[clang] [Clang] disallow use of attributes before extern template declarations (PR #136328)

[Oleksandr T. via cfe-commits]

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@@ -1049,6 +1049,7 @@ Parser::ParseExternalDeclaration(ParsedAttributes &Attrs,
 
   case tok::kw_extern:
     if (getLangOpts().CPlusPlus && NextToken().is(tok::kw_template)) {
+      ProhibitAttributes(Attrs);
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a-tarasyuk wrote:

@erichkeane, thanks for the feedback. I followed this part of the spec

> No [attribute-specifier-seq](https://eel.is/c++draft/dcl.attr.grammar#nt:attribute-specifier-seq) ([[dcl.attr.grammar]](https://eel.is/c++draft/dcl.attr.grammar)) shall appertain to an [explicit instantiation](https://eel.is/c++draft/temp.explicit#3.sentence-3).

I also checked how GCC handles this case, and it seems that it disallows all kinds of attrs.

Should non-standard attributes be applied to this `S<int>` (_if we are talking about the current test case_) declaration?



https://github.com/llvm/llvm-project/pull/136328