[Clang] add ext warning for missing return in 'main' for C89 mode (#134617)

Fixes #21650 

--- 

Clang currently inserts an implicit `return 0;` in `main()` when
compiling in `C89` mode, even though the `C89` standard doesn't require
this behavior. This patch changes that behavior by emitting a warning
instead of silently inserting the implicit return under `-pedantic`.

clang/docs/ReleaseNotes.rst

@@ -321,6 +321,7 @@ Improvements to Clang's diagnostics
 
 - ``-Wc++98-compat`` no longer diagnoses use of ``__auto_type`` or
   ``decltype(auto)`` as though it was the extension for ``auto``. (#GH47900)
+- Clang now issues a warning for missing return in ``main`` in C89 mode. (#GH21650)
 
 - Now correctly diagnose a tentative definition of an array with static
   storage duration in pedantic mode in C. (#GH50661)

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -1031,6 +1031,9 @@ 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'">,
     InGroup<MainReturnType>;
+def ext_main_no_return : Extension<
+    "implicit '0' return value from 'main' is a C99 extension">,
+    InGroup<MainReturnType>;
 def note_main_change_return_type : Note<"change return type to 'int'">;
 def err_main_surplus_args : Error<"too many parameters (%0) for 'main': "
     "must be 0, 2, or 3">;

clang/lib/Sema/AnalysisBasedWarnings.cpp

@@ -550,7 +550,8 @@ struct CheckFallThroughDiagnostics {
   unsigned FunKind; // TODO: use diag::FalloffFunctionKind
   SourceLocation FuncLoc;
 
-  static CheckFallThroughDiagnostics MakeForFunction(const Decl *Func) {
+  static CheckFallThroughDiagnostics MakeForFunction(Sema &S,
+                                                     const Decl *Func) {
     CheckFallThroughDiagnostics D;
     D.FuncLoc = Func->getLocation();
     D.diag_FallThrough_HasNoReturn = diag::warn_noreturn_has_return_expr;
@@ -564,8 +565,13 @@ struct CheckFallThroughDiagnostics {
 
     // Don't suggest that template instantiations be marked "noreturn"
     bool isTemplateInstantiation = false;
-    if (const FunctionDecl *Function = dyn_cast<FunctionDecl>(Func))
+    if (const FunctionDecl *Function = dyn_cast<FunctionDecl>(Func)) {
       isTemplateInstantiation = Function->isTemplateInstantiation();
+      if (!S.getLangOpts().CPlusPlus && !S.getLangOpts().C99 &&
+          Function->isMain()) {
+        D.diag_FallThrough_ReturnsNonVoid = diag::ext_main_no_return;
+      }
+    }
 
     if (!isVirtualMethod && !isTemplateInstantiation)
       D.diag_NeverFallThroughOrReturn = diag::warn_suggest_noreturn_function;
@@ -2737,15 +2743,14 @@ void clang::sema::AnalysisBasedWarnings::IssueWarnings(
   // Warning: check missing 'return'
   if (P.enableCheckFallThrough) {
     const CheckFallThroughDiagnostics &CD =
-        (isa<BlockDecl>(D)
-             ? CheckFallThroughDiagnostics::MakeForBlock()
-             : (isa<CXXMethodDecl>(D) &&
-                cast<CXXMethodDecl>(D)->getOverloadedOperator() == OO_Call &&
-                cast<CXXMethodDecl>(D)->getParent()->isLambda())
-                   ? CheckFallThroughDiagnostics::MakeForLambda()
-                   : (fscope->isCoroutine()
-                          ? CheckFallThroughDiagnostics::MakeForCoroutine(D)
-                          : CheckFallThroughDiagnostics::MakeForFunction(D)));
+        (isa<BlockDecl>(D) ? CheckFallThroughDiagnostics::MakeForBlock()
+         : (isa<CXXMethodDecl>(D) &&
+            cast<CXXMethodDecl>(D)->getOverloadedOperator() == OO_Call &&
+            cast<CXXMethodDecl>(D)->getParent()->isLambda())
+             ? CheckFallThroughDiagnostics::MakeForLambda()
+             : (fscope->isCoroutine()
+                    ? CheckFallThroughDiagnostics::MakeForCoroutine(D)
+                    : CheckFallThroughDiagnostics::MakeForFunction(S, D)));
     CheckFallThroughForBody(S, D, Body, BlockType, CD, AC);
   }
 

clang/lib/Sema/SemaDecl.cpp

@@ -16232,7 +16232,12 @@ Decl *Sema::ActOnFinishFunctionBody(Decl *dcl, Stmt *Body,
 
       // If the function implicitly returns zero (like 'main') or is naked,
       // don't complain about missing return statements.
-      if (FD->hasImplicitReturnZero() || FD->hasAttr<NakedAttr>())
+      // Clang implicitly returns 0 in C89 mode, but that's considered an
+      // extension. The check is necessary to ensure the expected extension
+      // warning is emitted in C89 mode.
+      if ((FD->hasImplicitReturnZero() &&
+           (getLangOpts().CPlusPlus || getLangOpts().C99 || !FD->isMain())) ||
+          FD->hasAttr<NakedAttr>())
         WP.disableCheckFallThrough();
 
       // MSVC permits the use of pure specifier (=0) on function definition,

clang/test/Sema/main-no-return-c89-1.c

@@ -0,0 +1,10 @@
+/* RUN: %clang_cc1 -std=c89 -fsyntax-only -verify -Wno-strict-prototypes -pedantic %s
+ * RUN: %clang_cc1 -std=c89 -fsyntax-only -verify -Wno-strict-prototypes -Wmain-return-type %s
+ * RUN: %clang_cc1 -std=c89 -fsyntax-only -verify=implicit-main-return -Wno-strict-prototypes -pedantic -Wno-main-return-type %s
+ */
+
+/* implicit-main-return-no-diagnostics */
+
+int main() {
+
+} /* expected-warning {{implicit '0' return value from 'main' is a C99 extension}} */

clang/test/Sema/main-no-return-c89-2.c

@@ -0,0 +1,11 @@
+/* RUN: %clang_cc1 -std=c89 -fsyntax-only -verify -Wno-strict-prototypes -Wmain-return-type %s
+ */
+
+/* expected-no-diagnostics */
+
+void exit(int);
+
+int main() {
+    if (1)
+        exit(1);
+}

clang/test/Sema/main-no-return-c89-3.c

@@ -0,0 +1,8 @@
+/* RUN: %clang_cc1 -std=c89 -fsyntax-only -verify -Wno-strict-prototypes -Wmain-return-type %s
+ */
+
+/* expected-no-diagnostics */
+
+int main() {
+    return 0;
+}