Enforce restrictions that 'main' is not allowed to be deleted, or to be used by

the program, in C++. (We allow the latter as an extension, since we've always
permitted it, and GCC does the same, and our supported C++ ABIs don't do
anything special in main.)

llvm-svn: 199782

Author: zygoloid

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -425,6 +425,7 @@ def ext_noreturn_main : ExtWarn<
 def note_main_remove_noreturn : Note<"remove '_Noreturn'">;
 def err_constexpr_main : Error<
   "'main' is not allowed to be declared constexpr">;
+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_returns_nonint : Error<"'main' must return 'int'">;
 def ext_main_returns_nonint : ExtWarn<"return type of 'main' is not 'int'">,
@@ -437,6 +438,8 @@ 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 ext_main_used : Extension<
+  "ISO C++ does not allow 'main' to be used by a program">, InGroup<Main>;
 
 /// parser diagnostics
 def ext_no_declarators : ExtWarn<"declaration does not declare anything">,

clang/lib/Sema/SemaDecl.cpp

@@ -6809,6 +6809,8 @@ Sema::ActOnFunctionDeclarator(Scope *S, Declarator &D, DeclContext *DC,
     }
 
     // If a function is defined as defaulted or deleted, mark it as such now.
+    // FIXME: Does this ever happen? ActOnStartOfFunctionDef forces the function
+    // definition kind to FDK_Definition.
     switch (D.getFunctionDefinitionKind()) {
       case FDK_Declaration:
       case FDK_Definition:
@@ -7670,8 +7672,9 @@ static SourceRange getResultSourceRange(const FunctionDecl *FD) {
 }
 
 void Sema::CheckMain(FunctionDecl* FD, const DeclSpec& DS) {
-  // C++11 [basic.start.main]p3:  A program that declares main to be inline,
-  //   static or constexpr is ill-formed.
+  // C++11 [basic.start.main]p3:
+  //   A program that [...] declares main to be inline, static or
+  //   constexpr is ill-formed.
   // C11 6.7.4p4:  In a hosted environment, no function specifier(s) shall
   //   appear in a declaration of main.
   // static main is not an error under C99, but we should warn about it.

clang/lib/Sema/SemaDeclCXX.cpp

@@ -11984,6 +11984,11 @@ void Sema::SetDeclDeleted(Decl *Dcl, SourceLocation DelLoc) {
     }
   }
 
+  // C++11 [basic.start.main]p3:
+  //   A program that defines main as deleted [...] is ill-formed.
+  if (Fn->isMain())
+    Diag(DelLoc, diag::err_deleted_main);
+
   Fn->setDeletedAsWritten();
 }
 

clang/lib/Sema/SemaExpr.cpp

@@ -264,13 +264,18 @@ bool Sema::DiagnoseUseOfDecl(NamedDecl *D, SourceLocation Loc,
       SmallVectorImpl<PartialDiagnosticAt> &Suppressed = Pos->second;
       for (unsigned I = 0, N = Suppressed.size(); I != N; ++I)
         Diag(Suppressed[I].first, Suppressed[I].second);
-      
+
       // Clear out the list of suppressed diagnostics, so that we don't emit
       // them again for this specialization. However, we don't obsolete this
       // entry from the table, because we want to avoid ever emitting these
       // diagnostics again.
       Suppressed.clear();
     }
+
+    // C++ [basic.start.main]p3:
+    //   The function 'main' shall not be used within a program.
+    if (cast<FunctionDecl>(D)->isMain())
+      Diag(Loc, diag::ext_main_used);
   }
 
   // See if this is an auto-typed variable whose initializer we are parsing.