diff --git a/clang/lib/Lex/ModuleMap.cpp b/clang/lib/Lex/ModuleMap.cpp
index 53b824baba58be..08381d0422058e 100644
--- a/clang/lib/Lex/ModuleMap.cpp
+++ b/clang/lib/Lex/ModuleMap.cpp
@@ -2173,7 +2173,7 @@ void ModuleMapParser::parseExternModuleDecl() {
   }
   if (auto File = SourceMgr.getFileManager().getFile(FileNameRef))
     Map.parseModuleMapFile(
-        *File, /*IsSystem=*/false,
+        *File, IsSystem,
         Map.HeaderInfo.getHeaderSearchOpts().ModuleMapFileHomeIsCwd
             ? Directory
             : (*File)->getDir(),
diff --git a/clang/test/Modules/fmodules-validate-once-per-build-session.c b/clang/test/Modules/fmodules-validate-once-per-build-session.c
index 840545bb59fc9b..0c97342a8d70ae 100644
--- a/clang/test/Modules/fmodules-validate-once-per-build-session.c
+++ b/clang/test/Modules/fmodules-validate-once-per-build-session.c
@@ -1,4 +1,5 @@
 #include "foo.h"
+#include "bar.h"
 
 // Clear the module cache.
 // RUN: rm -rf %t
@@ -9,51 +10,74 @@
 // Create a module.  We will use -I or -isystem to determine whether to treat
 // foo.h as a system header.
 // RUN: echo 'void meow(void);' > %t/Inputs/foo.h
+// RUN: echo 'void woof(void);' > %t/Inputs/bar.h
 // RUN: echo 'module Foo { header "foo.h" }' > %t/Inputs/module.map
+// RUN: echo 'extern module Bar "bar.modulemap"' >> %t/Inputs/module.map
+// RUN: echo 'module Bar { header "bar.h" }' > %t/Inputs/bar.modulemap
 
 // ===
 // Compile the module.
 // RUN: %clang_cc1 -cc1 -fmodules -fimplicit-module-maps -fdisable-module-hash -fmodules-cache-path=%t/modules-cache -fsyntax-only -isystem %t/Inputs -fbuild-session-timestamp=1390000000 -fmodules-validate-once-per-build-session %s
 // RUN: %clang_cc1 -cc1 -fmodules -fimplicit-module-maps -fdisable-module-hash -fmodules-cache-path=%t/modules-cache-user -fsyntax-only -I %t/Inputs -fbuild-session-timestamp=1390000000 -fmodules-validate-once-per-build-session %s
 // RUN: ls -R %t/modules-cache | grep Foo.pcm.timestamp
+// RUN: ls -R %t/modules-cache | grep Bar.pcm.timestamp
 // RUN: ls -R %t/modules-cache-user | grep Foo.pcm.timestamp
+// RUN: ls -R %t/modules-cache-user | grep Bar.pcm.timestamp
 // RUN: cp %t/modules-cache/Foo.pcm %t/modules-to-compare/Foo-before.pcm
+// RUN: cp %t/modules-cache/Bar.pcm %t/modules-to-compare/Bar-before.pcm
 // RUN: cp %t/modules-cache-user/Foo.pcm %t/modules-to-compare/Foo-before-user.pcm
+// RUN: cp %t/modules-cache-user/Bar.pcm %t/modules-to-compare/Bar-before-user.pcm
 
 // ===
 // Use it, and make sure that we did not recompile it.
 // RUN: %clang_cc1 -cc1 -fmodules -fimplicit-module-maps -fdisable-module-hash -fmodules-cache-path=%t/modules-cache -fsyntax-only -isystem %t/Inputs -fbuild-session-timestamp=1390000000 -fmodules-validate-once-per-build-session %s
 // RUN: %clang_cc1 -cc1 -fmodules -fimplicit-module-maps -fdisable-module-hash -fmodules-cache-path=%t/modules-cache-user -fsyntax-only -I %t/Inputs -fbuild-session-timestamp=1390000000 -fmodules-validate-once-per-build-session %s
 // RUN: ls -R %t/modules-cache | grep Foo.pcm.timestamp
+// RUN: ls -R %t/modules-cache | grep Bar.pcm.timestamp
 // RUN: ls -R %t/modules-cache-user | grep Foo.pcm.timestamp
+// RUN: ls -R %t/modules-cache-user | grep Bar.pcm.timestamp
 // RUN: cp %t/modules-cache/Foo.pcm %t/modules-to-compare/Foo-after.pcm
+// RUN: cp %t/modules-cache/Bar.pcm %t/modules-to-compare/Bar-after.pcm
 // RUN: cp %t/modules-cache-user/Foo.pcm %t/modules-to-compare/Foo-after-user.pcm
+// RUN: cp %t/modules-cache-user/Bar.pcm %t/modules-to-compare/Bar-after-user.pcm
 
