[clang] [modules] Add err_main_in_named_module (#146635)

Revival of https://github.com/llvm/llvm-project/pull/146247 which got
reverted for broken test.

Now that https://github.com/llvm/llvm-project/pull/146461 is merged to
allow `extern "C++"` for main, we can merge this change.

clang/docs/ReleaseNotes.rst

@@ -661,6 +661,9 @@ Improvements to Clang's diagnostics
   diagnostics when floating-point numbers had both width field and plus or space
   prefix specified. (#GH143951)
 
+- A warning is now emitted when ``main`` is attached to a named module,
+  which can be turned off with ``-Wno-main-attached-to-named-module``. (#GH146247)
+
 - Clang now avoids issuing `-Wreturn-type` warnings in some cases where
   the final statement of a non-void function is a `throw` expression, or
   a call to a function that is trivially known to always throw (i.e., its

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -1062,6 +1062,9 @@ def err_constexpr_main : Error<
   "'main' is not allowed to be declared %select{constexpr|consteval}0">;
 def err_deleted_main : Error<"'main' is not allowed to be deleted">;
 def err_mainlike_template_decl : Error<"%0 cannot be a template">;
+def warn_main_in_named_module
+    : ExtWarn<"'main' never has module linkage">,
+      InGroup<DiagGroup<"main-attached-to-named-module">>;
 def err_main_returns_nonint : Error<"'main' must return 'int'">;
 def ext_main_returns_nonint : ExtWarn<"return type of 'main' is not 'int'">,
     InGroup<MainReturnType>;

clang/lib/Sema/SemaDecl.cpp

@@ -12486,6 +12486,15 @@ void Sema::CheckMain(FunctionDecl *FD, const DeclSpec &DS) {
                                 : FixItHint());
       FD->setInvalidDecl(true);
     }
+
+    // [basic.start.main]p3:
+    // A program that declares a function main that belongs to the global scope
+    // and is attached to a named module is ill-formed.
+    if (FD->isInNamedModule()) {
+      const SourceLocation start = FD->getTypeSpecStartLoc();
+      Diag(start, diag::warn_main_in_named_module)
+          << FixItHint::CreateInsertion(start, "extern \"C++\" ", true);
+    }
   }
 
   // Treat protoless main() as nullary.

clang/test/Driver/autocomplete.c

@@ -111,6 +111,7 @@
 // RUN: %clang --autocomplete=-Wma | FileCheck %s -check-prefix=WARNING
 // WARNING: -Wmacro-redefined
 // WARNING-NEXT: -Wmain
+// WARNING-NEXT: -Wmain-attached-to-named-module
 // WARNING-NEXT: -Wmain-return-type
 // WARNING-NEXT: -Wmalformed-warning-check
 // WARNING-NEXT: -Wmany-braces-around-scalar-init

clang/test/SemaCXX/modules.cppm

@@ -41,6 +41,8 @@ struct S {
   export static int n; // expected-error {{expected member name or ';'}}
 };
 
+int main() {} // expected-warning {{'main' never has module linkage}}
+
 // FIXME: Exports of declarations without external linkage are disallowed.
 // Exports of declarations with non-external-linkage types are disallowed.