[clang] Add test for CWG1710 and related issues

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Summary

Those issues focus on template keyword being optional in certain type-only contexts (base specifiers, member initializers, typename specifiers), as opposed to be disallowed by the grammar, or required by some implementations. GCC accepts all the tests this patch touches since 10, others fail on various tests: https://godbolt.org/z/1M6KE3W1a

It should be noted that the wording in 1710 that resolves those issues has been substantially changed by P1787. I can't find the post-P1787 wording that covers those issues, but I can't find the intent of changing relevant behavior in P1787 either, so I assume that intent of the 1710 resolution is preserved somewhere.

This patch covers the following issues:
CWG314
CWG343
CWG1710
CWG1794
CWG1812

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ping

[cor3ntin]

Thanks.
These seem like important issues to fix. should we make a github issue to track them?

clang/test/CXX/drs/dr17xx.cpp

10

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Thank you for taking a look!
I found a bunch of issues pointing at the same diagnostics, but I'm not sure if they track exactly what I test here:
https://github.com/llvm/llvm-project/issues/17775
https://github.com/llvm/llvm-project/issues/36539
https://github.com/llvm/llvm-project/issues/38395
https://github.com/llvm/llvm-project/issues/57019

clang/test/CXX/drs/dr17xx.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++1z %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++2c %s -verify -fexceptions -fcxx-exceptions -pedantic-errors
+
+namespace dr1710 { // dr1710: no
+// FIXME: all of the following is well-formed
+template <typename T> struct D1 : T::template B<int>::template C<int> {};
+template <typename T> struct D2 : T::B<int>::template C<int> {};
+// expected-error@-1 {{use 'template' keyword to treat 'B' as a dependent template name}}
+template <typename T> struct D3 : T::template B<int>::C<int> {};
+// expected-error@-1 {{use 'template' keyword to treat 'C' as a dependent template name}}
+template <typename T> struct D4 : T::B<int>::C<int> {};
+// expected-error@-1 {{use 'template' keyword to treat 'B' as a dependent template name}}
+// expected-error@-2 {{use 'template' keyword to treat 'C' as a dependent template name}}
+} // namespace dr1710
 
 namespace dr1715 { // dr1715: 3.9
 #if __cplusplus >= 201103L
   struct B {
     template<class T> B(T, typename T::Q);
   };
 
   class S {
#if __cplusplus >= 201103L
   static_assert(!noexcept(B()), "");
 
   struct C { A a; C() noexcept(false) = default; };
   struct D { B b; D() noexcept(true) = default; };
   static_assert(!noexcept(C()), "");
   static_assert(noexcept(D()), "");
 #endif
 }
+
+namespace dr1794 { // dr1794: yes
+                   // NB: dup 1710
+#if __cplusplus >= 201103L
+template <template <typename> class Template> struct Internal {
+  template <typename Arg> using Bind = Template<Arg>;
+};
+
+template <template <typename> class Template, typename Arg>
+using Instantiate = Template<Arg>;
+
+template <template <typename> class Template, typename Argument>
+using Bind = Instantiate<Internal<Template>::template Bind, Argument>;
+#endif
+} // namespace dr1794

clang/test/CXX/drs/dr18xx.cpp

 // RUN: %clang_cc1 -std=c++98 -triple x86_64-unknown-unknown %s -verify -fexceptions -Wno-deprecated-builtins -fcxx-exceptions -pedantic-errors
 // RUN: %clang_cc1 -std=c++11 -triple x86_64-unknown-unknown %s -verify -fexceptions -Wno-deprecated-builtins -fcxx-exceptions -pedantic-errors
 // RUN: %clang_cc1 -std=c++14 -triple x86_64-unknown-unknown %s -verify -fexceptions -Wno-deprecated-builtins -fcxx-exceptions -pedantic-errors
 // RUN: %clang_cc1 -std=c++17 -triple x86_64-unknown-unknown %s -verify -fexceptions -Wno-deprecated-builtins -fcxx-exceptions -pedantic-errors
 // RUN: %clang_cc1 -std=c++20 -triple x86_64-unknown-unknown %s -verify -fexceptions -Wno-deprecated-builtins -fcxx-exceptions -pedantic-errors
 // RUN: %clang_cc1 -std=c++23 -triple x86_64-unknown-unknown %s -verify -fexceptions -Wno-deprecated-builtins -fcxx-exceptions -pedantic-errors
+// RUN: %clang_cc1 -std=c++2c -triple x86_64-unknown-unknown %s -verify -fexceptions -Wno-deprecated-builtins -fcxx-exceptions -pedantic-errors
 
