Fix collection of bound type variables in UnApply patterns

Author: odersky

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

@@ -720,7 +720,16 @@ class Inliner(call: tpd.Tree, rhsToInline: tpd.Tree)(implicit ctx: Context) {
                 foldOver(syms1, t)
               }
             }
-            val boundVars = getBoundVars(Nil, tpt)
+            var boundVars = getBoundVars(Nil, tpt)
+            // UnApply nodes with pattern bound variables translate to something like this
+            //   UnApply[t @ t](pats)(implicits): T[t]
+            // Need to traverse any binds in type arguments of the UnAppyl to get the set of
+            // all instantiable type variables. Test case is pos/inline-caseclass.scala.
+            pat1 match {
+              case UnApply(TypeApply(_, tpts), _, _) =>
+                for (tpt <- tpts) boundVars = getBoundVars(boundVars, tpt)
+              case _ =>
+            }
             for (bv <- boundVars) ctx.gadt.setBounds(bv, bv.info.bounds)
             scrut <:< tpt.tpe && {
               for (bv <- boundVars) {

tests/pos/inline-caseclass.scala

@@ -0,0 +1,19 @@
+trait Nat
+case object Z extends Nat
+case class S[N <: Nat](n: N) extends Nat
+
+object Test {
+  type Z = Z.type
+
+  inline def add(x: Nat, y: Int) <: Int = inline x match {
+    case Z => y
+    case S(x1) => add(x1, y) + 1
+  }
+
+  val x = S(S(Z))
+  val a: 2 = add(Z, 2)
+  inline val y = add(x, 2)
+  val z: 4 = y
+
+}
+