[clang] [Clang] disallow attributes on void parameters (PR #124920)

[Aaron Ballman via cfe-commits]

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@@ -50,8 +50,8 @@ struct testRecoverStrictnessStruct { };
 
 #pragma clang attribute pop
 
-#pragma clang attribute push (__attribute__((abi_tag("a"))), apply_to = any(function, record(unless(is_union)), variable, enum))
-// expected-error at -1 {{attribute 'abi_tag' cannot be applied to 'enum'}}
+#pragma clang attribute push (__attribute__((abi_tag("a"))), apply_to = any(function, record(unless(is_union)), variable, enum)) // expected-warning {{attribute 'abi_tag' cannot be applied to a 'void' parameter}}
+                                                                                                                                 // expected-error at -1 {{attribute 'abi_tag' cannot be applied to 'enum'}}
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AaronBallman wrote:

> @AaronBallman if we're considering changes to `Attr.td`, 

I don't think we need changes in Attr.td itself; the changes should be limited to just the tablegen bits (or the part consuming the tablegen bits).

> it seems that `[Var]` and `[ParmVar]` should be extended to `AttrSubject`, since `AttrSubjectMatcherRule` only accepts a list of `AttrSubject`. I'm unsure if this qualifies as a `Subject`, though...

Those already are an `AttrSubject`; we have attributes using them already.

> https://github.com/llvm/llvm-project/blob/50082773223b9eced296d8223ca4e4a79ecdeb9b/clang/include/clang/Basic/Attr.td#L560-L567

I think that's correct as-is.

> or explicitly add a helper to exclude void parameters and use it with
> 
> https://github.com/llvm/llvm-project/blob/d00579be39e8a470d7a0ff79ff6deadf9e003781/clang/lib/Sema/ParsedAttr.cpp#L172-L175

I think that's what needs to be changed to exclude `void` parameters (or somewhere nearby that call).

> > Side note: pragma clang attribute push diagnostics are THE WORST here...
> 
> I suppose the diagnostic for void parameters will be skipped if the rules exclude them, or should the diagnostic still handle this case?

The goal is to get rid of the diagnostics about `void` parameters. e.g., `// expected-warning {{attribute 'abi_tag' cannot be applied to a 'void' parameter}}` shouldn't be fired.

https://github.com/llvm/llvm-project/pull/124920