Don't unsoundly maximise types to Any/Nothing

Also fix the span of the new pattern bound symbol,
which was failing pickling tests.

Author: dwijnand

compiler/src/dotty/tools/dotc/typer/Applications.scala

@@ -1341,7 +1341,7 @@ trait Applications extends Compatibility {
             // Constraining only fails if the pattern cannot possibly match,
             // but useless pattern checks detect more such cases, so we simply rely on them instead.
             withMode(Mode.GadtConstraintInference)(TypeComparer.constrainPatternType(unapplyArgType, selType))
-            val patternBound = maximizeType(unapplyArgType, tree.span)
+            val patternBound = maximizeType(unapplyArgType, unapplyFn.span.endPos)
             if (patternBound.nonEmpty) unapplyFn = addBinders(unapplyFn, patternBound)
             unapp.println(i"case 2 $unapplyArgType ${ctx.typerState.constraint}")
             unapplyArgType

compiler/src/dotty/tools/dotc/typer/Inferencing.scala

@@ -405,8 +405,8 @@ object Inferencing {
     val patternBindings = new mutable.ListBuffer[(Symbol, TypeParamRef)]
     vs foreachBinding { (tvar, v) =>
       if !tvar.isInstantiated then
-        if (v == 1) tvar.instantiate(fromBelow = false)
-        else if (v == -1) tvar.instantiate(fromBelow = true)
+        if (v == 1 && tvar.hasUpperBound) tvar.instantiate(fromBelow = false)
+        else if (v == -1 && tvar.hasLowerBound) tvar.instantiate(fromBelow = true)
         else {
           val bounds = TypeComparer.fullBounds(tvar.origin)
           if bounds.hi <:< bounds.lo || bounds.hi.classSymbol.is(Final) then

tests/neg/i14983.scala

@@ -0,0 +1,13 @@
+sealed trait Tree[+A]
+final case class Leaf[+B](v: B)        extends Tree[B]
+final case class Node[+C](xs: List[C]) extends Tree[List[C]]
+
+object Test:
+  def meth[X](tree: Tree[X]): X = tree match
+    case Leaf(v) => v
+    case Node(x) => List("boom") // error
+
+  def main(args: Array[String]): Unit =
+    val tree = Node(List(42))
+    val res = meth(tree)
+    assert(res.head == 42) // was: ClassCastException: class java.lang.String cannot be cast to class java.lang.Integer

tests/run/i14983.scala

@@ -0,0 +1,13 @@
+sealed trait Tree[+A]
+final case class Leaf[+B](v: B)        extends Tree[B]
+final case class Node[+C](xs: List[C]) extends Tree[List[C]]
+
+object Test:
+  def meth[X](tree: Tree[X]): X = tree match
+    case Leaf(v) => v
+    case Node(x) => x // ok
+
+  def main(args: Array[String]): Unit =
+    val tree = Node(List(42))
+    val res = meth(tree)
+    assert(res.head == 42) // ok