[clang] Add test for CWG1837

[Endill]

Summary

P1787: CWG1837 is resolved by restricting this to referring to the innermost enclosing class.
Wording: see changes to [expr.prim.this] and [expr.prim.lambda].

[Endill]

minor fix

[erichkeane]

This seems right based on the paper listed/standardeeze. Thanks!

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++2a -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++2b -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 dr1813 { // dr1813: 7
   struct B { int i; };

 #if __cplusplus >= 201103L
   int a;
   auto x = [] (int a) {
 #pragma clang __debug dump a // CHECK: ParmVarDecl
   };
 #endif
 }
 
+namespace dr1837 { // dr1837: 3.3
+#if __cplusplus >= 201103L
+  template <typename T>
+  struct Fish { static const bool value = true; };
+
+  struct Other {
+    int p();
+    auto q() -> decltype(p()) *;
+  };
+
+  class Outer {
+    friend auto Other::q() -> decltype(this->p()) *; // expected-error {{invalid use of 'this'}}
+    int g();
+    int f() {
+      extern void f(decltype(this->g()) *);
+      struct Inner {
+        static_assert(Fish<decltype(this->g())>::value, ""); // expected-error {{invalid use of 'this'}}
+        enum { X = Fish<decltype(this->f())>::value }; // expected-error {{invalid use of 'this'}}
+        struct Inner2 : Fish<decltype(this->g())> { }; // expected-error {{invalid use of 'this'}}
+        friend void f(decltype(this->g()) *); // expected-error {{invalid use of 'this'}}
+        friend auto Other::q() -> decltype(this->p()) *; // expected-error {{invalid use of 'this'}}
+      };
+      return 0;
+    }
+  };
+
+  struct A {
+    int f();
+    bool b = [] {
+      struct Local {
+        static_assert(sizeof(this->f()) == sizeof(int), "");
+      };
+    };
+  };
+#endif
+}
+
 namespace dr1872 { // dr1872: 9
 #if __cplusplus >= 201103L
   template<typename T> struct A : T {
     constexpr int f() const { return 0; }
   };
   struct X {};
   struct Y { virtual int f() const; };
   struct Z : virtual X {};

clang/www/cxx_dr_status.html

<tr id="1836">
     <td>CD5</td>
     <td>Use of class type being defined in <I>trailing-return-type</I></td>
     <td class="none" align="center">Unknown</td>
   </tr>
   <tr id="1837">
     <td><a href="https://wg21.link/cwg1837">1837</a></td>
     <td>CD6</td>
     <td>Use of <TT>this</TT> in <TT>friend</TT> and local class declarations</td>
-    <td class="none" align="center">Unknown</td>
+    <td class="full" align="center">Clang 3.3</td>
   </tr>
   <tr id="1838">
     <td><a href="https://wg21.link/cwg1838">1838</a></td>
     <td>CD4</td>
     <td>Definition via <I>unqualified-id</I> and <I>using-declaration</I></td>
     <td class="none" align="center">Unknown</td>
   </tr>
   <tr id="1839">