[clang] Add tests for Core issues about friend templates (#94288)

This patch covers the following Core issues:
[CWG1862](https://cplusplus.github.io/CWG/issues/1862.html) "Determining
“corresponding members” for friendship"
[CWG1918](https://cplusplus.github.io/CWG/issues/1918.html) "`friend`
templates with dependent scopes"
[CWG1945](https://cplusplus.github.io/CWG/issues/1945.html) "Friend
declarations naming members of class templates in non-templates"

All of them were resolved by CWG1862, where the current wording of
[[temp.friend] p5](https://eel.is/c++draft/temp.friend#5) comes from:
> A template friend declaration may declare a member of a dependent type
to be a friend[.](https://eel.is/c++draft/temp.friend#5.sentence-1) The
friend declaration shall declare a function or specify a type with an
[elaborated-type-specifier](https://eel.is/c++draft/dcl.type.elab#nt:elaborated-type-specifier),
in either case with a
[nested-name-specifier](https://eel.is/c++draft/expr.prim.id.qual#nt:nested-name-specifier)
ending with a
[simple-template-id](https://eel.is/c++draft/temp.names#nt:simple-template-id),
C, whose
[template-name](https://eel.is/c++draft/temp.names#nt:template-name)
names a class
template[.](https://eel.is/c++draft/temp.friend#5.sentence-2) The
template parameters of the template friend declaration shall be
deducible from C
([[temp.deduct.type]](https://eel.is/c++draft/temp.deduct.type))[.](https://eel.is/c++draft/temp.friend#5.sentence-3)
In this case, a member of a specialization S of the class template is a
friend of the class granting friendship if deduction of the template
parameters of C from S succeeds, and substituting the deduced template
arguments into the friend declaration produces a declaration that
corresponds to the member of the
specialization[.](https://eel.is/c++draft/temp.friend#5.sentence-4)

A useful example is provided after the paragraph:
https://eel.is/c++draft/temp.friend#example-4.

Neither of Core issues is implemented, because we don't support
dependent nested friend specifiers.

---------

Co-authored-by: Shafik Yaghmour <shafik.yaghmour@intel.com>

3 files changed, 134 insertions, 3 deletions

diff --git a/clang/test/CXX/drs/cwg18xx.cpp b/clang/test/CXX/drs/cwg18xx.cpp
index b71a81b..054ce5a 100644
--- a/clang/test/CXX/drs/cwg18xx.cpp
+++ b/clang/test/CXX/drs/cwg18xx.cpp
@@ -373,6 +373,98 @@ namespace cwg1837 { // cwg1837: 3.3
 #endif
 }
 
+namespace cwg1862 { // cwg1862: no
+template<class T>
+struct A {
+  struct B {
+    void e();
+  };
+  
+  void f();
+  
+  struct D {
+    void g();
+  };
+  
+  T h();
+
+  template<T U>
+  T i();
+};
+
+template<>
+struct A<int> {
+  struct B {
+    void e();
+  };
+  
+  int f();
+  
+  struct D {
+    void g();
+  };
+  
+  template<int U>
+  int i();
+};
+
+template<>
+struct A<float*> {
+  int* h();
+};
+
+class C {
+  int private_int;
+
+  template<class T>
+  friend struct A<T>::B;
+  // expected-warning@-1 {{dependent nested name specifier 'A<T>::' for friend class declaration is not supported; turning off access control for 'C'}}
+
+  template<class T>
+  friend void A<T>::f();
+  // expected-warning@-1 {{dependent nested name specifier 'A<T>::' for friend class declaration is not supported; turning off access control for 'C'}}
+
+  // FIXME: this is ill-formed, because A<T>​::​D does not end with a simple-template-id
+  template<class T>
+  friend void A<T>::D::g();
+  // expected-warning@-1 {{dependent nested name specifier 'A<T>::D::' for friend class declaration is not supported; turning off access control for 'C'}}
+  
+  template<class T>
+  friend int *A<T*>::h();
+  // expected-warning@-1 {{dependent nested name specifier 'A<T *>::' for friend class declaration is not supported; turning off access control for 'C'}}
+  
+  template<class T>
+  template<T U>
+  friend T A<T>::i();
+  // expected-warning@-1 {{dependent nested name specifier 'A<T>::' for friend class declaration is not supported; turning off access control for 'C'}}
+};
+
+C c;
+
+template<class T>
+void A<T>::B::e() { (void)c.private_int; }
+void A<int>::B::e() { (void)c.private_int; }
+
+template<class T>
+void A<T>::f() { (void)c.private_int; }
+int A<int>::f() { (void)c.private_int; return 0; }
+
+// FIXME: both definition of 'D::g' are not friends, so they don't have access to 'private_int' 
+template<class T>
+void A<T>::D::g() { (void)c.private_int; }
+void A<int>::D::g() { (void)c.private_int; }
+
+template<class T>
+T A<T>::h() { (void)c.private_int; }
+int* A<float*>::h() { (void)c.private_int; return 0; }
+
+template<class T>
+template<T U>
+T A<T>::i() { (void)c.private_int; }
+template<int U>
+int A<int>::i() { (void)c.private_int; }
+} // namespace cwg1862
+
 namespace cwg1872 { // cwg1872: 9
 #if __cplusplus >= 201103L
   template<typename T> struct A : T {
diff --git a/clang/test/CXX/drs/cwg19xx.cpp b/clang/test/CXX/drs/cwg19xx.cpp
index 3e4f828..762787a 100644
--- a/clang/test/CXX/drs/cwg19xx.cpp
+++ b/clang/test/CXX/drs/cwg19xx.cpp
@@ -80,6 +80,30 @@ namespace cwg1909 { // cwg1909: 3.7
   };
 }
 
