[clang] [Clang] emit -Wunused-variable warning for unused structured bindings without the [[maybe_unused]] attribute (PR #127061)

[Oleksandr T. via cfe-commits]

https://github.com/a-tarasyuk updated https://github.com/llvm/llvm-project/pull/127061

>From a76ee008bdb87655da465e21d09c840edecc2b1b Mon Sep 17 00:00:00 2001
From: Oleksandr T <oleksandr.tarasiuk at outlook.com>
Date: Thu, 13 Feb 2025 15:24:09 +0200
Subject: [PATCH 1/2] [Clang] emit -Wunused-variable warning for unused
 structured bindings without the [[maybe_unused]] attribute

---
 clang/docs/ReleaseNotes.rst            |  2 ++
 clang/lib/Sema/SemaDecl.cpp            |  9 +++++----
 clang/test/SemaCXX/unused-bindings.cpp | 17 +++++++++++++++++
 clang/test/SemaCXX/unused.cpp          |  3 ++-
 4 files changed, 26 insertions(+), 5 deletions(-)
 create mode 100644 clang/test/SemaCXX/unused-bindings.cpp

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 6344c4b36e357..4f20415ec006d 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -160,6 +160,8 @@ Bug Fixes to C++ Support
 - Clang is now better at keeping track of friend function template instance contexts. (#GH55509)
 - The initialization kind of elements of structured bindings
   direct-list-initialized from an array is corrected to direct-initialization.
+- Clang now emits the ``-Wunused-variable`` warning when some structured bindings are unused
+  and the ``[[maybe_unused]]`` attribute is not applied. (#GH125810)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index 6eedc77ed20a0..19a73a66be5af 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -1921,13 +1921,14 @@ static bool ShouldDiagnoseUnusedDecl(const LangOptions &LangOpts,
     // For a decomposition declaration, warn if none of the bindings are
     // referenced, instead of if the variable itself is referenced (which
     // it is, by the bindings' expressions).
-    bool IsAllPlaceholders = true;
+    bool IsAllIgnored = true;
     for (const auto *BD : DD->bindings()) {
-      if (BD->isReferenced() || BD->hasAttr<UnusedAttr>())
+      if (BD->isReferenced())
         return false;
-      IsAllPlaceholders = IsAllPlaceholders && BD->isPlaceholderVar(LangOpts);
+      IsAllIgnored = IsAllIgnored && (BD->isPlaceholderVar(LangOpts) ||
+                                      BD->hasAttr<UnusedAttr>());
     }
-    if (IsAllPlaceholders)
+    if (IsAllIgnored)
       return false;
   } else if (!D->getDeclName()) {
     return false;
diff --git a/clang/test/SemaCXX/unused-bindings.cpp b/clang/test/SemaCXX/unused-bindings.cpp
new file mode 100644
index 0000000000000..01f2126133a20
--- /dev/null
+++ b/clang/test/SemaCXX/unused-bindings.cpp
@@ -0,0 +1,17 @@
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c++2c -Wunused %s
+
+namespace GH125810 {
+struct S {
+  int a, b;
+};
+
+void t(S s) {
+  auto &[_, _] = s;
+  auto &[a1, _] = s; // expected-warning {{unused variable '[a1, _]'}}
+  auto &[_, b2] = s; // expected-warning {{unused variable '[_, b2]'}}
+
+  auto &[a3 [[maybe_unused]], b3 [[maybe_unused]]] = s;
+  auto &[a4, b4 [[maybe_unused]]] = s; // expected-warning {{unused variable '[a4, b4]'}}
+  auto &[a5 [[maybe_unused]], b5] = s; // expected-warning {{unused variable '[a5, b5]'}}
+}
+}
diff --git a/clang/test/SemaCXX/unused.cpp b/clang/test/SemaCXX/unused.cpp
index 1f40c1b1ca903..ab728069f2faf 100644
--- a/clang/test/SemaCXX/unused.cpp
+++ b/clang/test/SemaCXX/unused.cpp
@@ -114,7 +114,8 @@ namespace maybe_unused_binding {
 
 void test() {
   struct X { int a, b; } x;
-  auto [a [[maybe_unused]], b] = x; // expected-warning {{an attribute specifier sequence attached to a structured binding declaration is a C++2c extension}}
+  auto [a [[maybe_unused]], b] = x; // expected-warning {{an attribute specifier sequence attached to a structured binding declaration is a C++2c extension}} \
+                                    // expected-warning {{unused variable '[a, b]'}}
 }
 
 }

>From c600c2a2378310c3f2fc82edaca66ca80b40284d Mon Sep 17 00:00:00 2001
From: Oleksandr T <oleksandr.tarasiuk at outlook.com>
Date: Fri, 14 Feb 2025 13:43:53 +0200
Subject: [PATCH 2/2] update test to use std c++26

---
 clang/test/SemaCXX/unused-bindings.cpp | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/clang/test/SemaCXX/unused-bindings.cpp b/clang/test/SemaCXX/unused-bindings.cpp
index 01f2126133a20..83250dd4b0a11 100644
--- a/clang/test/SemaCXX/unused-bindings.cpp
+++ b/clang/test/SemaCXX/unused-bindings.cpp
@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 -fsyntax-only -verify -std=c++2c -Wunused %s
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c++26 -Wunused %s
 
 namespace GH125810 {
 struct S {