Sema: Allow "case let value as [Int]" and remove the diagnostic that it is not implemented

Author: simanerush

lib/Sema/TypeCheckPattern.cpp

@@ -1384,16 +1384,7 @@ Pattern *TypeChecker::coercePatternToType(ContextualPattern pattern,
     // Valid checks.
     case CheckedCastKind::ArrayDowncast:
     case CheckedCastKind::DictionaryDowncast:
-    case CheckedCastKind::SetDowncast: {
-      diags.diagnose(IP->getLoc(),
-                     diag::isa_collection_downcast_pattern_value_unimplemented,
-                     IP->getCastType());
-      IP->setType(ErrorType::get(Context));
-      if (Pattern *sub = IP->getSubPattern())
-        sub->forEachVariable([](VarDecl *VD) { VD->setInvalid(); });
-      return P;
-    }
-
+    case CheckedCastKind::SetDowncast:
     case CheckedCastKind::ValueCast:
       IP->setCastKind(castKind);
       break;

test/Parse/matching_patterns.swift

@@ -289,16 +289,16 @@ case (let (_, _, _)) + 1:
   ()
 }
 
-// FIXME: We don't currently allow subpatterns for "isa" patterns that
-// require interesting conditional downcasts.
+// "isa" patterns.
 class Base { }
 class Derived : Base { }
 
 
 switch [Derived(), Derived(), Base()] {
-case let ds as [Derived]: // expected-error{{collection downcast in cast pattern is not implemented; use an explicit downcast to '[Derived]' instead}}
+case let ds as [Derived]:
+  // expected-warning@-1 {{immutable value 'ds' was never used; consider replacing with '_' or removing it}}
   ()
-case is [Derived]: // expected-error{{collection downcast in cast pattern is not implemented; use an explicit downcast to '[Derived]' instead}}
+case is [Derived]:
   ()
 
 default:

test/SILGen/switch.swift

@@ -635,6 +635,23 @@ func test_isa_class_2(x: B) -> AnyObject {
 }
 // CHECK: } // end sil function '$s6switch16test_isa_class_21xyXlAA1BC_tF'
 
+// CHECK-LABEL: sil hidden [ossa] @$s6switch14test_isa_array2psySayAA1P_pG_tF : $@convention(thin) (@guaranteed Array<P>) -> () {
+func test_isa_array(ps: Array<P>) {
+  switch ps {
+  case is [X]:
+    // CHECK: [[IS_X]]:
+    a()
+  case is [Y]:
+    // CHECK: [[IS_Y]]:
+    b()
+
+  case _:
+    d()
+  }
+  e()
+}
+// CHECK: } // end sil function '$s6switch14test_isa_array2psySayAA1P_pG_tF'
+
 enum MaybePair {
   case Neither
   case Left(Int)