[Clang] disallow # operators in attribute argument lists

[a-tarasyuk]

Fixes #147217

---

This PR addresses the parsing of attribute arguments by diagnosing and disallowing the # operator

[llvmbot]

@llvm/pr-subscribers-clang

Author: Oleksandr T. (a-tarasyuk)

Changes

Fixes #147217

---

This PR addresses the parsing of attribute arguments by diagnosing and disallowing the # operator

---

Full diff: https://github.com/llvm/llvm-project/pull/147308.diff

4 Files Affected:

• (modified) clang/docs/ReleaseNotes.rst (+1)
• (modified) clang/include/clang/Basic/DiagnosticParseKinds.td (+2)
• (modified) clang/lib/Parse/ParseDecl.cpp (+7)
• (modified) clang/test/Parser/cxx0x-attributes.cpp (+5)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index a6be59f1d6bd7..b8032ee9c03da 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -673,6 +673,7 @@ Improvements to Clang's diagnostics
   false positives in exception-heavy code, though only simple patterns
   are currently recognized.
 
+- Clang now rejects ``#`` operators in attribute argument lists. (#GH147217)
 
 Improvements to Clang's time-trace
 ----------------------------------
diff --git a/clang/include/clang/Basic/DiagnosticParseKinds.td b/clang/include/clang/Basic/DiagnosticParseKinds.td
index 6c30da376dafb..b39e2a7359c22 100644
--- a/clang/include/clang/Basic/DiagnosticParseKinds.td
+++ b/clang/include/clang/Basic/DiagnosticParseKinds.td
@@ -830,6 +830,8 @@ def err_ms_property_expected_comma_or_rparen : Error<
   "expected ',' or ')' at end of property accessor list">;
 def err_ms_property_initializer : Error<
   "property declaration cannot have a default member initializer">;
+def err_invalid_attribute_argument
+    : Error<"'%0' is not allowed in attribute argument lists">;
 
 def err_assume_attr_expects_cond_expr : Error<
   "use of this expression in an %0 attribute requires parentheses">;
diff --git a/clang/lib/Parse/ParseDecl.cpp b/clang/lib/Parse/ParseDecl.cpp
index 7e739e09b15e8..059636653723c 100644
--- a/clang/lib/Parse/ParseDecl.cpp
+++ b/clang/lib/Parse/ParseDecl.cpp
@@ -488,6 +488,13 @@ unsigned Parser::ParseAttributeArgsCommon(
   bool AttributeHasVariadicIdentifierArg =
       attributeHasVariadicIdentifierArg(*AttrName, Form.getSyntax(), ScopeName);
 
+  if (Tok.is(tok::hash) || Tok.is(tok::hashhash)) {
+    Diag(Tok.getLocation(), diag::err_invalid_attribute_argument)
+        << PP.getSpelling(Tok);
+    SkipUntil(tok::r_paren, StopAtSemi);
+    return 0;
+  }
+
   // Interpret "kw_this" as an identifier if the attributed requests it.
   if (ChangeKWThisToIdent && Tok.is(tok::kw_this))
     Tok.setKind(tok::identifier);
diff --git a/clang/test/Parser/cxx0x-attributes.cpp b/clang/test/Parser/cxx0x-attributes.cpp
index 372a373a49ec5..d343cd4f3a93e 100644
--- a/clang/test/Parser/cxx0x-attributes.cpp
+++ b/clang/test/Parser/cxx0x-attributes.cpp
@@ -477,3 +477,8 @@ namespace P2361 {
 }
 
 alignas(int) struct AlignAsAttribute {}; // expected-error {{misplaced attributes; expected attributes here}}
+
+namespace GH147217 {
+  [[clang::annotate(#)]] void a();      // expected-error {{'#' is not allowed in attribute argument lists}}
+  [[clang::annotate(##)]] void b();     // expected-error {{'##' is not allowed in attribute argument lists}}
+}

[cor3ntin]

There is divergence between C and C++, I sent a mail to core https://lists.isocpp.org/liaison/2025/07/1549.php

[AaronBallman]

LGTM!

[AaronBallman]

Actually, the changes still LGTM, but let's hold off on landing the changes until we give WG21 a chance to weigh in. It's possible we'll end up converging and not need to have different behaviors.