in a number of cases, just so it can be used to
restore, for the cases in which it must be restored. The recent change
exposed the underlying pieces of the GADT constraint and allowed those
to be restored as components. Now we still do that, but as a packaged up
GadtConstraint that we created once, eagerly. That also makes sure that
invariants in components are upheld, like the mappings.

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