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

[via cfe-commits]

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

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.

>From 87bc9f798ab3429c019bafeba3023ae5afdfec26 Mon Sep 17 00:00:00 2001
From: Sirraide <aeternalmail at gmail.com>
Date: Mon, 27 May 2024 17:16:55 +0200
Subject: [PATCH 1/3] [Clang] [Sema] Diagnose unexpanded parameter packs in
 attributes

---
 clang/lib/Parse/ParseDecl.cpp           | 5 +++++
 clang/lib/Sema/SemaTemplateVariadic.cpp | 3 +++
 2 files changed, 8 insertions(+)

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.
     //------------------------------------------------------------------------

>From 8d36b67f39157e2178fdd9d0a3805acc51526fa1 Mon Sep 17 00:00:00 2001
From: Sirraide <aeternalmail at gmail.com>
Date: Mon, 27 May 2024 17:19:02 +0200
Subject: [PATCH 2/3] Add release note

---
 clang/docs/ReleaseNotes.rst | 1 +
 1 file changed, 1 insertion(+)

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
 ^^^^^^^^^^^^^^^^^^^^^^^^^

>From 6f1f76c026731b9933c06e24d9fee7a2066f865d Mon Sep 17 00:00:00 2001
From: Sirraide <aeternalmail at gmail.com>
Date: Mon, 27 May 2024 17:20:12 +0200
Subject: [PATCH 3/3] Actually commit the tests

---
 .../test/SemaCXX/attribute-pack-expansion.cpp | 20 +++++++++++++++++++
 1 file changed, 20 insertions(+)
 create mode 100644 clang/test/SemaCXX/attribute-pack-expansion.cpp

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}}
+}