[clang] Fix a segfault when M is a nullptr

Author: matts1

clang/include/clang/Basic/Module.h

@@ -888,7 +888,7 @@ class VisibleModuleSet {
 
   /// Get the location at which the import of a module was triggered.
   SourceLocation getImportLoc(const Module *M) const {
-    return M->getVisibilityID() < ImportLocs.size()
+    return M && M->getVisibilityID() < ImportLocs.size()
                ? ImportLocs[M->getVisibilityID()]
                : SourceLocation();
   }

clang/test/Modules/pr130712.cppm

@@ -0,0 +1,33 @@
+// RUN: split-file %s %t
+
+// There are two requirements here to result in the owner of a macro being null.
+// 1) There must be a configuration mismatch between a header and a file it depends on
+// 2) -fmodules-local-submodule-visibility must be enabled.
+
+// Unfortunately, libcxx is compiled with exceptions, but the sysroot it depends on is likely compiled without exceptions.
+
+// RUN: %clang_cc1 -I%t -emit-module -o %t/a.pcm -fmodules %t/module.modulemap -fmodule-name=a -fmodules-local-submodule-visibility 
+// RUN: %clang_cc1 -fexceptions -Wno-module-file-config-mismatch -I%t -emit-module -o %t/b.pcm -fmodules %t/module.modulemap -fmodule-name=b -fmodules-local-submodule-visibility -fmodule-file=%t/a.pcm
+// RUN: %clang_cc1 -fexceptions -Wno-module-file-config-mismatch -I%t -emit-module -o %t/c.pcm -fmodules %t/module.modulemap -fmodule-name=c -fmodules-local-submodule-visibility -fmodule-file=%t/a.pcm -fmodule-file=%t/b.pcm
+
+//--- module.modulemap
+module a { header "a.h" }
+module b { header "b.h" }
+module c { header "c.h" }
+
+//--- a.h
+#ifndef A_H
+#define A_H
+#endif
+
+//--- b.h
+#ifndef B_H
+#define B_H
+
+#include <a.h>
+
+#endif
+
+//--- c.h
+#include <a.h>
+#include <b.h>