 #if __cplusplus < 201103L
 // expected-error@+1 {{variadic macro}}
 #define static_assert(...) __extension__ _Static_assert(__VA_ARGS__)
 #endif
 
+namespace dr1812 { // dr1812: no
+                   // NB: dup 1710
+#if __cplusplus >= 201103L
+template <typename T> struct A {
+  using B = typename T::C<int>;
+  // expected-error@-1 {{use 'template' keyword to treat 'C' as a dependent template name}}
+};
+#endif
+} // namespace dr1812
+
 namespace dr1813 { // dr1813: 7
   struct B { int i; };
   struct C : B {};
   struct D : C {};
   struct E : D { char : 4; };
 
   static_assert(__is_standard_layout(B), "");
   static_assert(__is_standard_layout(C), "");

clang/test/CXX/drs/dr3xx.cpp

 namespace dr313 { // dr313: dup 299 c++11
   struct A { operator int() const; };
   int *p = new int[A()];
 #if __cplusplus < 201103L
   // FIXME: should this be available in c++98 mode? expected-error@-2 {{extension}}
 #endif
 }
 
-namespace dr314 { // FIXME 314: dup 1710
-  template<typename T> struct A {
-    template<typename U> struct B {};
-  };
-  template<typename T> struct C : public A<T>::template B<T> {
-    C() : A<T>::template B<T>() {}
-  };
-}
+namespace dr314 { // dr314: no
+                  // NB: dup 1710
+template <typename T> struct A {
+  template <typename U> struct B {};
+template <typename T> struct C : public A<T>::template B<T> {
+template <typename T> struct C2 : public A<T>::B<T> {
+  // expected-error@-1 {{use 'template' keyword to treat 'B' as a dependent template name}}
+  C2() : A<T>::B<T>() {}
+  // expected-error@-1 {{use 'template' keyword to treat 'B' as a dependent template name}}
+};
+} // namespace dr314
 
 // dr315: na
 // dr316: sup 1004
 
 namespace dr317 { // dr317: 3.5
   void f() {} // expected-note {{previous}}
   inline void f(); // expected-error {{inline declaration of 'f' follows non-inline definition}}
 
namespace dr341 {
   extern "C" void dr341_d(); // expected-error {{declaration of 'dr341_d' with C language linkage conflicts with declaration in global scope}}
 
   namespace A { extern "C" int dr341_e; } // expected-note {{previous}}
   namespace B { extern "C" void dr341_e(); } // expected-error {{redefinition of 'dr341_e' as different kind of symbol}}
 }
 
 // dr342: na
 
-namespace dr343 { // FIXME 343: no
+namespace dr343 { // dr343: no
   // FIXME: dup 1710
   template<typename T> struct A {
     template<typename U> struct B {};
   };
   // FIXME: In these contexts, the 'template' keyword is optional.
   template<typename T> struct C : public A<T>::B<T> { // expected-error {{use 'template'}}
     C() : A<T>::B<T>() {} // expected-error {{use 'template'}}
   };

