[clang] Add test for CWG2811 "Clarify "use" of main"

[Endilll]

This patch covers CWG2811 "Clarify "use" of main", basically adding a test for -Wmain, focusing on usages of main in unevaluated contexts.

To my understanding, the diagnostic message is based on the wording, so I updated it based on the new wording. I also replaces "ISO C++ requires" with a phrasing that explicitly says "extension".

[llvmbot]

@llvm/pr-subscribers-clang

Author: Vlad Serebrennikov (Endilll)

Changes

This patch covers CWG2811 "Clarify "use" of main", basically adding a test for -Wmain, focusing on usages of main in unevaluated contexts.

To my understanding, the diagnostic message is based on the wording, so I updated it based on the new wording. I also replaces "ISO C++ requires" with a phrasing that explicitly says "extension".

---

Full diff: https://github.com/llvm/llvm-project/pull/96168.diff

4 Files Affected:

• (modified) clang/include/clang/Basic/DiagnosticSemaKinds.td (+1-1)
• (modified) clang/test/CXX/basic/basic.start/basic.start.init/p3.cpp (+2-2)
• (modified) clang/test/CXX/drs/cwg28xx.cpp (+19)
• (modified) clang/www/cxx_dr_status.html (+1-1)

diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
index 14736784cff5f..b747d2637bd5f 100644
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -964,7 +964,7 @@ def err_main_global_variable :
 def warn_main_redefined : Warning<"variable named 'main' with external linkage "
     "has undefined behavior">, InGroup<Main>;
 def ext_main_used : Extension<
-  "ISO C++ does not allow 'main' to be used by a program">, InGroup<Main>;
+  "expressions that refer to 'main' are an extension">, InGroup<Main>;
 
 /// parser diagnostics
 def ext_no_declarators : ExtWarn<"declaration does not declare anything">,
diff --git a/clang/test/CXX/basic/basic.start/basic.start.init/p3.cpp b/clang/test/CXX/basic/basic.start/basic.start.init/p3.cpp
index 506232ebacc4c..8f215172235c2 100644
--- a/clang/test/CXX/basic/basic.start/basic.start.init/p3.cpp
+++ b/clang/test/CXX/basic/basic.start/basic.start.init/p3.cpp
@@ -16,9 +16,9 @@ int main(int argc, char **argv)
   = delete; // expected-error {{'main' is not allowed to be deleted}}
 #else
 {
-  int (*pmain)(int, char**) = &main; // expected-error {{ISO C++ does not allow 'main' to be used by a program}}
+  int (*pmain)(int, char**) = &main; // expected-error {{expressions that refer to 'main' are an extension}}
 
   if (argc)
-    main(0, 0); // expected-error {{ISO C++ does not allow 'main' to be used by a program}}
+    main(0, 0); // expected-error {{expressions that refer to 'main' are an extension}}
 }
 #endif
diff --git a/clang/test/CXX/drs/cwg28xx.cpp b/clang/test/CXX/drs/cwg28xx.cpp
index da81eccc8dc22..049d90a1f7b20 100644
--- a/clang/test/CXX/drs/cwg28xx.cpp
+++ b/clang/test/CXX/drs/cwg28xx.cpp
@@ -6,6 +6,25 @@
 // RUN: %clang_cc1 -std=c++23 -pedantic-errors -verify=expected,since-cxx20,since-cxx23 %s
 // RUN: %clang_cc1 -std=c++2c -pedantic-errors -verify=expected,since-cxx20,since-cxx23,since-cxx26 %s
 
+
+int main() {} // required for cwg2811
+
+namespace cwg2811 { // cwg2811: 3.5
+#if __cplusplus >= 201103L
+void f() {
+  (void)[&] {
+    using T = decltype(main);
+    // expected-error@-1 {{expressions that refer to 'main' are an extension}}
+  };
+  using T2 = decltype(main);
+  // expected-error@-1 {{expressions that refer to 'main' are an extension}}
+}
+
+using T = decltype(main);
+// expected-error@-1 {{expressions that refer to 'main' are an extension}}
+#endif
+} // namespace cwg2811
+
 namespace cwg2819 { // cwg2819: 19 tentatively ready 2023-12-01
 #if __cpp_constexpr >= 202306L
   constexpr void* p = nullptr;
diff --git a/clang/www/cxx_dr_status.html b/clang/www/cxx_dr_status.html
index dac38cedfcb75..d6b7c32dd844d 100755
--- a/clang/www/cxx_dr_status.html
+++ b/clang/www/cxx_dr_status.html
@@ -16675,7 +16675,7 @@ <h2 id="cxxdr">C++ defect report implementation status</h2>
     <td><a href="https://cplusplus.github.io/CWG/issues/2811.html">2811</a></td>
     <td>DR</td>
     <td>Clarify "use" of main</td>
-    <td class="unknown" align="center">Unknown</td>
+    <td class="full" align="center">Clang 3.5</td>
   </tr>
   <tr class="open" id="2812">
     <td><a href="https://cplusplus.github.io/CWG/issues/2812.html">2812</a></td>

[cor3ntin]

I think I prefer the current wording, it's clearer that it's not conforming (at the very least we should say something like "clang extension". and maybe "referring to main is a clang extension".
LGTM otherwise

do we have test that this is fine?

namespace foo {
    int main();
    using a = decltype(&main);
}```

[Endilll]

it's clearer that it's not conforming (at the very least we should say something like "clang extension"

As someone who was a user of Clang not so long ago, I have to say that the only thing "ISO C++ requires" conveyed to me was "you're doing something wrong". For me it never had a clear interpretation "you're asking for an extension, but we're not giving it to you". On the other hand, I believe the word "extension" is clearly understood to describe something outside of ISO C++, somewhat by definition. At least that would match my understanding 3-4 years ago.

CC @AaronBallman I remember we discussed it offline a while ago.

[Endilll]

do we have test that this is fine?

namespace foo {
    int main();
    using a = decltype(&main);
}```

Thank you, I added the test.

[cor3ntin]

LGTM if @AaronBallman is happy

("expressions that... are an extensions" still sounds weird to me, cf previous comment)

[AaronBallman]

LGTM!