[clang] Add test for CWG253

[Endill]

Summary

Also mark CWG78 as superseded by CWG2536.

CWG253 is concerned with users required to explicitly declare defaulted default constructor when they want to declare local object without initializer, when there's nothing to initialize (e.g. a struct without non-static data members).

https://cplusplus.github.io/CWG/issues/253.html
https://cplusplus.github.io/CWG/issues/78.html

[Endill]

Update cxx_dr_status.html

[shafik]

clang/test/CXX/drs/dr0xx.cpp

1022

This is issue 1380 and the issue is whether we want static initialization to happen before constant initialization or whether constant initialization excludes zero-init.

I think dr77 is now part of cwg 2536 and we need to wait for the resolution of that in order to know what to do here.

[Endill]

@shafik Thank you for letting me know! Now I wonder if it was possible to realize by myself without prior knowledge of CWG 2536, because I feel like some cross-references are missing.
Can we proceed if I strip CWG 78 out, leaving only CWG 253?

[shafik]

In D151634#4383428, @Endill wrote:

@shafik Thank you for letting me know! Now I wonder if it was possible to realize by myself without prior knowledge of CWG 2536, because I feel like some cross-references are missing.
Can we proceed if I strip CWG 78 out, leaving only CWG 253?

I don't think there was a way to figure this out w/o knowing or luck.

I think just leave out 78 for now.

clang/test/CXX/drs/dr2xx.cpp

686

Note this was fixed via: https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0490r0.html

[Endill]

Mark CWG78 as superseded by 2536
Expand CWG253 test

@shafik I think rather than leave 78 out, we better mark it as superseded, so that we don't forget about it.

[shafik]

clang/test/CXX/drs/dr0xx.cpp

1022

I was mistaken and completely missed: https://eel.is/c++draft/dcl.init#general-8.sentence-2

DR 78 is just repeating what we have in: https://eel.is/c++draft/basic.start#static

The wording has changed a lot since DR 78.

[Endill]

clang/test/CXX/drs/dr0xx.cpp

1022

Can you please elaborate how does your conclusion affect this patch? Because I feel a bit lost at this point.
Was my initial analysis correct, and we should say that this DR is not available in Clang?

[shafik]

clang/test/CXX/drs/dr0xx.cpp

1022

No, so this DR was just clarifying that static objects will not have an indeterminate value if they don't have an initializer. So we can consider this NA or you can add a test with a static global w/o an init and show it has zero value.

[Endill]

clang/test/CXX/drs/dr0xx.cpp

1022

I'd like to focus on the following points:

1. test case is correct, because we're not supposed to issue a diagnostics for static objects without initializers
2. when we stop issuing a diagnostics, we should write a codegen test that ensures k is initialized with 0
3. this DR is not superseded, so we should mark it as no

Do you agree with everything above?

clang/test/CXX/drs/dr0xx.cpp

 // RUN: %clang_cc1 -std=c++98 %s -verify -fexceptions -fcxx-exceptions -pedantic-errors -Wno-bind-to-temporary-copy
 // RUN: %clang_cc1 -std=c++11 %s -verify -fexceptions -fcxx-exceptions -pedantic-errors -triple %itanium_abi_triple
 // RUN: %clang_cc1 -std=c++14 %s -verify -fexceptions -fcxx-exceptions -pedantic-errors -triple %itanium_abi_triple
 // RUN: %clang_cc1 -std=c++17 %s -verify -fexceptions -fcxx-exceptions -pedantic-errors -triple %itanium_abi_triple
 // RUN: %clang_cc1 -std=c++20 %s -verify -fexceptions -fcxx-exceptions -pedantic-errors -triple %itanium_abi_triple
 // RUN: %clang_cc1 -std=c++23 %s -verify -fexceptions -fcxx-exceptions -pedantic-errors -triple %itanium_abi_triple
+// RUN: %clang_cc1 -std=c++2c %s -verify -fexceptions -fcxx-exceptions -pedantic-errors -triple %itanium_abi_triple
 
 namespace dr1 { // dr1: no
   namespace X { extern "C" void dr1_f(int a = 1); }
   namespace Y { extern "C" void dr1_f(int a = 1); }
   using X::dr1_f; using Y::dr1_f;
   void g() {
     dr1_f(0);
     // FIXME: This should be rejected, due to the ambiguous default argument.

 
 namespace dr77 { // dr77: yes
   struct A {
     struct B {};
     friend struct B;
   };
 }
 
-namespace dr78 { // dr78: sup ????
+namespace dr78 { // dr78: sup 2536
   // Under DR78, this is valid, because 'k' has static storage duration, so is
   // zero-initialized.
   const int k; // expected-error {{default initialization of an object of const}}
 }
 
