[clang][modules][deps] Implicit modules are out of date when their explicit imports are (#138920)

The dependency scanner mixes implicitly- and explicitly-built modules.
When an implicitly-built module imports an explicitly-built one, we
never run the modification time validation checks, resulting in an
out-of-date module cache. This PR fixes that by only skipping the
modification time validation checks when both the imported module and
its importer are built explicitly.

rdar://150230022

1 files changed, 5 insertions, 3 deletions

diff --git a/clang/lib/Serialization/ModuleManager.cpp b/clang/lib/Serialization/ModuleManager.cpp
index e3d7ff4..7f3f246 100644
--- a/clang/lib/Serialization/ModuleManager.cpp
+++ b/clang/lib/Serialization/ModuleManager.cpp
@@ -110,10 +110,12 @@ ModuleManager::addModule(StringRef FileName, ModuleKind Type,
   // Look for the file entry. This only fails if the expected size or
   // modification time differ.
   OptionalFileEntryRef Entry;
-  const bool IgnoreModTime =
-      (Type == MK_ExplicitModule || Type == MK_PrebuiltModule);
+  bool IgnoreModTime = Type == MK_ExplicitModule || Type == MK_PrebuiltModule;
+  if (ImportedBy)
+    IgnoreModTime &= ImportedBy->Kind == MK_ExplicitModule ||
+                     ImportedBy->Kind == MK_PrebuiltModule;
   if (IgnoreModTime) {
-    // If we're not expecting to pull this file out of the module cache, it
+    // If neither this file nor the importer are in the module cache, this file
     // might have a different mtime due to being moved across filesystems in
     // a distributed build. The size must still match, though. (As must the
     // contents, but we can't check that.)