 // RUN: diff %t/modules-to-compare/Foo-before.pcm %t/modules-to-compare/Foo-after.pcm
+// RUN: diff %t/modules-to-compare/Bar-before.pcm %t/modules-to-compare/Bar-after.pcm
 // RUN: diff %t/modules-to-compare/Foo-before-user.pcm %t/modules-to-compare/Foo-after-user.pcm
+// RUN: diff %t/modules-to-compare/Bar-before-user.pcm %t/modules-to-compare/Bar-after-user.pcm
 
 // ===
 // Change the sources.
 // RUN: echo 'void meow2(void);' > %t/Inputs/foo.h
+// RUN: echo 'module Bar { header "bar.h" export * }' > %t/Inputs/bar.modulemap
 
 // ===
-// Use the module, and make sure that we did not recompile it if foo.h is a
-// system header, even though the sources changed.
+// Use the module, and make sure that we did not recompile it if foo.h or
+// module.map are system files, even though the sources changed.
 // RUN: %clang_cc1 -cc1 -fmodules -fimplicit-module-maps -fdisable-module-hash -fmodules-cache-path=%t/modules-cache -fsyntax-only -isystem %t/Inputs -fbuild-session-timestamp=1390000000 -fmodules-validate-once-per-build-session %s
 // RUN: %clang_cc1 -cc1 -fmodules -fimplicit-module-maps -fdisable-module-hash -fmodules-cache-path=%t/modules-cache-user -fsyntax-only -I %t/Inputs -fbuild-session-timestamp=1390000000 -fmodules-validate-once-per-build-session %s
 // RUN: ls -R %t/modules-cache | grep Foo.pcm.timestamp
+// RUN: ls -R %t/modules-cache | grep Bar.pcm.timestamp
 // RUN: ls -R %t/modules-cache-user | grep Foo.pcm.timestamp
+// RUN: ls -R %t/modules-cache-user | grep Bar.pcm.timestamp
 // RUN: cp %t/modules-cache/Foo.pcm %t/modules-to-compare/Foo-after.pcm
+// RUN: cp %t/modules-cache/Bar.pcm %t/modules-to-compare/Bar-after.pcm
 // RUN: cp %t/modules-cache-user/Foo.pcm %t/modules-to-compare/Foo-after-user.pcm
+// RUN: cp %t/modules-cache-user/Bar.pcm %t/modules-to-compare/Bar-after-user.pcm
 
 // RUN: diff %t/modules-to-compare/Foo-before.pcm %t/modules-to-compare/Foo-after.pcm
+// RUN: diff %t/modules-to-compare/Bar-before.pcm %t/modules-to-compare/Bar-after.pcm
 // When foo.h is a user header, we will always validate it.
 // RUN: not diff %t/modules-to-compare/Foo-before-user.pcm %t/modules-to-compare/Foo-after-user.pcm
+// RUN: not diff %t/modules-to-compare/Bar-before-user.pcm %t/modules-to-compare/Bar-after-user.pcm
 
 // ===
 // Recompile the module if the today's date is before 01 January 2100.
 // RUN: %clang_cc1 -cc1 -fmodules -fimplicit-module-maps -fdisable-module-hash -fmodules-cache-path=%t/modules-cache -fsyntax-only -isystem %t/Inputs -fbuild-session-timestamp=4102441200 -fmodules-validate-once-per-build-session %s
 // RUN: ls -R %t/modules-cache | grep Foo.pcm.timestamp
+// RUN: ls -R %t/modules-cache | grep Bar.pcm.timestamp
 // RUN: cp %t/modules-cache/Foo.pcm %t/modules-to-compare/Foo-after.pcm
+// RUN: cp %t/modules-cache/Bar.pcm %t/modules-to-compare/Bar-after.pcm
 
 // RUN: not diff %t/modules-to-compare/Foo-before.pcm %t/modules-to-compare/Foo-after.pcm
+// RUN: not diff %t/modules-to-compare/Bar-before.pcm %t/modules-to-compare/Bar-after.pcm