Warn about non-int main() results in GNU C mode instead of erroring.

Based on a patch by Vasiliy Korchagin!

llvm-svn: 150500

clang/include/clang/Basic/DiagnosticGroups.td

@@ -104,6 +104,7 @@ def LocalTypeTemplateArgs : DiagGroup<"local-type-template-args",
                                       [CXX98CompatLocalTypeTemplateArgs]>;
 def MalformedWarningCheck : DiagGroup<"malformed-warning-check">;
 def Main : DiagGroup<"main">;
+def MainReturnType : DiagGroup<"main-return-type">;
 def MissingBraces : DiagGroup<"missing-braces">;
 def MissingDeclarations: DiagGroup<"missing-declarations">;
 def : DiagGroup<"missing-format-attribute">;

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -346,6 +346,8 @@ def err_constexpr_main : Error<
   "'main' is not allowed to be declared constexpr">;
 def err_main_template_decl : Error<"'main' 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'">,
+    InGroup<MainReturnType>;
 def err_main_surplus_args : Error<"too many parameters (%0) for 'main': "
     "must be 0, 2, or 3">;
 def warn_main_one_arg : Warning<"only one parameter on 'main' declaration">,

clang/lib/Sema/SemaDecl.cpp

@@ -5794,9 +5794,23 @@ void Sema::CheckMain(FunctionDecl* FD, const DeclSpec& DS) {
 
   QualType T = FD->getType();
   assert(T->isFunctionType() && "function decl is not of function type");
-  const FunctionType* FT = T->getAs<FunctionType>();
+  const FunctionType* FT = T->castAs<FunctionType>();
 
-  if (!Context.hasSameUnqualifiedType(FT->getResultType(), Context.IntTy)) {
+  // All the standards say that main() should should return 'int'.
+  if (Context.hasSameUnqualifiedType(FT->getResultType(), Context.IntTy)) {
+    // In C and C++, main magically returns 0 if you fall off the end;
+    // set the flag which tells us that.
+    // This is C++ [basic.start.main]p5 and C99 5.1.2.2.3.
+    FD->setHasImplicitReturnZero(true);
+
+  // In C with GNU extensions we allow main() to have non-integer return
+  // type, but we should warn about the extension, and we disable the
+  // implicit-return-zero rule.
+  } else if (getLangOptions().GNUMode && !getLangOptions().CPlusPlus) {
+    Diag(FD->getTypeSpecStartLoc(), diag::ext_main_returns_nonint);
+
+  // Otherwise, this is just a flat-out error.
+  } else {
     Diag(FD->getTypeSpecStartLoc(), diag::err_main_returns_nonint);
     FD->setInvalidDecl(true);
   }
@@ -7259,16 +7273,11 @@ Decl *Sema::ActOnFinishFunctionBody(Decl *dcl, Stmt *Body,
 
   if (FD) {
     FD->setBody(Body);
-    if (FD->isMain()) {
-      // C and C++ allow for main to automagically return 0.
-      // Implements C++ [basic.start.main]p5 and C99 5.1.2.2.3.
-      FD->setHasImplicitReturnZero(true);
+
+    // If the function implicitly returns zero (like 'main') or is naked,
+    // don't complain about missing return statements.
+    if (FD->hasImplicitReturnZero() || FD->hasAttr<NakedAttr>())
       WP.disableCheckFallThrough();
-    } else if (FD->hasAttr<NakedAttr>()) {
-      // If the function is marked 'naked', don't complain about missing return
-      // statements.
-      WP.disableCheckFallThrough();
-    }
 
     // MSVC permits the use of pure specifier (=0) on function definition,
     // defined at class scope, warn about this non standard construct.

clang/test/Sema/c89.c

@@ -89,3 +89,5 @@ extern int printf(__const char *__restrict __format, ...);
 void test16() {
   printg("Hello, world!\n"); /* expected-warning {{implicit declaration of function 'printg'}} */
 }
+
+void main() {} /* expected-error {{'main' must return 'int'}} */

clang/test/Sema/gnu89.c

@@ -1,3 +1,5 @@
 // RUN: %clang_cc1 %s -std=gnu89 -pedantic -fsyntax-only -verify
 
 int f(int restrict);
+
+void main() {} // expected-warning {{return type of 'main' is not 'int'}}