Make expressions using GADTs type check in later phases

GADT logic is lost after PatMat. To make code typecheck, the typer
should already insert casts where a subtype check succeeded because
it involved bounds established by a GADT comparison.

Author: odersky

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

@@ -76,6 +76,19 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
     myNothingType
   }
 
+  /** Indicates whether a previous subtype check used GADT bounds */
+  var GADTused = false
+
+  /** Record that GADT bounds of `sym` were used in a subtype check.
+   *  But exclude constructor type parameters, as these are aliased
+   *  to the corresponding class parameters, which does not constitute
+   *  a true usage of a GADT symbol.
+   */
+  private def GADTusage(sym: Symbol) = {
+    if (!sym.owner.isConstructor) GADTused = true
+    true
+  }
+
   // Subtype testing `<:<`
 
   def topLevelSubType(tp1: Type, tp2: Type): Boolean = {
@@ -325,7 +338,8 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
         val gbounds2 = ctx.gadt.bounds(tp2.symbol)
         (gbounds2 != null) &&
           (isSubTypeWhenFrozen(tp1, gbounds2.lo) ||
-            narrowGADTBounds(tp2, tp1, isUpper = false))
+            narrowGADTBounds(tp2, tp1, isUpper = false)) &&
+          GADTusage(tp2.symbol)
       }
       ((frozenConstraint || !isCappable(tp1)) && isSubType(tp1, lo2) ||
         compareGADT ||
@@ -507,7 +521,8 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
             val gbounds1 = ctx.gadt.bounds(tp1.symbol)
             (gbounds1 != null) &&
               (isSubTypeWhenFrozen(gbounds1.hi, tp2) ||
-               narrowGADTBounds(tp1, tp2, isUpper = true))
+               narrowGADTBounds(tp1, tp2, isUpper = true)) &&
+              GADTusage(tp1.symbol)
           }
           isSubType(hi1, tp2) || compareGADT
         case _ =>

src/dotty/tools/dotc/typer/Typer.scala

@@ -1679,6 +1679,7 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
         else
           missingArgs
       case _ =>
+        ctx.typeComparer.GADTused = false
         if (ctx.mode is Mode.Pattern) {
           tree match {
             case _: RefTree | _: Literal if !isVarPattern(tree) =>
@@ -1687,7 +1688,9 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
           }
           tree
         }
-        else if (tree.tpe <:< pt) tree
+        else if (tree.tpe <:< pt)
+          if (ctx.typeComparer.GADTused) tree.asInstance(pt)
+          else tree
         else if (wtp.isInstanceOf[MethodType]) missingArgs
         else {
           typr.println(i"adapt to subtype ${tree.tpe} !<:< $pt")