Check value casts are to non-foreign types (#2754)

Per the discussion in SR-1612, we don’t have a robust mechanism for
checking this and should warn about it.

Signed-off-by: Robert Widmann <[email protected]>

Author: CodaFi

include/swift/AST/DiagnosticsSema.def

@@ -665,8 +665,12 @@ ERROR(init_not_instance_member,none,
       "a new object of the same dynamic type", ())
 ERROR(super_initializer_not_in_initializer,none,
       "'super.init' cannot be called outside of an initializer", ())
+
 WARNING(isa_is_always_true,none, "'%0' test is always true",
         (StringRef))
+WARNING(isa_is_foreign_check,none,
+      "'is' test is always true because %0 is a Core Foundation type",
+      (Type))
 WARNING(conditional_downcast_coercion,none,
       "conditional cast from %0 to %1 always succeeds",
       (Type, Type))

lib/Sema/CSApply.cpp

@@ -2857,12 +2857,20 @@ namespace {
         tc.diagnose(expr->getLoc(), diag::isa_is_always_true, "is");
         expr->setCastKind(castKind);
         break;
+      case CheckedCastKind::ValueCast:
+        // Check the cast target is a non-foreign type
+        if (auto cls = toType->getAs<ClassType>()) {
+          if (cls->getDecl()->isForeign()) {
+            tc.diagnose(expr->getLoc(), diag::isa_is_foreign_check, toType);
+          }
+        }
+        expr->setCastKind(castKind);
+        break;
       case CheckedCastKind::ArrayDowncast:
       case CheckedCastKind::DictionaryDowncast:
       case CheckedCastKind::DictionaryDowncastBridged:
       case CheckedCastKind::SetDowncast:
       case CheckedCastKind::SetDowncastBridged:
-      case CheckedCastKind::ValueCast:
       case CheckedCastKind::BridgeFromObjectiveC:
         // Valid checks.
         expr->setCastKind(castKind);

test/Misc/misc_diagnostics.swift

@@ -148,4 +148,8 @@ func tuple_splat2(_ q : (a : Int, b : Int)) {
   tuple_splat2(1, b: 2)    // expected-error {{extra argument 'b' in call}}
 }
 
+// SR-1612: Type comparison of foreign types is always true.
+func is_foreign(a: AnyObject) -> Bool {
+  return a is CGColor // expected-warning {{'is' test is always true because 'CGColor' is a Core Foundation type}}
+}