[Clang] Strengthen checks for main to meet [basic.start.main]p3’s requirements (llvm#101853)

Fixes llvm#101512.

Author: a-tarasyuk

clang/docs/ReleaseNotes.rst

@@ -160,6 +160,8 @@ Improvements to Clang's diagnostics
 - Clang now always preserves the template arguments as written used
   to specialize template type aliases.
 
+- Clang now diagnoses the use of ``main`` in an ``extern`` context as invalid according to [basic.start.main] p3. Fixes #GH101512.
+
 Improvements to Clang's time-trace
 ----------------------------------
 

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -985,11 +985,14 @@ def err_main_arg_wrong : Error<"%select{first|second|third|fourth}0 "
 def warn_main_returns_bool_literal : Warning<"bool literal returned from "
     "'main'">, InGroup<Main>;
 def err_main_global_variable :
-    Error<"main cannot be declared as global variable">;
+    Error<"main cannot be declared as a variable %select{in the global scope|with C language linkage}0">;
 def warn_main_redefined : Warning<"variable named 'main' with external linkage "
     "has undefined behavior">, InGroup<Main>;
 def ext_main_used : Extension<
     "referring to 'main' within an expression is a Clang extension">, InGroup<Main>;
+def ext_main_invalid_linkage_specification : ExtWarn<
+  "'main' should not be "
+  "'extern \"%select{C|C++}0\"'">, InGroup<Main>;
 
 /// parser diagnostics
 def ext_no_declarators : ExtWarn<"declaration does not declare anything">,

clang/lib/AST/Decl.cpp

@@ -3292,11 +3292,9 @@ bool FunctionDecl::isImmediateFunction() const {
 }
 
 bool FunctionDecl::isMain() const {
-  const TranslationUnitDecl *tunit =
-    dyn_cast<TranslationUnitDecl>(getDeclContext()->getRedeclContext());
-  return tunit &&
-         !tunit->getASTContext().getLangOpts().Freestanding &&
-         isNamed(this, "main");
+  return isNamed(this, "main") && !getLangOpts().Freestanding &&
+         (getDeclContext()->getRedeclContext()->isTranslationUnit() ||
+          isExternC());
 }
 
 bool FunctionDecl::isMSVCRTEntryPoint() const {

clang/lib/Sema/SemaDecl.cpp

@@ -7353,6 +7353,15 @@ void emitReadOnlyPlacementAttrWarning(Sema &S, const VarDecl *VD) {
   }
 }
 
+// Checks if VD is declared at global scope or with C language linkage.
+static bool isMainVar(DeclarationName Name, VarDecl *VD) {
+  return Name.getAsIdentifierInfo() &&
+         Name.getAsIdentifierInfo()->isStr("main") &&
+         !VD->getDescribedVarTemplate() &&
+         (VD->getDeclContext()->getRedeclContext()->isTranslationUnit() ||
+          VD->isExternC());
+}
+
 NamedDecl *Sema::ActOnVariableDeclarator(
     Scope *S, Declarator &D, DeclContext *DC, TypeSourceInfo *TInfo,
     LookupResult &Previous, MultiTemplateParamsArg TemplateParamLists,
@@ -8053,14 +8062,15 @@ NamedDecl *Sema::ActOnVariableDeclarator(
   }
 
   // Special handling of variable named 'main'.
-  if (Name.getAsIdentifierInfo() && Name.getAsIdentifierInfo()->isStr("main") &&
-      NewVD->getDeclContext()->getRedeclContext()->isTranslationUnit() &&
-      !getLangOpts().Freestanding && !NewVD->getDescribedVarTemplate()) {
-
-    // C++ [basic.start.main]p3
-    // A program that declares a variable main at global scope is ill-formed.
+  if (!getLangOpts().Freestanding && isMainVar(Name, NewVD)) {
+    // C++ [basic.start.main]p3:
+    //   A program that declares
+    //    - a variable main at global scope, or
+    //    - an entity named main with C language linkage (in any namespace)
+    //   is ill-formed
     if (getLangOpts().CPlusPlus)
-      Diag(D.getBeginLoc(), diag::err_main_global_variable);
+      Diag(D.getBeginLoc(), diag::err_main_global_variable)
+          << NewVD->isExternC();
 
