Allow refineUsingParent to infer GADT bounds

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

@@ -1945,8 +1945,11 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
       val tparam = tr.symbol
       gadts.println(i"narrow gadt bound of $tparam: ${tparam.info} from ${if (isUpper) "above" else "below"} to $bound ${bound.toString} ${bound.isRef(tparam)}")
       if (bound.isRef(tparam)) false
-      else if (isUpper) gadtAddUpperBound(tparam, bound)
-      else gadtAddLowerBound(tparam, bound)
+      else
+        val savedGadt = ctx.gadt.fresh
+        val success = if isUpper then gadtAddUpperBound(tparam, bound) else gadtAddLowerBound(tparam, bound)
+        if !success then ctx.gadt.restore(savedGadt)
+        success
     }
   }
 

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

@@ -712,7 +712,7 @@ object TypeOps:
 
     val childTp = if (child.isTerm) child.termRef else child.typeRef
 
-    inContext(ctx.fresh.setExploreTyperState().setFreshGADTBounds) {
+    inContext(ctx.fresh.setExploreTyperState().setFreshGADTBounds.addMode(Mode.GadtConstraintInference)) {
       instantiateToSubType(childTp, parent).dealias
     }
   }
@@ -829,6 +829,13 @@ object TypeOps:
     val tvars = tp1.typeParams.map { tparam => newTypeVar(tparam.paramInfo.bounds) }
     val protoTp1 = inferThisMap.apply(tp1).appliedTo(tvars)
 
+    val getAbstractSymbols = new TypeAccumulator[List[Symbol]]:
+      def apply(xs: List[Symbol], tp: Type) = tp.dealias match
+        case tp: TypeRef if !tp.symbol.isClass => foldOver(tp.symbol :: xs, tp)
+        case tp                                => foldOver(xs, tp)
+    val syms2 = getAbstractSymbols(Nil, tp2).reverse
+    if syms2.nonEmpty then ctx.gadt.addToConstraint(syms2)
+
     // If parent contains a reference to an abstract type, then we should
     // refine subtype checking to eliminate abstract types according to
     // variance. As this logic is only needed in exhaustivity check,

compiler/test/dotty/tools/vulpix/ParallelTesting.scala

@@ -168,7 +168,7 @@ trait ParallelTesting extends RunnerOrchestration { self =>
   ) extends TestSource {
     def sourceFiles: Array[JFile] = files.filter(isSourceFile)
 
-    override def toString() = outDir.toString
+    override def toString() = sourceFiles match { case Array(f) => f.getPath case _ => outDir.getPath }
   }
 
   /** A test source whose files will be compiled separately according to their

tests/neg-custom-args/fatal-warnings/suppressed-type-test-warnings.scala

@@ -18,10 +18,12 @@ object Test {
   def err2[A, B](value: Foo[A, B], a: A => Int): B = value match {
     case b: Bar[B] => // spurious // error
       b.x
+    case _ => ??? // avoid fatal inexhaustivity warnings suppressing the uncheckable warning
   }
 
   def fail[A, B](value: Foo[A, B], a: A => Int): B = value match {
     case b: Bar[Int] => // error
       b.x
+    case _ => ??? // avoid fatal inexhaustivity warnings suppressing the uncheckable warning
   }
 }

tests/pos/i13548.scala

@@ -0,0 +1,6 @@
+// scalac: -Werror
+sealed abstract class Foo[N, A]
+final case class Bar[B](foo: Foo[B, B]) extends Foo[B, B]
+class Test:
+  def pmat[P, C](scr: Foo[P, C]): C = scr match
+    case Bar(foo) => pmat(foo)

tests/pos/i15289.scala

@@ -0,0 +1,6 @@
+// scalac: -Werror
+sealed abstract class Foo[A, B]
+final case class Bar[C](baz: C) extends Foo[C, C]
+
+class Test:
+  def m1[X](f1: Foo[X, String]): String = f1 match { case Bar(_) => "" }