+namespace cwg1918 { // cwg1918: no
+template<typename T> struct A {
+  class B {
+    class C {};
+  };
+};
+class X {
+  static int x;
+  // FIXME: this is ill-formed, because A<T>::B::C does not end with a simple-template-id
+  template <typename T>
+  friend class A<T>::B::C;
+  // expected-warning@-1 {{dependent nested name specifier 'A<T>::B::' for friend class declaration is not supported; turning off access control for 'X'}}
+};
+template<> struct A<int> {
+  typedef struct Q B;
+};
+struct Q {
+  class C {
+    // FIXME: 'f' is not a friend, so 'X::x' is not accessible
+    int f() { return X::x; }
+  };
+};
+} // namespace cwg1918
+
 namespace cwg1940 { // cwg1940: 3.5
 #if __cplusplus >= 201103L
 static union {
@@ -121,6 +145,21 @@ derived d2(42, 9);
 #endif
 }
 
+namespace cwg1945 { // cwg1945: no
+template<typename T> struct A {
+  class B {
+    class C {};
+  };
+};
+class X {
+  static int x;
+  // FIXME: this is ill-formed, because A<T>::B::C does not end with a simple-template-id
+  template <typename T>
+  friend class A<T>::B::C;
+  // expected-warning@-1 {{dependent nested name specifier 'A<T>::B::' for friend class declaration is not supported; turning off access control for 'X'}}
+};
+} // namespace cwg1945
+
 namespace cwg1947 { // cwg1947: 3.5
 #if __cplusplus >= 201402L
 unsigned o = 0'01;  // ok
diff --git a/clang/www/cxx_dr_status.html b/clang/www/cxx_dr_status.html
index 4d94ac5..e5f7941 100755
--- a/clang/www/cxx_dr_status.html
+++ b/clang/www/cxx_dr_status.html
@@ -10980,7 +10980,7 @@ and <I>POD class</I></td>
     <td><a href="https://cplusplus.github.io/CWG/issues/1862.html">1862</a></td>
     <td>CD5</td>
     <td>Determining &#8220;corresponding members&#8221; for friendship</td>
-    <td class="unknown" align="center">Unknown</td>
+    <td class="none" align="center">No</td>
   </tr>
   <tr id="1863">
     <td><a href="https://cplusplus.github.io/CWG/issues/1863.html">1863</a></td>
@@ -11316,7 +11316,7 @@ and <I>POD class</I></td>
     <td><a href="https://cplusplus.github.io/CWG/issues/1918.html">1918</a></td>
     <td>CD5</td>
     <td><TT>friend</TT> templates with dependent scopes</td>
-    <td class="unknown" align="center">Unknown</td>
+    <td class="none" align="center">No</td>
   </tr>
   <tr class="open" id="1919">
     <td><a href="https://cplusplus.github.io/CWG/issues/1919.html">1919</a></td>
@@ -11478,7 +11478,7 @@ and <I>POD class</I></td>
     <td><a href="https://cplusplus.github.io/CWG/issues/1945.html">1945</a></td>
     <td>CD5</td>
     <td>Friend declarations naming members of class templates in non-templates</td>
-    <td class="unknown" align="center">Unknown</td>
+    <td class="none" align="center">No</td>
   </tr>
   <tr id="1946">
     <td><a href="https://cplusplus.github.io/CWG/issues/1946.html">1946</a></td>