Record constraint against tvar parameters

Instead of using TypeVar#underlying, which uses Constraint#instType,
relying only on TypeVar#inst.  That way we can record constraints
against the original parameter and _then_ maximise the type, which
results in `Bar[String]` instead of `Bar[X]`, in both cases with `X :=
String` recorded in the GADT constraint.

Author: dwijnand

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

@@ -252,8 +252,7 @@ trait ConstraintHandling {
       val bound = legalBound(param, rawBound, isUpper)
       val oldBounds @ TypeBounds(lo, hi) = constraint.nonParamBounds(param)
       val equalBounds = (if isUpper then lo else hi) eq bound
-      val isGadt = ctx.mode.is(Mode.GadtConstraintInference)
-      if equalBounds && !isGadt && !bound.existsPart(_ eq param, StopAt.Static) then
+      if equalBounds && !bound.existsPart(_ eq param, StopAt.Static) then
         // The narrowed bounds are equal and not recursive,
         // so we can remove `param` from the constraint.
         constraint = constraint.replace(param, bound)
@@ -268,12 +267,8 @@ trait ConstraintHandling {
           homogenizeArgs = Config.alignArgsInAnd
           try
             trustBounds = false
-            if isUpper then
-              if isGadt && !hi.isAny && hi.isValueTypeOrWildcard && bound.isValueTypeOrWildcard then oldBounds.derivedTypeBounds(lo, AndType(hi, bound))
-              else oldBounds.derivedTypeBounds(lo, hi & bound)
-            else
-              if isGadt && !lo.isNothingType && lo.isValueTypeOrWildcard && bound.isValueTypeOrWildcard then oldBounds.derivedTypeBounds(OrType(lo, bound, soft = true), hi)
-              else oldBounds.derivedTypeBounds(lo | bound, hi)
+            if isUpper then oldBounds.derivedTypeBounds(lo, hi & bound)
+            else oldBounds.derivedTypeBounds(lo | bound, hi)
           finally
             homogenizeArgs = savedHomogenizeArgs
             trustBounds = savedTrustBounds

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

@@ -162,16 +162,13 @@ class OrderingConstraint(private val boundsMap: ParamBounds,
 
   def contains(pt: TypeLambda): Boolean = boundsMap(pt) != null
 
-  def contains(param: TypeParamRef): Boolean = {
-    val entries = boundsMap(param.binder)
-    entries != null && isBounds(entries(param.paramNum))
-  }
+  def contains(param: TypeParamRef): Boolean = boundsMap(param.binder) != null
 
   def contains(tvar: TypeVar): Boolean = {
     val origin = tvar.origin
     val entries = boundsMap(origin.binder)
     val pnum = origin.paramNum
-    entries != null && isBounds(entries(pnum)) && (typeVar(entries, pnum) eq tvar)
+    entries != null && (typeVar(entries, pnum) eq tvar)
   }
 
 // ---------- Dependency handling ----------------------------------------------
@@ -548,7 +545,7 @@ class OrderingConstraint(private val boundsMap: ParamBounds,
 
   def forallParams(p: TypeParamRef => Boolean): Boolean =
     boundsMap.forallBinding { (poly, entries) =>
-      !0.until(paramCount(entries)).exists(i => isBounds(entries(i)) && !p(poly.paramRefs(i)))
+      (0 until paramCount(entries)).forall(i => p(poly.paramRefs(i)))
     }
 
   def foreachParam(p: (TypeLambda, Int) => Unit): Unit =

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

@@ -117,7 +117,7 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
   protected def gadtAddLowerBound(sym: Symbol, b: Type): Boolean = ctx.gadt.addBound(sym, b, isUpper = false)
   protected def gadtAddUpperBound(sym: Symbol, b: Type): Boolean = ctx.gadt.addBound(sym, b, isUpper = true)
 
-  protected def typeVarInstance(tvar: TypeVar)(using Context): Type = tvar.underlying
+  protected def typeVarInstance(tvar: TypeVar)(using Context): Type = tvar.inst.orElse(tvar.origin)
 
   // Subtype testing `<:<`
 
@@ -1201,7 +1201,7 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
       def compareLower(tycon2bounds: TypeBounds, tyconIsTypeRef: Boolean): Boolean =
         if ((tycon2bounds.lo `eq` tycon2bounds.hi) && !tycon2bounds.isInstanceOf[MatchAlias])
           if (tyconIsTypeRef) recur(tp1, tp2.superTypeNormalized)
-          else recur(tp1, tycon2bounds.lo.applyIfParameterized(args2))
+          else isSubApproxHi(tp1, tycon2bounds.lo.applyIfParameterized(args2))
         else
           fallback(tycon2bounds.lo)
 

tests/pos/i15289.scala

@@ -18,14 +18,14 @@ class Test:
 // max: Bar[X]
 
 // what's more correct?
-// * unapp arg tpe = Bar[X]       gadt: X =:= String
-// * unapp arg tpe = Bar[String]  gadt: X =:= String
+// * unapp arg tpe = Bar[X]       gadt: X := String
+// * unapp arg tpe = Bar[String]  gadt: X := String
 
 // after fix:
 // ptc: Bar[C] aka Foo[C, C] vs Foo[X, String]
-// ptc:  C >: X       ~>  cstr: C >:  X
-// ptc:  C <: X       ~>  cstr: C =:= X
-// ptc:  C >: String  ~>  cstr: C >: X | String <: X           gadt: X >:  String
-// ptc:  C <: String  ~>  cstr: C >: X | String <: X & String  gadt: X =:= String
+// ptc:  C >: X       ~>  cstr: C >: X
+// ptc:  C <: X       ~>  cstr: C >: X <: X
+// ptc:  C >: String  ~>  cstr: C >: X | String <: X       gadt: X >: String
+// ptc:  C <: String  ~>  cstr: C >: X | String <: String  gadt: X := String
 // ptc: Bar[C]
 // max: Bar[String]