[clang] [Clang] Don't treat a declaration that declares nothing as an invalid statement (PR #224682)

[Aaron Ballman via cfe-commits]

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@@ -231,6 +231,11 @@ StmtResult Parser::ParseStatementOrDeclarationAfterAttributes(
                                    GNUAttrs.Range.getBegin());
       } else if (GNUAttrs.Range.getBegin().isValid())
         DeclStart = GNUAttrs.Range.getBegin();
+      // A declaration that declares nothing (`int;`) yields no Decl but still
+      // occupies the statement position; unlike a pragma, ParseStatement() must
+      // not skip it.
+      if (!Decl)
+        return Actions.ActOnNullStmt(PrevTokLocation);
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AaronBallman wrote:

> I tried EmptyDecl, but it can't appear inside a function or class today. 

`EmptyDecl` would be wrong anyway, that's modeling this: https://eel.is/c++draft/dcl#nt:empty-declaration

> An invalid unnamed decl would make any struct containing `int;`

Ah, that's a good point, drat.

> So I'd like to keep this PR as the crash fix.

@efriedma-quic is there some better way to signal to CodeGen "don't emit code for this AST node?" so that we could keep a declaration AST node?

https://github.com/llvm/llvm-project/pull/224682