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

Close #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.

Author: ChuanqiXu9

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -11693,6 +11693,8 @@ def err_module_not_defined : Error<
 def err_module_redeclaration : Error<
   "translation unit contains multiple module declarations">;
 def note_prev_module_declaration : Note<"previous module declaration is here">;
+def err_module_declaration_missing : Error<
+  "missing 'export module' declaration in module interface unit">;
 def err_module_declaration_missing_after_global_module_introducer : Error<
   "missing 'module' declaration at end of global module fragment "
   "introduced here">;

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.

clang/test/CXX/module/dcl.dcl/dcl.module/dcl.module.interface/p1.cppm

@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 -std=c++20 %s -verify -o /dev/null
+// RUN: %clang_cc1 -std=c++20 %s -verify -emit-module-interface -o /dev/null
 // RUN: %clang_cc1 -std=c++20 %s -DINTERFACE -verify -emit-module-interface -o %t
 // RUN: %clang_cc1 -std=c++20 %s -DIMPLEMENTATION -verify -fmodule-file=A=%t -o /dev/null
 //
@@ -15,6 +15,8 @@ module A; // #module-decl
   // expected-error@-2 {{missing 'export' specifier in module declaration while building module interface}}
   #define INTERFACE
  #endif
+#else // Not in a module
+// expected-error@* {{missing 'export module' declaration in module interface unit}}
 #endif
 
 #ifndef INTERFACE

clang/test/Modules/pr101398.cppm

@@ -0,0 +1,5 @@
+// RUN: mkdir -p %t
+// RUN: %clang -std=c++20 -xc++-module %s -Xclang -verify --precompile -o %t/tmp.pcm
+// not modules
+
+// expected-error@* {{missing 'export module' declaration in module interface unit}}