[Sema] main can't be declared as global variable, in C++.

So, we now reject that. We also warn for any external-linkage global
variable named main in C, because it results in undefined behavior.

PR:	  24309
Differential Revision:	http://reviews.llvm.org/D11658
Reviewed by:	rsmith

llvm-svn: 245051

1 files changed, 16 insertions, 0 deletions

diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index 7c54986f568..c3fdd900cd0 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -6111,6 +6111,22 @@ Sema::ActOnVariableDeclarator(Scope *S, Declarator &D, DeclContext *DC,
     }
   }
 
+  // Special handling of variable named 'main'.
+  if (Name.isIdentifier() && 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().CPlusPlus)
+      Diag(D.getLocStart(), diag::err_main_global_variable);
+
+    // In C, and external-linkage variable named main results in undefined
+    // behavior.
+    else if (NewVD->hasExternalFormalLinkage())
+      Diag(D.getLocStart(), diag::warn_main_redefined);
+  }
+
   if (D.isRedeclaration() && !Previous.empty()) {
     checkDLLAttributeRedeclaration(
         *this, dyn_cast<NamedDecl>(Previous.getRepresentativeDecl()), NewVD,