[Clang] disallow use of attributes before extern template declarations

[a-tarasyuk]

Fixes #79893

---

This PR addresses the issue of attributes being incorrectly allowed on extern template declarations

[[deprecated]] extern template struct S<int>;

[llvmbot]

@llvm/pr-subscribers-clang

Author: Oleksandr T. (a-tarasyuk)

Changes

Fixes #79893

---

This PR addresses the issue of attributes being incorrectly allowed on extern template declarations

[[deprecated]] extern template struct S<int>;

---

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

3 Files Affected:

• (modified) clang/docs/ReleaseNotes.rst (+3)
• (modified) clang/lib/Parse/Parser.cpp (+1)
• (added) clang/test/Parser/extern-template-attributes.cpp (+6)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index f5cd1fbeabcfe..cf0ec3e0bbf29 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -301,6 +301,9 @@ related warnings within the method body.
   particularly relevant for AMDGPU targets, where they map to corresponding IR
   metadata.
 
+- Clang now disallows the use of attributes applied before an
+  ``extern template`` declaration (#GH79893).
+
 Improvements to Clang's diagnostics
 -----------------------------------
 
diff --git a/clang/lib/Parse/Parser.cpp b/clang/lib/Parse/Parser.cpp
index d528664bca352..25f8ed29d15fe 100644
--- a/clang/lib/Parse/Parser.cpp
+++ b/clang/lib/Parse/Parser.cpp
@@ -1049,6 +1049,7 @@ Parser::ParseExternalDeclaration(ParsedAttributes &Attrs,
 
   case tok::kw_extern:
     if (getLangOpts().CPlusPlus && NextToken().is(tok::kw_template)) {
+      ProhibitAttributes(Attrs);
       // Extern templates
       SourceLocation ExternLoc = ConsumeToken();
       SourceLocation TemplateLoc = ConsumeToken();
diff --git a/clang/test/Parser/extern-template-attributes.cpp b/clang/test/Parser/extern-template-attributes.cpp
new file mode 100644
index 0000000000000..0912ad69806db
--- /dev/null
+++ b/clang/test/Parser/extern-template-attributes.cpp
@@ -0,0 +1,6 @@
+// RUN: %clang_cc1 -std=c++17 -verify %s
+
+template <class>
+struct S {};
+
+[[deprecated]] extern template struct S<int>; // expected-error {{an attribute list cannot appear here}}

[erichkeane]

I think we're ok with this now, LGTM.