[clang] Implement CWG 2397

This patch implements CWG 2397, allowing array of placeholder type
to be valid.

See also https://wg21.cmeerw.net/cwg/issue2397.

Reviewed By: aaron.ballman, #clang-language-wg

Differential Revision: https://reviews.llvm.org/D147909

clang/docs/ReleaseNotes.rst

@@ -103,6 +103,8 @@ C++2b Feature Support
 
 Resolutions to C++ Defect Reports
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+- Implemented `DR2397 <https://wg21.link/CWG2397>`_ which allows ``auto`` specifier for pointers
+  and reference to arrays.
 
 C Language Changes
 ------------------

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -2314,8 +2314,6 @@ def err_auto_variable_cannot_appear_in_own_initializer : Error<
 def err_binding_cannot_appear_in_own_initializer : Error<
   "binding %0 cannot appear in the initializer of its own "
   "decomposition declaration">;
-def err_illegal_decl_array_of_auto : Error<
-  "'%0' declared as array of %1">;
 def err_new_array_of_auto : Error<
   "cannot allocate array of 'auto'">;
 def err_auto_not_allowed : Error<

clang/lib/Sema/SemaType.cpp

@@ -5130,17 +5130,6 @@ static TypeSourceInfo *GetFullTypeForDeclarator(TypeProcessingState &state,
           D.setInvalidType(true);
         }
       }
-      const AutoType *AT = T->getContainedAutoType();
-      // Allow arrays of auto if we are a generic lambda parameter.
-      // i.e. [](auto (&array)[5]) { return array[0]; }; OK
-      if (AT && D.getContext() != DeclaratorContext::LambdaExprParameter) {
-        // We've already diagnosed this for decltype(auto).
-        if (!AT->isDecltypeAuto())
-          S.Diag(DeclType.Loc, diag::err_illegal_decl_array_of_auto)
-              << getPrintableNameForEntity(Name) << T;
-        T = QualType();
-        break;
-      }
 
       // Array parameters can be marked nullable as well, although it's not
       // necessary if they're marked 'static'.

clang/test/CXX/dcl.decl/dcl.meaning/dcl.array/cwg2397.cpp

@@ -0,0 +1,25 @@
+// RUN: %clang_cc1 -fsyntax-only -verify %s -std=c++11
+// RUN: %clang_cc1 -fsyntax-only -verify %s -std=c++14
+
+void f() {
+  int a[5];
+  auto (*b)[5] = &a;
+  auto (&c)[5] = a;
+  auto (&&d)[5] = static_cast<int(&&)[5]>(a);
+  auto e[] = {0}; // expected-error{{cannot deduce actual type for variable 'e' with type 'auto[]' from initializer list}}
+  static_assert(__is_same(decltype(b), int (*)[5]), "");
+  static_assert(__is_same(decltype(c), int (&)[5]), "");
+  static_assert(__is_same(decltype(d), int (&&)[5]), "");
+}
+
+#if __cplusplus >= 201402L
+
+constexpr int g() {
+  int a[] = {1, 2, 3};
+  auto (&b)[3] = a;
+  return b[1];
+}
+
+static_assert(g() == 2, "");
+
+#endif

clang/test/CXX/dcl.decl/dcl.meaning/dcl.array/p1-cxx0x.cpp

@@ -2,6 +2,6 @@
 
 void f() {
   int b[5];
-  auto a[5] = b; // expected-error{{'a' declared as array of 'auto'}}
-  auto *c[5] = b; // expected-error{{'c' declared as array of 'auto *'}}
+  auto a[5] = b; // expected-error{{variable 'a' with type 'auto[5]' has incompatible initializer of type 'int[5]'}}
+  auto *c[5] = b; // expected-error{{variable 'c' with type 'auto *[5]' has incompatible initializer of type 'int[5]'}}
 }

clang/test/CXX/drs/dr23xx.cpp

@@ -213,3 +213,14 @@ namespace dr2396 { // dr2396: no
   // void g(A a) { a.operator decltype(B()) B::*(); }
   // void g2(A a) { a.operator B decltype(B())::*(); }
 }
+
+#if __cplusplus >= 201103L
+namespace dr2397 { // dr2397: 17
+  void foo() {
+    int a[5];
+
+    auto (&b)[5] = a;
+    auto (*c)[5] = &a;
+  }
+} // namespace dr2397
+#endif

clang/www/cxx_dr_status.html

@@ -14189,7 +14189,7 @@ and <I>POD class</I></td>
     <td><a href="https://wg21.link/cwg2397">2397</a></td>
     <td>CD6</td>
     <td><TT>auto</TT> specifier for pointers and references to arrays</td>
-    <td class="none" align="center">Unknown</td>
+    <td class="unreleased" align="center">Clang 17</td>
   </tr>
   <tr class="open" id="2398">
     <td><a href="https://wg21.link/cwg2398">2398</a></td>