[C++20] [Modules] Emit Errors when compiling a non-module source as module (#102565)

Close https://github.com/llvm/llvm-project/issues/101398

The root cause of the issue is that I removed the codes before and
failed to recognize it in time and this was not found for a long time
due to it only crashes with invalid codes.

1 files changed, 12 insertions, 0 deletions

diff --git a/clang/lib/Sema/Sema.cpp b/clang/lib/Sema/Sema.cpp
index 19d8692..633b822 100644
--- a/clang/lib/Sema/Sema.cpp
+++ b/clang/lib/Sema/Sema.cpp
@@ -1272,6 +1272,18 @@ void Sema::ActOnEndOfTranslationUnit() {
                                    Module::ExplicitGlobalModuleFragment) {
     Diag(ModuleScopes.back().BeginLoc,
          diag::err_module_declaration_missing_after_global_module_introducer);
+  } else if (getLangOpts().getCompilingModule() ==
+                 LangOptions::CMK_ModuleInterface &&
+             // We can't use ModuleScopes here since ModuleScopes is always
+             // empty if we're compiling the BMI.
+             !getASTContext().getCurrentNamedModule()) {
+    // If we are building a module interface unit, we should have seen the
+    // module declaration.
+    //
+    // FIXME: Make a better guess as to where to put the module declaration.
+    Diag(getSourceManager().getLocForStartOfFile(
+             getSourceManager().getMainFileID()),
+         diag::err_module_declaration_missing);
   }
 
   // Now we can decide whether the modules we're building need an initializer.