     // In C, and external-linkage variable named main results in undefined
     // behavior.
@@ -12211,7 +12221,18 @@ bool Sema::CheckFunctionDeclaration(Scope *S, FunctionDecl *NewFD,
   return Redeclaration;
 }
 
-void Sema::CheckMain(FunctionDecl* FD, const DeclSpec& DS) {
+void Sema::CheckMain(FunctionDecl *FD, const DeclSpec &DS) {
+  // [basic.start.main]p3
+  //    The main function shall not be declared with a linkage-specification.
+  if (FD->isExternCContext() ||
+      (FD->isExternCXXContext() &&
+       FD->getDeclContext()->getRedeclContext()->isTranslationUnit())) {
+    Diag(FD->getLocation(), diag::ext_main_invalid_linkage_specification)
+        << FD->getLanguageLinkage();
+    FD->setInvalidDecl();
+    return;
+  }
+
   // C++11 [basic.start.main]p3:
   //   A program that [...] declares main to be inline, static or
   //   constexpr is ill-formed.

clang/test/CXX/basic/basic.start/basic.start.main/p3.cpp

@@ -8,9 +8,12 @@
 // RUN: %clang_cc1 -fsyntax-only -verify %s -DTEST8
 // RUN: %clang_cc1 -fsyntax-only -verify %s -DTEST9
 // RUN: %clang_cc1 -fsyntax-only -verify %s -DTEST10 -ffreestanding
+// RUN: %clang_cc1 -fsyntax-only -verify -pedantic %s -DTEST11
+// RUN: %clang_cc1 -fsyntax-only -verify -pedantic %s -DTEST12
+// RUN: %clang_cc1 -fsyntax-only -verify -pedantic %s -DTEST13
 
 #if TEST1
-int main; // expected-error{{main cannot be declared as global variable}}
+int main; // expected-error{{main cannot be declared as a variable in the global scope}}
 
 #elif TEST2
 // expected-no-diagnostics
@@ -46,7 +49,7 @@ namespace foo {
 #elif TEST8
 void z(void)
 {
-  extern int main;  // expected-error{{main cannot be declared as global variable}}
+  extern int main;  // expected-error{{main cannot be declared as a variable in the global scope}}}
 }
 
 #elif TEST9
@@ -61,6 +64,64 @@ int q(void)
 // expected-no-diagnostics
 int main;
 
+#elif TEST11
+extern "C" {
+  namespace Y {
+    int main; // expected-error {{main cannot be declared as a variable with C language linkage}}}
+  }
+}
+namespace ns {
+  extern "C" int main; // expected-error {{main cannot be declared as a variable with C language linkage}}
+}
+
+#elif TEST12
+extern "C" struct A { int main(); }; // ok
+
+namespace c {
+  extern "C" void main(); // expected-warning {{'main' should not be 'extern "C"'}}
+}
+
+extern "C" {
+  namespace Z {
+    void main(); // expected-warning {{'main' should not be 'extern "C"'}}
+  }
+}
+
+namespace ns {
+  extern "C" struct A {
+    int main; // ok
+  };
+
+  extern "C" struct B {
+    int main(); // ok
+  };
+}
+
+#elif TEST13
+extern "C++" {
+  int main(); // expected-warning {{'main' should not be 'extern "C++"'}}
+}
+
+extern "C" {
+  int main(); // expected-warning {{'main' should not be 'extern "C"'}}
+}
+
+extern "C" int main(); // expected-warning {{'main' should not be 'extern "C"'}}
+extern "C++" int main(); // expected-warning {{'main' should not be 'extern "C++"'}}
+
+namespace ns1 {
+  extern "C++" int main(); // ok
+  extern "C" {
+    extern "C++" {
+      int main(void *); // ok
+    }
+  }
+}
+
+namespace ns2 {
+  extern "C++" void main() {} // ok
+}
+
 #else
 #error Unknown Test
 #endif