Sema: Fix recent regression with invalid subscripts

A SubscriptExpr can represent both the result of parsing a subscript and
the result of type checking / overload resolution. In the former case,
we don't have an associated decl, triggering an assert when FindCapturedVars
tries visits an invalid SubscriptExpr inside of an Objective-C extension.

Fixes <rdar://problem/26283886>.

Author: slavapestov

lib/Sema/TypeCheckExpr.cpp

@@ -1053,14 +1053,14 @@ namespace {
       // Referring to a class-constrained generic or metatype
       // doesn't require its type metadata.
       if (auto declRef = dyn_cast<DeclRefExpr>(E))
-        return !declRef->getDecl()->isObjC()
-          && !E->getType()->hasRetainablePointerRepresentation()
-          && !E->getType()->is<AnyMetatypeType>();
+        return (!declRef->getDecl()->isObjC()
+                && !E->getType()->hasRetainablePointerRepresentation()
+                && !E->getType()->is<AnyMetatypeType>());
 
       // Loading classes or metatypes doesn't require their metadata.
       if (isa<LoadExpr>(E))
-        return !E->getType()->hasRetainablePointerRepresentation()
-          && !E->getType()->is<AnyMetatypeType>();
+        return (!E->getType()->hasRetainablePointerRepresentation()
+                && !E->getType()->is<AnyMetatypeType>());
 
       // Accessing @objc members doesn't require type metadata.
       if (auto memberRef = dyn_cast<MemberRefExpr>(E))
@@ -1078,13 +1078,14 @@ namespace {
       }
 
       if (auto subscriptExpr = dyn_cast<SubscriptExpr>(E)) {
-        return !subscriptExpr->getDecl().getDecl()->isObjC();
+        return (subscriptExpr->hasDecl() &&
+                !subscriptExpr->getDecl().getDecl()->isObjC());
       }
 
       // Getting the dynamic type of a class doesn't require type metadata.
       if (isa<DynamicTypeExpr>(E))
-        return !E->getType()->castTo<AnyMetatypeType>()->getInstanceType()
-          ->hasRetainablePointerRepresentation();
+        return (!E->getType()->castTo<AnyMetatypeType>()->getInstanceType()
+                    ->hasRetainablePointerRepresentation());
 
       // Building a fixed-size tuple doesn't require type metadata.
       // Approximate this for the purposes of being able to invoke @objc methods
@@ -1122,8 +1123,8 @@ namespace {
 
       // Opening an @objc existential or metatype is a no-op.
       if (auto open = dyn_cast<OpenExistentialExpr>(E))
-        return !open->getSubExpr()->getType()->isObjCExistentialType()
-          && !open->getSubExpr()->getType()->is<AnyMetatypeType>();
+        return (!open->getSubExpr()->getType()->isObjCExistentialType()
+                && !open->getSubExpr()->getType()->is<AnyMetatypeType>());
 
       // Erasure to an ObjC existential or between metatypes doesn't require
       // type metadata.

test/ClangModules/objc_bridging_generics.swift

@@ -234,6 +234,14 @@ extension AnimalContainer {
     _ = #selector(AnimalContainer.doesntUseGenericParam3)
     _ = #selector(AnimalContainer.doesntUseGenericParam4)
   }
+
+  // rdar://problem/26283886
+  func funcWithWrongArgType(x: NSObject) {}
+
+  func crashWithInvalidSubscript(x: NSArray) {
+    _ = funcWithWrongArgType(x: x[12])
+    // expected-error@-1{{'AnyObject' is not convertible to 'NSObject'; did you mean to use 'as!' to force downcast?}}
+  }
 }
 
 extension PettableContainer {