[Mirror] Handle fields of thin metatypes.

Fields containing metatypes with no possible subtypes are thin i.e. they have no storage. There is only one possible value they can have: the corresponding type. Mirror attempted to copy the nonexistent field value from the nonexistent storage, producing garbage. Instead, special-case thin metatypes and copy the instance type out of the metatype metadata rather than trying to copy it from the field.

rdar://108280543

stdlib/public/runtime/ReflectionMirror.cpp

@@ -171,6 +171,29 @@ static AnyReturn copyFieldContents(OpaqueValue *fieldData,
   auto ownership = fieldType.getReferenceOwnership();
   auto *destContainer = type->allocateBoxForExistentialIn(&outValue.Buffer);
 
+  // If the field's type is a thin metatype, then there's no actual data at
+  // fieldData, and we need to obtain the metatype value from the field type.
+  if (auto *metatype = dyn_cast<MetatypeMetadata>(type)) {
+    switch (metatype->InstanceType->getKind()) {
+    case MetadataKind::Struct:
+    case MetadataKind::Enum:
+    case MetadataKind::Optional:
+    case MetadataKind::Tuple:
+    case MetadataKind::Function:
+    case MetadataKind::Existential: {
+      // These kinds don't have subtypes and thus have thin representations.
+      auto asOpaque = const_cast<OpaqueValue *>(
+          reinterpret_cast<const OpaqueValue *>(&metatype->InstanceType));
+      type->vw_initializeWithCopy(destContainer, asOpaque);
+      return AnyReturn(outValue);
+    }
+
+    default:
+      // Other kinds have subtypes and will not have a thin representation.
+      break;
+    }
+  }
+
   if (ownership.isStrong()) {
     type->vw_initializeWithCopy(destContainer, fieldData);
   }

test/stdlib/Mirror.swift

@@ -27,6 +27,7 @@
 // REQUIRES: reflection
 
 // rdar://96439408
+// UNSUPPORTED: back_deployment_runtime
 // UNSUPPORTED: use_os_stdlib
 
 import StdlibUnittest
@@ -1669,6 +1670,48 @@ mirrors.test("MetatypeMirror") {
   }
 }
 
+class MetatypeExampleClass {}
+class MetatypeExampleSubclass: MetatypeExampleClass {}
+final class MetatypeExampleFinalClass {}
+enum MetatypeExampleEnum {}
+struct MetatypeContainer {
+  var before = 42
+  var before2 = 43
+  var structType = String.self
+  var enumType = MetatypeExampleEnum.self
+  var tupleType = (Int, String, AnyObject).self
+  var functionType = (() -> Void).self
+  var classType = MetatypeExampleClass.self
+  var subclassType: MetatypeExampleClass.Type = MetatypeExampleSubclass.self
+  var finalClassType = MetatypeExampleFinalClass.self
+  var existentialType: (any Any).Type = Any.self
+  var existentialType2: Any.Type = Any.self
+  var after = 45
+}
+
+mirrors.test("MetatypeFields") {
+  var output = ""
+  let container = MetatypeContainer()
+  dump(container, to: &output)
+  expectEqual("""
+    ▿ Mirror.MetatypeContainer
+      - before: 42
+      - before2: 43
+      - structType: Swift.String #0
+      - enumType: Mirror.MetatypeExampleEnum #1
+      - tupleType: (Swift.Int, Swift.String, Swift.AnyObject) #2
+      - functionType: () -> () #3
+      - classType: Mirror.MetatypeExampleClass #4
+      - subclassType: Mirror.MetatypeExampleSubclass #5
+      - finalClassType: Mirror.MetatypeExampleFinalClass #6
+      - existentialType: Any #7
+      - existentialType2: Any #7
+      - after: 45
+
+    """,
+    output)
+}
+
 //===--- Tuples -----------------------------------------------------------===//
 //===----------------------------------------------------------------------===//