diff --git a/clang/lib/Serialization/ModuleManager.cpp b/clang/lib/Serialization/ModuleManager.cpp
--- a/clang/lib/Serialization/ModuleManager.cpp
+++ b/clang/lib/Serialization/ModuleManager.cpp
@@ -132,15 +132,40 @@
     return Missing;
   }
 
+  // The ModuleManager's use of FileEntry nodes as the keys for its map of
+  // loaded modules is less than ideal. Uniqueness for FileEntry nodes is
+  // maintained by FileManager, which in turn uses inode numbers on hosts
+  // that support that. When coupled with the module cache's proclivity for
+  // turning over and deleting stale PCMs, this means entries for different
+  // module files can wind up reusing the same underlying inode. When this
+  // happens, subsequent accesses to the Modules map will disagree on the
+  // ModuleFile associated with a given file. In general, it is not sufficient
+  // to resolve this conundrum with a type like FileEntryRef that stores the
+  // name of the FileEntry node on first access because of path canonicalization
+  // issues. However, the paths constructed for implicit module builds are
+  // fully under Clang's control. We *can*, therefore, rely on their structure
+  // being consistent across operating systems and across subsequent accesses
+  // to the Modules map.
+  auto implicitModuleNamesMatch = [](ModuleKind Kind,
+                                     const ModuleFile *MF,
+                                     const FileEntry *Entry) -> bool {
+    if (Kind != MK_ImplicitModule) {
+      return true;
+    }
+    return Entry->getName() == MF->FileName;
+  };
+
   // Check whether we already loaded this module, before
   if (ModuleFile *ModuleEntry = Modules.lookup(Entry)) {
-    // Check the stored signature.
-    if (checkSignature(ModuleEntry->Signature, ExpectedSignature, ErrorStr))
-      return OutOfDate;
-
-    Module = ModuleEntry;
-    updateModuleImports(*ModuleEntry, ImportedBy, ImportLoc);
-    return AlreadyLoaded;
+    if (implicitModuleNamesMatch(Type, ModuleEntry, Entry)) {
+      // Check the stored signature.
+      if (checkSignature(ModuleEntry->Signature, ExpectedSignature, ErrorStr))
+        return OutOfDate;
+
+      Module = ModuleEntry;
+      updateModuleImports(*ModuleEntry, ImportedBy, ImportLoc);
+      return AlreadyLoaded;
+    }
   }
 
   // Allocate a new module.