IRGen: Correctly set and honor the "is reflectable" bit on structs and classes.

If we only emit an opaque reflection record for a struct or class, then we can't reflect its fields. We failed both to clear the "is reflectable" bit in the context descriptor for non-reflectable structs, and to check for the bit before trying to present a struct's fields as children in the runtime. rdar://problem/41274260

Author: jckarter

lib/IRGen/GenMeta.cpp

@@ -700,7 +700,10 @@ namespace {
     uint16_t getKindSpecificFlags() {
       TypeContextDescriptorFlags flags;
 
-      flags.setIsReflectable(true); // struct always reflectable
+      // Structs are reflectable unless we emit them with opaque reflection
+      // metadata.
+      flags.setIsReflectable(
+                            !IGM.shouldEmitOpaqueTypeMetadataRecord(getType()));
 
       getClangImportedFlags(flags);
       return flags.getOpaqueValue();

stdlib/public/runtime/ReflectionMirror.mm

@@ -257,11 +257,21 @@ AnyReturn subscript(intptr_t i, const char **outName,
 
 // Implementation for structs.
 struct StructImpl : ReflectionMirrorImpl {
+  bool isReflectable() {
+    const auto *Struct = static_cast<const StructMetadata *>(type);
+    const auto &Description = Struct->getDescription();
+    return Description->getTypeContextDescriptorFlags().isReflectable();
+  }
+
   char displayStyle() {
     return 's';
   }
 
   intptr_t count() {
+    if (!isReflectable()) {
+      return 0;
+    }
+
     auto *Struct = static_cast<const StructMetadata *>(type);
     return Struct->getDescription()->NumFields;
   }
@@ -399,11 +409,20 @@ AnyReturn subscript(intptr_t i, const char **outName,
 
 // Implementation for classes.
 struct ClassImpl : ReflectionMirrorImpl {
+  bool isReflectable() {
+    const auto *Class = static_cast<const ClassMetadata *>(type);
+    const auto &Description = Class->getDescription();
+    return Description->getTypeContextDescriptorFlags().isReflectable();
+  }
+
   char displayStyle() {
     return 'c';
   }
 
   intptr_t count() {
+    if (!isReflectable())
+      return 0;
+
     auto *Clas = static_cast<const ClassMetadata*>(type);
     auto count = Clas->getDescription()->NumFields;
 

test/stdlib/Reflection_objc.swift

@@ -14,6 +14,7 @@
 
 import Swift
 import Foundation
+import simd
 
 #if os(macOS)
 import AppKit
@@ -284,6 +285,11 @@ testQLO(HasStringQLO.self)
 // CHECK-NEXT: HasStringQLO overboard
 // CHECK-NEXT: CanaryBase overboard
 
+// simd types get no reflection info, so should have no mirror children
+let x = float4(0)
+print("float4 has \(Mirror(reflecting: x).children.count) children")
+// CHECK-NEXT: float4 has 0 children
+
 // CHECK-LABEL: and now our song is done
 print("and now our song is done")