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

Fixes #125810 

---

This patch resolves an issue in Clang where the `-Wunused-variable`
warning was suppressed for structured bindings with elements marked
`[[maybe_unused]]`, causing the entire declaration to be treated as used
and preventing the warning from being emitted.

Author: a-tarasyuk

clang/docs/ReleaseNotes.rst

@@ -348,6 +348,8 @@ Bug Fixes to C++ Support
 - Correctly diagnoses if unresolved using declarations shadows template paramters (#GH129411)
 - Clang was previously coalescing volatile writes to members of volatile base class subobjects.
   The issue has been addressed by propagating qualifiers during derived-to-base conversions in the AST. (#GH127824)
+- 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
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Sema/SemaDecl.cpp

@@ -1934,13 +1934,14 @@ 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;

clang/test/SemaCXX/unused-bindings.cpp

@@ -0,0 +1,17 @@
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c++26 -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]'}}
+}
+}

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]'}}
 }
 
 }