clang/www/cxx_dr_status.html

<tr id="313">
     <td>dup</td>
     <td>Class with single conversion function to integral as array size in <TT>new</TT></td>
     <td class="full" align="center">Duplicate of <a href="#299">299</a> (C++11 onwards)</td>
   </tr>
   <tr id="314">
     <td><a href="https://cplusplus.github.io/CWG/issues/314.html">314</a></td>
     <td>C++17</td>
     <td><TT>template</TT> in base class specifier</td>
-    <td class="none" align="center">Unknown</td>
+    <td class="none" align="center">No</td>
   </tr>
   <tr id="315">
     <td><a href="https://cplusplus.github.io/CWG/issues/315.html">315</a></td>
     <td>NAD</td>
     <td>Is call of static member function through null pointer undefined?</td>
     <td class="na" align="center">N/A</td>
   </tr>
   <tr id="316">
<tr id="342">
     <td>CD3</td>
     <td>Terminology: "indirection" versus "dereference"</td>
     <td class="na" align="center">N/A</td>
   </tr>
   <tr id="343">
     <td><a href="https://cplusplus.github.io/CWG/issues/343.html">343</a></td>
     <td>C++17</td>
     <td>Make <TT>template</TT> optional in contexts that require a type</td>
-    <td class="none" align="center">Unknown</td>
+    <td class="none" align="center">No</td>
   </tr>
   <tr id="344">
     <td><a href="https://cplusplus.github.io/CWG/issues/344.html">344</a></td>
     <td>CD3</td>
     <td>Naming destructors</td>
     <td class="none" align="center">Duplicate of <a href="#1435">1435</a></td>
   </tr>
   <tr id="345">
<tr class="open" id="1709">
     <td>drafting</td>
     <td>Stringizing raw string literals containing newline</td>
     <td align="center">Not resolved</td>
   </tr>
   <tr id="1710">
     <td><a href="https://cplusplus.github.io/CWG/issues/1710.html">1710</a></td>
     <td>C++17</td>
     <td>Missing <TT>template</TT> keyword in <I>class-or-decltype</I></td>
-    <td class="none" align="center">Unknown</td>
+    <td class="none" align="center">No</td>
   </tr>
   <tr id="1711">
     <td><a href="https://cplusplus.github.io/CWG/issues/1711.html">1711</a></td>
     <td>CD6</td>
     <td>Missing specification of variable template partial specializations</td>
     <td class="none" align="center">Unknown</td>
   </tr>
   <tr id="1712">
<tr id="1793">
     <td>CD4</td>
     <td><TT>thread_local</TT> in explicit specializations</td>
     <td class="none" align="center">Unknown</td>
   </tr>
   <tr id="1794">
     <td><a href="https://cplusplus.github.io/CWG/issues/1794.html">1794</a></td>
     <td>C++17</td>
     <td><TT>template</TT> keyword and alias templates</td>
-    <td class="none" align="center">Unknown</td>
+    <td class="full" align="center">Yes</td>
   </tr>
   <tr id="1795">
     <td><a href="https://cplusplus.github.io/CWG/issues/1795.html">1795</a></td>
     <td>CD4</td>
     <td>Disambiguating <I>original-namespace-definition</I> and <I>extension-namespace-definition</I></td>
     <td class="none" align="center">Unknown</td>
   </tr>
   <tr id="1796">
<tr id="1811">
     <td>CD4</td>
     <td>Lookup of deallocation function in a virtual destructor definition</td>
     <td class="none" align="center">Unknown</td>
   </tr>
   <tr id="1812">
     <td><a href="https://cplusplus.github.io/CWG/issues/1812.html">1812</a></td>
     <td>C++17</td>
     <td>Omission of <TT>template</TT> in a <I>typename-specifier</I></td>
-    <td class="none" align="center">Unknown</td>
+    <td class="none" align="center">No</td>
   </tr>
   <tr id="1813">
     <td><a href="https://cplusplus.github.io/CWG/issues/1813.html">1813</a></td>
     <td>CD4</td>
     <td>Direct vs indirect bases in standard-layout classes</td>
     <td class="full" align="center">Clang 7</td>
   </tr>
   <tr id="1814">