Fix #8972: Handle applications in Unapply fun

Author: nicolasstucki

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

@@ -149,10 +149,12 @@ object Applications {
   }
 
   def unapplyArgs(unapplyResult: Type, unapplyFn: Tree, args: List[untpd.Tree], pos: SourcePosition)(using Context): List[Type] = {
-
-    val unapplyName = unapplyFn match            // tolerate structural `unapply`, which does not have a symbol
-      case TypeApply(fn: RefTree, _) => fn.name
-      case fn: RefTree => fn.name
+    def getName(fn: Tree): Name =
+      fn match
+        case TypeApply(fn, _) => getName(fn)
+        case Apply(fn, _) => getName(fn)
+        case fn: RefTree => fn.name
+    val unapplyName = getName(unapplyFn) // tolerate structural `unapply`, which does not have a symbol
 
     def getTp = extractorMemberType(unapplyResult, nme.get, pos)
 

tests/pos/i8972.scala

@@ -0,0 +1,9 @@
+trait Num:
+  type Nat
+
+object IsInt:
+  def unapply(using num: Num)(sc: num.Nat): Option[Int] = ???
+
+def test(using num: Num)(x: num.Nat) =
+  x match
+    case IsInt(i) =>