[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


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@245051 91177308-0d34-0410-b5e6-96231b3b80d8

include/clang/Basic/DiagnosticSemaKinds.td

@@ -510,6 +510,10 @@ def warn_main_one_arg : Warning<"only one parameter on 'main' declaration">,
 def err_main_arg_wrong : Error<"%select{first|second|third|fourth}0 "
     "parameter of 'main' (%select{argument count|argument array|environment|"
     "platform-specific data}0) must be of type %1">;
+def err_main_global_variable :
+    Error<"main cannot be declared as 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>;
 

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,

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

@@ -0,0 +1,66 @@
+// RUN: %clang_cc1 -fsyntax-only -verify %s -DTEST1
+// RUN: %clang_cc1 -fsyntax-only -verify %s -DTEST2
+// RUN: %clang_cc1 -fsyntax-only -verify %s -DTEST3
+// RUN: %clang_cc1 -fsyntax-only -verify %s -DTEST4
+// RUN: %clang_cc1 -fsyntax-only -verify %s -std=c++14 -DTEST5
+// RUN: %clang_cc1 -fsyntax-only -verify %s -std=c++14 -DTEST6
+// RUN: %clang_cc1 -fsyntax-only -verify %s -DTEST7
+// 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
+
+#if TEST1
+int main; // expected-error{{main cannot be declared as global variable}}
+
+#elif TEST2
+// expected-no-diagnostics
+int f () {
+  int main;
+  return main;
+}
+
+#elif TEST3
+// expected-no-diagnostics
+void x(int main) {};
+int y(int main);
+
+#elif TEST4
+// expected-no-diagnostics
+class A {
+  static int main;
+};
+
+#elif TEST5
+// expected-no-diagnostics
+template<class T> constexpr T main;
+
+#elif TEST6
+extern template<class T> constexpr T main; //expected-error{{expected unqualified-id}}
+
+#elif TEST7
+// expected-no-diagnostics
+namespace foo {
+  int main;
+}
+
+#elif TEST8
+void z(void)
+{
+  extern int main;  // expected-error{{main cannot be declared as global variable}}
+}
+
+#elif TEST9
+// expected-no-diagnostics
+int q(void)
+{
+  static int main;
+  return main;
+}
+
+#elif TEST10
+// expected-no-diagnostics
+int main;
+
+#else
+#error Unknown Test
+#endif

test/Sema/warn-extern-main.c

@@ -0,0 +1,56 @@
+// RUN: %clang_cc1 -fsyntax-only -verify %s -DTEST1
+// RUN: %clang_cc1 -fsyntax-only -verify %s -DTEST2
+// RUN: %clang_cc1 -fsyntax-only -verify %s -DTEST3
+// RUN: %clang_cc1 -fsyntax-only -verify %s -DTEST4
+// RUN: %clang_cc1 -fsyntax-only -verify %s -DTEST5
+// RUN: %clang_cc1 -fsyntax-only -verify %s -DTEST6
+// RUN: %clang_cc1 -fsyntax-only -verify %s -DTEST7
+// RUN: %clang_cc1 -fsyntax-only -verify %s -DTEST8
+// RUN: %clang_cc1 -fsyntax-only -verify %s -DTEST9 -ffreestanding
+
+#if TEST1
+int main; // expected-warning{{variable named 'main' with external linkage has undefined behavior}}
+
+#elif TEST2
+extern int main; // expected-warning{{variable named 'main' with external linkage has undefined behavior}}
+
+#elif TEST3
+// expected-no-diagnostics
+void x() {
+  static int main;
+}
+
+#elif TEST4
+void x() {
+  extern int main; // expected-warning{{variable named 'main' with external linkage has undefined behavior}}
+}
+
+#elif TEST5
+// expected-no-diagnostics
+void x() {
+  int main;
+}
+
+#elif TEST6
+// expected-no-diagnostics
+static int main;
+
+#elif TEST7
+// expected-no-diagnostics
+void x() {
+  auto int main;
+}
+
+#elif TEST8
+// expected-no-diagnostics
+void x() {
+  register int main;
+}
+
+#elif TEST9
+// expected-no-diagnostics
+int main;
+
+#else
+#error Unknown Test
+#endif