Check GADT ordering for HKTs

Author: Linyxus

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

@@ -1045,10 +1045,18 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
                     }
                   }
 
+                  var usedGadtOrdering: Boolean = false
+                  def byGadtOrdering: Boolean =
+                    ctx.gadt.contains(tycon1sym)
+                    && ctx.gadt.contains(tycon2sym)
+                    && ctx.gadt.isLess(tycon1sym, tycon2sym)
+                    && { usedGadtOrdering = true; true }
+
                   val res = (
                     tycon1sym == tycon2sym && isSubPrefix(tycon1.prefix, tycon2.prefix)
                     || byGadtBounds(tycon1sym, tycon2, fromAbove = true)
                     || byGadtBounds(tycon2sym, tycon1, fromAbove = false)
+                    || byGadtOrdering
                   ) && {
                     // There are two cases in which we can assume injectivity.
                     // First we check if either sym is a class.
@@ -1058,9 +1066,11 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
                     // 2) if we touched GADTs, then the _other_ symbol (class syms
                     //    cannot have GADT constraints), the one w/ GADT cstrs,
                     //    must be instantiated, making the two tycons equal
+                    // 3) if we used GADT ordering, which means that neither symbol
+                    //    has been instantiated, we should not assume injectivity
                     val tyconIsInjective =
                       (tycon1sym.isClass || tycon2sym.isClass)
-                      && (!touchedGADTs || gadtIsInstantiated)
+                      && (!touchedGADTs || gadtIsInstantiated && !usedGadtOrdering)
 
                     inFrozenGadtIf(!tyconIsInjective) {
                       if tycon1sym == tycon2sym && tycon1sym.isAliasType then