GADTs: freeze when joining LHS union

These approximations shouldn't be used to derive GADTs constraints, just as the
.widenSingleton approx shouldn't.

Author: abgruszecki

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

@@ -431,18 +431,25 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
           case Some(b) => return b
           case None =>
 
+        /** `recur` shouldn't normally be used with approximated types, according to its
+          * documentation. In the specific examples where this function is
+          * called, it is sound to do so as long as we freeze GADTs.
+          */
+        def recurInFrozenGadt(tp1: Type, tp2: Type) =
+          inFrozenGadt { recur(tp1, tp2) }
+
         def widenOK =
           (tp2.widenSingletons eq tp2)
           && (tp1.widenSingletons ne tp1)
-          && inFrozenGadt { recur(tp1.widenSingletons, tp2) }
+          && recurInFrozenGadt(tp1.widenSingletons, tp2)
 
         def joinOK = tp2.dealiasKeepRefiningAnnots match {
           case tp2: AppliedType if !tp2.tycon.typeSymbol.isClass =>
             // If we apply the default algorithm for `A[X] | B[Y] <: C[Z]` where `C` is a
             // type parameter, we will instantiate `C` to `A` and then fail when comparing
             // with `B[Y]`. To do the right thing, we need to instantiate `C` to the
             // common superclass of `A` and `B`.
-            recur(tp1.join, tp2)
+            recurInFrozenGadt(tp1.join, tp2)
           case _ =>
             false
         }
@@ -469,7 +476,7 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
         widenOK
         || joinOK
         || (tp1.isSoft || constrainRHSVars(tp2)) && recur(tp11, tp2) && recur(tp12, tp2)
-        || containsAnd(tp1) && recur(tp1.join, tp2)
+        || containsAnd(tp1) && recurInFrozenGadt(tp1.join, tp2)
      case tp1: MatchType =>
         val reduced = tp1.reduced
         if (reduced.exists) recur(reduced, tp2) else thirdTry