[clang][modules] Correctly set module map systemness

clang/lib/Frontend/CompilerInstance.cpp

@@ -1360,10 +1360,17 @@ static bool compileModule(CompilerInstance &ImportingInstance,
 
     StringRef ModuleMapFilePath = ModuleMapFile->getNameAsRequested();
 
+    // Use the systemness of the module map as parsed instead of using the
+    // IsSystem attribute of the module. If the module has [system] but the
+    // module map is not in a system path, then this would incorrectly parse
+    // any other modules in that module map as system too.
+    const SrcMgr::SLocEntry &SLoc = SourceMgr.getSLocEntry(ModuleMapFID);
+    bool IsSystem = isSystem(SLoc.getFile().getFileCharacteristic());
+
     // Use the module map where this module resides.
     Result = compileModuleImpl(
         ImportingInstance, ImportLoc, Module->getTopLevelModuleName(),
-        FrontendInputFile(ModuleMapFilePath, IK, +Module->IsSystem),
+        FrontendInputFile(ModuleMapFilePath, IK, IsSystem),
         ModMap.getModuleMapFileForUniquing(Module)->getName(), ModuleFileName);
   } else {
     // FIXME: We only need to fake up an input file here as a way of

clang/test/Modules/transitive-system.test

@@ -0,0 +1,27 @@
+// RUN: rm -rf %t
+// RUN: split-file %s %t
+
+// RUN: sed "s|DIR|%/t|g" %t/cdb.json.template > %t/cdb.json
+// RUN: clang-scan-deps -compilation-database %t/cdb.json -format experimental-full -module-name=direct > %t/result1.json
+// RUN: rm -rf %t/cache
+// RUN: clang-scan-deps -compilation-database %t/cdb.json -format experimental-full -module-name=transitive > %t/result2.json
+// RUN: %deps-to-rsp %t/result1.json --module-name transitive > %t/1.rsp
+// RUN: %deps-to-rsp %t/result2.json --module-name transitive > %t/2.rsp
+// RUN: diff %t/1.rsp %t/2.rsp
+
+//--- module.modulemap
+module direct [system] { header "direct.h" }
+module transitive { header "transitive.h" }
+
+//--- direct.h
+#include "transitive.h"
+
+//--- transitive.h
+// empty
+
+//--- cdb.json.template
+[{
+  "file": "",
+  "directory": "DIR",
+  "command": "clang -fmodules -fmodules-cache-path=DIR/cache -I DIR -x c"
+}]