Index: lib/Basic/VirtualFileSystem.cpp
===================================================================
--- lib/Basic/VirtualFileSystem.cpp
+++ lib/Basic/VirtualFileSystem.cpp
@@ -1281,7 +1281,8 @@
     }
   }
 
-  std::unique_ptr<Entry> parseEntry(yaml::Node *N, RedirectingFileSystem *FS) {
+  std::unique_ptr<Entry> parseEntry(yaml::Node *N, RedirectingFileSystem *FS,
+                                    bool IsRootEntry) {
     auto *M = dyn_cast<yaml::MappingNode>(N);
     if (!M) {
       error(N, "expected mapping node for file or directory entry");
@@ -1302,6 +1303,7 @@
     std::vector<std::unique_ptr<Entry>> EntryArrayContents;
     std::string ExternalContentsPath;
     std::string Name;
+    yaml::Node *NameValueNode;
     auto UseExternalName = RedirectingFileEntry::NK_NotSet;
     EntryKind Kind;
 
@@ -1321,6 +1323,7 @@
         if (!parseScalarString(I.getValue(), Value, Buffer))
           return nullptr;
 
+        NameValueNode = I.getValue();
         if (FS->UseCanonicalizedPaths) {
           SmallString<256> Path(Value);
           // Guarantee that old YAML files containing paths with ".." and "."
@@ -1357,7 +1360,8 @@
         }
 
         for (auto &I : *Contents) {
-          if (std::unique_ptr<Entry> E = parseEntry(&I, FS))
+          if (std::unique_ptr<Entry> E =
+                  parseEntry(&I, FS, /*IsRootEntry*/ false))
             EntryArrayContents.push_back(std::move(E));
           else
             return nullptr;
@@ -1418,6 +1422,13 @@
       return nullptr;
     }
 
+    if (IsRootEntry && !sys::path::is_absolute(Name)) {
+      assert(NameValueNode && "Name presence should be checked earlier");
+      error(NameValueNode,
+            "entry with relative path at the root level is not discoverable");
+      return nullptr;
+    }
+
     // Remove trailing slash(es), being careful not to remove the root path
     StringRef Trimmed(Name);
     size_t RootPathLen = sys::path::root_path(Trimmed).size();
@@ -1500,7 +1511,8 @@
         }
 
         for (auto &I : *Roots) {
-          if (std::unique_ptr<Entry> E = parseEntry(&I, FS))
+          if (std::unique_ptr<Entry> E =
+                  parseEntry(&I, FS, /*IsRootEntry*/ true))
             RootEntries.push_back(std::move(E));
           else
             return false;
Index: unittests/Basic/VirtualFileSystemTest.cpp
===================================================================
--- unittests/Basic/VirtualFileSystemTest.cpp
+++ unittests/Basic/VirtualFileSystemTest.cpp
@@ -1468,3 +1468,35 @@
   }
   EXPECT_EQ(I, E);
 }
+
+TEST_F(VFSFromYAMLTest, RelativePaths) {
+  IntrusiveRefCntPtr<DummyFileSystem> Lower(new DummyFileSystem());
+  // Filename at root level without a parent directory.
+  IntrusiveRefCntPtr<vfs::FileSystem> FS = getFromYAMLString(
+      "{ 'roots': [\n"
+      "  { 'type': 'file', 'name': 'file-not-in-directory.h',\n"
+      "    'external-contents': '//root/external/file'\n"
+      "  }\n"
+      "] }", Lower);
+  EXPECT_EQ(nullptr, FS.get());
+
+  // Relative file path.
+  FS = getFromYAMLString(
+      "{ 'roots': [\n"
+      "  { 'type': 'file', 'name': 'relative/file/path.h',\n"
+      "    'external-contents': '//root/external/file'\n"
+      "  }\n"
+      "] }", Lower);
+  EXPECT_EQ(nullptr, FS.get());
+
+  // Relative directory path.
+  FS = getFromYAMLString(
+       "{ 'roots': [\n"
+       "  { 'type': 'directory', 'name': 'relative/directory/path.h',\n"
+       "    'contents': []\n"
+       "  }\n"
+       "] }", Lower);
+  EXPECT_EQ(nullptr, FS.get());
+
+  EXPECT_EQ(3, NumDiagnostics);
+}