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

[Aaron Ballman via cfe-commits]

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@@ -16232,7 +16232,9 @@ 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>())
+      if ((FD->hasImplicitReturnZero() &&
+           (getLangOpts().CPlusPlus || getLangOpts().C99 || !FD->isMain())) ||
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AaronBallman wrote:

This change looks wrong to me. We shouldn't have to look at the language options and whether the function is or isn't `main`, the point is to rely on `hasImplicitReturnZero()` alone. Pretty sure the fallthrough checker is what should be handling silencing the diagnostic.

https://github.com/llvm/llvm-project/pull/134617