 // dr79: na
 
 namespace dr80 { // dr80: yes

clang/test/CXX/drs/dr2xx.cpp

 // RUN: %clang_cc1 -std=c++98 %s -verify -fexceptions -fcxx-exceptions -pedantic-errors
 // RUN: %clang_cc1 -std=c++11 %s -verify -fexceptions -fcxx-exceptions -pedantic-errors
 // RUN: %clang_cc1 -std=c++14 %s -verify -fexceptions -fcxx-exceptions -pedantic-errors
 // RUN: %clang_cc1 -std=c++17 %s -verify -fexceptions -fcxx-exceptions -pedantic-errors
 // RUN: %clang_cc1 -std=c++20 %s -verify -fexceptions -fcxx-exceptions -pedantic-errors
 // RUN: %clang_cc1 -std=c++23 %s -verify -fexceptions -fcxx-exceptions -pedantic-errors
+// RUN: %clang_cc1 -std=c++2b %s -verify -fexceptions -fcxx-exceptions -pedantic-errors
 
 // PR13819 -- __SIZE_TYPE__ is incompatible.
 typedef __SIZE_TYPE__ size_t; // expected-error 0-1 {{extension}}
 
 #if __cplusplus < 201103L
 #define fold(x) (__builtin_constant_p(x) ? (x) : (x))
 #else
 #define fold
namespace dr252 { // dr252: yes
 
   struct G {
     void operator delete(void*, size_t);
     virtual ~G();
   };
   G::~G() {}
 }
 
+namespace dr253 { // dr253: 3.9
+struct X {};
+struct Y {
+  X x;
+};
+
+struct Z {
+  operator int() const { return 0; }
+};
+
+void f() {
+  const X x1;
+  const X x2 = { };
+  const Z z1;      
+  const Z z2 = { };
+}
+} // namespace dr253
+
 namespace dr254 { // dr254: yes
   template<typename T> struct A {
     typedef typename T::type type; // ok even if this is a typedef-name, because
                                    // it's not an elaborated-type-specifier
     typedef struct T::type foo; // expected-error {{typedef 'type' cannot be referenced with a struct specifier}}
   };
   struct B { struct type {}; };
   struct C { typedef struct {} type; }; // expected-note {{here}}

clang/www/cxx_dr_status.html

<tr id="77">
     <td>CD1</td>
     <td>The definition of friend does not allow nested classes to be friends</td>
     <td class="full" align="center">Yes</td>
   </tr>
   <tr id="78">
     <td><a href="https://cplusplus.github.io/CWG/issues/78.html">78</a></td>
     <td>CD1</td>
     <td>Section 8.5 paragraph 9 should state it only applies to non-static objects</td>
-    <td class="none" align="center">Superseded by <a href="#????">????</a></td>
+    <td class="none" align="center">Superseded by <a href="#2536">2536</a></td>
   </tr>
   <tr id="79">
     <td><a href="https://cplusplus.github.io/CWG/issues/79.html">79</a></td>
     <td>dup</td>
     <td>Alignment and placement new</td>
     <td class="na" align="center">N/A</td>
   </tr>
   <tr id="80">
<tr id="252">
     <td>CD1</td>
     <td>Looking up deallocation functions in virtual destructors</td>
     <td class="full" align="center">Yes</td>
   </tr>
   <tr id="253">
     <td><a href="https://cplusplus.github.io/CWG/issues/253.html">253</a></td>
     <td>C++17</td>
     <td>Why must empty or fully-initialized const objects be initialized?</td>
-    <td class="none" align="center">Unknown</td>
+    <td class="full" align="center">Clang 3.9</td>
   </tr>
   <tr id="254">
     <td><a href="https://cplusplus.github.io/CWG/issues/254.html">254</a></td>
     <td>CD1</td>
     <td>Definitional problems with <I>elaborated-type-specifier</I>s</td>
     <td class="full" align="center">Yes</td>
   </tr>
   <tr id="255">
<tr id="496">
     <td>CD3</td>
     <td>Is a volatile-qualified type really a POD?</td>
     <td class="full" align="center">Superseded by <a href="#2094">2094</a></td>
   </tr>
   <tr id="497">
     <td><a href="https://cplusplus.github.io/CWG/issues/497.html">497</a></td>
     <td>CD1</td>
     <td>Missing required initialization in example</td>
-    <td class="none" align="center">Superseded by <a href="#253">253</a></td>
+    <td class="full" align="center">Superseded by <a href="#253">253</a></td>
   </tr>
   <tr class="open" id="498">
     <td><a href="https://cplusplus.github.io/CWG/issues/498.html">498</a></td>
     <td>open</td>
     <td>Storage class specifiers in definitions of class members</td>
     <td align="center">Not resolved</td>
   </tr>
   <tr id="499">