[clang] [Clang] [Sema] Diagnose unexpanded parameter packs in attributes (PR #93482)

[via cfe-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: None (Sirraide)

<details>
<summary>Changes</summary>

Call `DiagnoseUnexpandedParameterPack` when we parse an expression argument to an attribute and check for implicit code in the `CollectUnexpandedParameterPacksVisitor` so we can actually find unexpanded packs in attributes that end up applied to lambda call operators.

This fixes #<!-- -->93269.

---
Full diff: https://github.com/llvm/llvm-project/pull/93482.diff


4 Files Affected:

- (modified) clang/docs/ReleaseNotes.rst (+1) 
- (modified) clang/lib/Parse/ParseDecl.cpp (+5) 
- (modified) clang/lib/Sema/SemaTemplateVariadic.cpp (+3) 
- (added) clang/test/SemaCXX/attribute-pack-expansion.cpp (+20) 


``````````diff
diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 58e407a724a9c..37a664b14fab1 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -801,6 +801,7 @@ Bug Fixes to C++ Support
   packs. (#GH93076)
 - Fixed a regression introduced in Clang 18 causing a static function overloading a non-static function
   with the same parameters not to be diagnosed. (Fixes #GH93456).
+- Clang now diagnoses unexpanded parameter packs in attributes. (Fixes #GH93269).
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/clang/lib/Parse/ParseDecl.cpp b/clang/lib/Parse/ParseDecl.cpp
index 86e8a6b7ee0ed..c528917437332 100644
--- a/clang/lib/Parse/ParseDecl.cpp
+++ b/clang/lib/Parse/ParseDecl.cpp
@@ -467,6 +467,11 @@ bool Parser::ParseAttributeArgumentList(
       break;
     }
 
+    if (Actions.DiagnoseUnexpandedParameterPack(Expr.get())) {
+      SawError = true;
+      break;
+    }
+
     Exprs.push_back(Expr.get());
 
     if (Tok.isNot(tok::comma))
diff --git a/clang/lib/Sema/SemaTemplateVariadic.cpp b/clang/lib/Sema/SemaTemplateVariadic.cpp
index 0b20604665068..7a44b978aacdb 100644
--- a/clang/lib/Sema/SemaTemplateVariadic.cpp
+++ b/clang/lib/Sema/SemaTemplateVariadic.cpp
@@ -66,6 +66,9 @@ namespace {
 
     bool shouldWalkTypesOfTypeLocs() const { return false; }
 
+    // We need this so we can find e.g. attributes on lambdas.
+    bool shouldVisitImplicitCode() const { return true; }
+
     //------------------------------------------------------------------------
     // Recording occurrences of (unexpanded) parameter packs.
     //------------------------------------------------------------------------
diff --git a/clang/test/SemaCXX/attribute-pack-expansion.cpp b/clang/test/SemaCXX/attribute-pack-expansion.cpp
new file mode 100644
index 0000000000000..a339e68c09648
--- /dev/null
+++ b/clang/test/SemaCXX/attribute-pack-expansion.cpp
@@ -0,0 +1,20 @@
+// RUN: %clang_cc1 -std=c++20 -fsyntax-only -verify %s
+
+template <bool... vals>
+void f() __attribute((diagnose_if(vals, "message", "error"))) { // expected-error {{expression contains unexpanded parameter pack 'vals'}}
+  [] () __attribute((diagnose_if(vals, "message", "error"))) {}(); // expected-error {{expression contains unexpanded parameter pack 'vals'}}
+  [] () __attribute((diagnose_if(vals..., "message", "error"))) {}(); // expected-error {{attribute 'diagnose_if' does not support argument pack expansion}}
+  [] <bool ...inner> () __attribute((diagnose_if(inner, "message", "error"))) {}(); // expected-error {{expression contains unexpanded parameter pack 'inner'}}
+  ([] <bool ...inner> () __attribute((diagnose_if(inner, "message", "error"))) {}(), ...); // expected-error {{expression contains unexpanded parameter pack 'inner'}} \
+                                                                                           // expected-error {{pack expansion does not contain any unexpanded parameter packs}}
+
+  // This is fine, so check that we're actually emitting an error
+  // due to the 'diagnose_if'.
+  ([] () __attribute((diagnose_if(vals, "foobar", "error"))) {}(), ...); // expected-error {{foobar}} expected-note {{from 'diagnose_if'}}
+}
+
+void g() {
+  f<>();
+  f<false>();
+  f<true, true>(); // expected-note {{in instantiation of}}
+}

``````````

</details>


https://github.com/llvm/llvm-project/pull/93482