[clang][modules][deps] Implicit modules are out of date when their explicit imports are

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.)

clang/test/ClangScanDeps/modules-pch-common-stale.c

@@ -0,0 +1,77 @@
+// Test that modifications to a common header (imported from both a PCH and a TU)
+// cause rebuilds of dependent modules imported from the TU on incremental build.
+
+// RUN: rm -rf %t
+// RUN: split-file %s %t
+
+//--- module.modulemap
+module mod_common { header "mod_common.h" }
+module mod_tu { header "mod_tu.h" }
+module mod_tu_extra { header "mod_tu_extra.h" }
+
+//--- mod_common.h
+#define MOD_COMMON_MACRO 0
+
+//--- mod_tu.h
+#include "mod_common.h"
+#if MOD_COMMON_MACRO
+#include "mod_tu_extra.h"
+#endif
+
+//--- mod_tu_extra.h
+
+//--- prefix.h
+#include "mod_common.h"
+
+//--- tu.c
+#include "mod_tu.h"
+
+// Clean: scan the PCH.
+// RUN: clang-scan-deps -format experimental-full -o %t/deps_pch.json -- \
+// RUN:     %clang -x c-header %t/prefix.h -o %t/prefix.h.pch -F %t \
+// RUN:     -fmodules -fimplicit-module-maps -fmodules-cache-path=%t/cache
+
+// Clean: build the PCH.
+// RUN: %deps-to-rsp %t/deps_pch.json --module-name mod_common > %t/mod_common.rsp
+// RUN: %deps-to-rsp %t/deps_pch.json --tu-index 0 > %t/pch.rsp
+// RUN: %clang @%t/mod_common.rsp
+// RUN: %clang @%t/pch.rsp
+
+// Clean: scan the TU.
+// RUN: clang-scan-deps -format experimental-full -o %t/deps_tu.json -- \
+// RUN:     %clang -c %t/tu.c -o %t/tu.o -include %t/prefix.h -F %t \
+// RUN:     -fmodules -fimplicit-module-maps -fmodules-cache-path=%t/cache
+
+// Clean: build the TU.
+// RUN: %deps-to-rsp %t/deps_tu.json --module-name mod_tu > %t/mod_tu.rsp
+// RUN: %deps-to-rsp %t/deps_tu.json --tu-index 0 > %t/tu.rsp
+// RUN: %clang @%t/mod_tu.rsp
+// RUN: %clang @%t/tu.rsp
+
+// Incremental: modify the common module.
+// RUN: echo "#define MOD_COMMON_MACRO 1" > %t/mod_common.h
+
+// Incremental: scan the PCH.
+// RUN: clang-scan-deps -format experimental-full -o %t/deps_pch.json -- \
+// RUN:     %clang -x c-header %t/prefix.h -o %t/prefix.h.pch -F %t \
+// RUN:     -fmodules -fimplicit-module-maps -fmodules-cache-path=%t/cache
+
+// Incremental: build the PCH.
+// RUN: %deps-to-rsp %t/deps_pch.json --module-name mod_common > %t/mod_common.rsp
+// RUN: %deps-to-rsp %t/deps_pch.json --tu-index 0 > %t/pch.rsp
+// RUN: %clang @%t/mod_common.rsp
+// RUN: %clang @%t/pch.rsp
+
+// Incremental: scan the TU. This needs to invalidate modules imported from the
+//              TU that depend on modules imported from the PCH.
+// RUN: clang-scan-deps -format experimental-full -o %t/deps_tu.json -- \
+// RUN:     %clang -c %t/tu.c -o %t/tu.o -include %t/prefix.h -F %t \
+// RUN:     -fmodules -fimplicit-module-maps -fmodules-cache-path=%t/cache
+
+// Incremental: build the TU.
+// RUN: %deps-to-rsp %t/deps_tu.json --module-name mod_tu_extra > %t/mod_tu_extra.rsp
+// RUN: %deps-to-rsp %t/deps_tu.json --module-name mod_tu > %t/mod_tu.rsp
+// RUN: %deps-to-rsp %t/deps_tu.json --tu-index 0 > %t/tu.rsp
+// RUN: %clang @%t/mod_tu_extra.rsp
+// RUN: %clang @%t/mod_tu.rsp
+// RUN: %clang @%t/tu.rsp