[clang] [modules] Add err_main_in_named_module

[ashwinbanwari]

Close #146229

As the issue said, main shouldn't be in any modules.

new error output:

/my/code/directory/main.cpp:3:1: error: 'main' cannot be attached to a named module
    3 | int main() {
      | ^
1 error generated.

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[llvmbot]

@llvm/pr-subscribers-clang

Author: Ashwin Banwari (ashwinbanwari)

Changes

Close #146229

As the issue said, main shouldn't be in any modules.

New Error Output:

/my/code/directory/main.cpp:3:1: error: 'main' cannot be attached to a named module
    3 | int main() {
      | ^
1 error generated.

---

Full diff: https://github.com/llvm/llvm-project/pull/146247.diff

2 Files Affected:

• (modified) clang/include/clang/Basic/DiagnosticSemaKinds.td (+2)
• (modified) clang/lib/Sema/SemaDecl.cpp (+8)

diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
index 5062505cf3c01..ce9017ded0087 100644
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -1062,6 +1062,8 @@ 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 err_main_in_named_module
+    : Error<"'main' cannot be attached to a 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>;
diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index e1cccf068b5aa..c4ddfda9f447f 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -12490,6 +12490,14 @@ void Sema::CheckMain(FunctionDecl *FD, const DeclSpec &DS) {
                                 : FixItHint());
       FD->setInvalidDecl(true);
     }
+
+    // In C++ [basic.start.main]p3, it is said a program attaching main to a
+    // named module is ill-formed.
+    if (FD->isInNamedModule()) {
+      Diag(FD->getTypeSpecStartLoc(), diag::err_main_in_named_module)
+          << FixItHint();
+      FD->setInvalidDecl(true);
+    }
   }
 
   // Treat protoless main() as nullary.

[Mr-Anyone]

You should add a test case and a release note for this patch to be accepted.

[MikailBag]

It seems that clang trunk warns on extern "C++" main, which is now allowed after P3618.

I think it will be helpful for the UX to land this change after that warning is fixed.

(Disclaimer: I'm not clang contributor, just clang user)

[ChuanqiXu9]

I prefer a warning instead of hard error. There are already users who uses this.