[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`.

1 files changed, 6 insertions, 1 deletions

diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index 540f5f2..1c30458 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/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,