[Clang] Add test for CWG2285 "Issues with structured bindings" (llvm#126421)

The resolution of [CWG2285](https://wg21.link/cwg2285) adds the point of
declaration of a structured binding, and was implemented in
llvm@bdb84f3
.

Drive-by changes: modify comment and diagnostic messages mentioned in
CWG2285.

Author: zwuis

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -551,7 +551,7 @@ def err_decomp_decl_constraint : Error<
 def err_decomp_decl_parens : Error<
   "decomposition declaration cannot be declared with parentheses">;
 def err_decomp_decl_template : Error<
-  "decomposition declaration template not supported">;
+  "decomposition declaration cannot be a template">;
 def err_decomp_decl_not_alone : Error<
   "decomposition declaration must be the only declaration in its group">;
 def err_decomp_decl_requires_init : Error<

clang/lib/Sema/SemaDeclCXX.cpp

@@ -733,8 +733,11 @@ Sema::ActOnDecompositionDeclarator(Scope *S, Declarator &D,
   }
 
   if (!TemplateParamLists.empty()) {
-    // FIXME: There's no rule against this, but there are also no rules that
-    // would actually make it usable, so we reject it for now.
+    // C++17 [temp]/1:
+    //   A template defines a family of class, functions, or variables, or an
+    //   alias for a family of types.
+    //
+    // Structured bindings are not included.
     Diag(TemplateParamLists.front()->getTemplateLoc(),
          diag::err_decomp_decl_template);
     return nullptr;

clang/test/CXX/drs/cwg22xx.cpp

@@ -1,10 +1,10 @@
 // RUN: %clang_cc1 -std=c++98 -triple x86_64-unknown-unknown %s -verify=expected -fexceptions -fcxx-exceptions -pedantic-errors
 // RUN: %clang_cc1 -std=c++11 -triple x86_64-unknown-unknown %s -verify=expected,since-cxx11 -fexceptions -fcxx-exceptions -pedantic-errors
 // RUN: %clang_cc1 -std=c++14 -triple x86_64-unknown-unknown %s -verify=expected,since-cxx11 -fexceptions -fcxx-exceptions -pedantic-errors
-// RUN: %clang_cc1 -std=c++17 -triple x86_64-unknown-unknown %s -verify=expected,since-cxx11 -fexceptions -fcxx-exceptions -pedantic-errors
-// RUN: %clang_cc1 -std=c++20 -triple x86_64-unknown-unknown %s -verify=expected,since-cxx11 -fexceptions -fcxx-exceptions -pedantic-errors
-// RUN: %clang_cc1 -std=c++23 -triple x86_64-unknown-unknown %s -verify=expected,since-cxx11 -fexceptions -fcxx-exceptions -pedantic-errors
-// RUN: %clang_cc1 -std=c++2c -triple x86_64-unknown-unknown %s -verify=expected,since-cxx11 -fexceptions -fcxx-exceptions -pedantic-errors
+// RUN: %clang_cc1 -std=c++17 -triple x86_64-unknown-unknown %s -verify=expected,since-cxx11,since-cxx17 -fexceptions -fcxx-exceptions -pedantic-errors
+// RUN: %clang_cc1 -std=c++20 -triple x86_64-unknown-unknown %s -verify=expected,since-cxx11,since-cxx17 -fexceptions -fcxx-exceptions -pedantic-errors
+// RUN: %clang_cc1 -std=c++23 -triple x86_64-unknown-unknown %s -verify=expected,since-cxx11,since-cxx17 -fexceptions -fcxx-exceptions -pedantic-errors
+// RUN: %clang_cc1 -std=c++2c -triple x86_64-unknown-unknown %s -verify=expected,since-cxx11,since-cxx17 -fexceptions -fcxx-exceptions -pedantic-errors
 
 
 namespace cwg2211 { // cwg2211: 8
@@ -196,6 +196,17 @@ void g() {
 #endif
 } // namespace cwg2277
 
+namespace cwg2285 { // cwg2285: 4
+// Note: Clang 4 implements this DR but it set a wrong value of `__cplusplus`
+#if __cplusplus >= 201703L
+  void test() {
+    using T = int[1];
+    auto [a] = T{a};
+    // since-cxx17-error@-1 {{binding 'a' cannot appear in the initializer of its own decomposition declaration}}
+  }
+#endif
+} // namespace cwg2285
+
 namespace cwg2292 { // cwg2292: 9
 #if __cplusplus >= 201103L
   template<typename T> using id = T;

clang/test/Parser/cxx1z-decomposition.cpp

@@ -136,8 +136,8 @@ namespace MultiDeclarator {
 
 namespace Template {
   int n[3];
-  // FIXME: There's no actual rule against this...
-  template<typename T> auto [a, b, c] = n; // expected-error {{decomposition declaration template not supported}}
+  // Structured binding template is not allowed.
+  template<typename T> auto [a, b, c] = n; // expected-error {{decomposition declaration cannot be a template}}
 }
 
 namespace Init {

clang/www/cxx_dr_status.html

@@ -13537,7 +13537,7 @@ <h2 id="cxxdr">C++ defect report implementation status</h2>
     <td><a href="https://cplusplus.github.io/CWG/issues/2285.html">2285</a></td>
     <td>CD5</td>
     <td>Issues with structured bindings</td>
-    <td class="unknown" align="center">Unknown</td>
+    <td class="full" align="center">Clang 4</td>
   </tr>
   <tr id="2286">
     <td><a href="https://cplusplus.github.io/CWG/issues/2286.html">2286</a></td>