[clang][modules] Guard against bad -fmodule-file mappings (#132059)

clang/lib/Serialization/ASTReader.cpp

@@ -3318,6 +3318,18 @@ ASTReader::ReadControlBlock(ModuleFile &F,
                                 Loaded, StoredSize, StoredModTime,
                                 StoredSignature, Capabilities);
 
+      // Check the AST we just read from ImportedFile contains a different
+      // module than we expected (ImportedName). This can occur for C++20
+      // Modules when given a mismatch via -fmodule-file=<name>=<file>
+      if (IsImportingStdCXXModule) {
+        if (const auto *Imported =
+                getModuleManager().lookupByFileName(ImportedFile);
+            Imported != nullptr && Imported->ModuleName != ImportedName) {
+          Diag(diag::err_failed_to_find_module_file) << ImportedName;
+          Result = Missing;
+        }
+      }
+
       // If we diagnosed a problem, produce a backtrace.
       bool recompilingFinalized = Result == OutOfDate &&
                                   (Capabilities & ARR_OutOfDate) &&

clang/test/Modules/fmodule-file-mismatch.cppm

@@ -0,0 +1,48 @@
+// RUN: rm -rf %t
+// RUN: split-file %s %t
+// RUN: cd %t
+
+// Related to issue #132059
+
+// Precompile the module dependencies correctly 
+// RUN: %clang_cc1 -std=c++20 -emit-module-interface a.cppm -o a.pcm
+// RUN: %clang_cc1 -std=c++20 -emit-module-interface b.cppm -o b.pcm \
+// RUN:     -fmodule-file=A=a.pcm
+
+// Verify that providing incorrect mappings via
+// `-fmodule-file=<name>=<path/to/bmi>` does not crash the compiler when loading
+// a module that imports the incorrectly mapped module.
+// RUN: not %clang_cc1 -std=c++20 main1.cpp -fmodule-file=A=b.pcm
+
+//--- a.cppm
+export module A;
+
+export int a() {
+  return 41;
+}
+
+//--- b.cppm
+export module B;
+import A;
+
+export int b() {
+  return a() + 1;
+}
+
+//--- main1.cpp
+import A;
+
+int main() {
+  return a();
+}
+
+// Test again for the case where the BMI is first loaded correctly
+// RUN: not %clang_cc1 -std=c++20 main2.cpp-fmodule-file=B=b.pcm \
+// RUN:     -fmodule-file=A=b.pcm
+
+//--- main2.cpp
+import B;
+
+int main() {
+  return b();
+}