[clang] Add tests from CWG156 to CWG1111 (dual-scope lookup for conve…

…rsion-function-ids) (llvm#121654)

This patch adds test from
[CWG156](https://cplusplus.github.io/CWG/issues/156.html) to
[CWG1111](https://cplusplus.github.io/CWG/issues/1111.html) test, and
downgrades the latter to partial availability. The most relevant piece
of current wording is
[[basic.lookup.unqual]/5](https://eel.is/c++draft/basic.lookup#unqual-5):
> An unqualified name that is a component name
([[expr.prim.id.unqual]](https://eel.is/c++draft/expr.prim.id.unqual))
of a
[type-specifier](https://eel.is/c++draft/dcl.type.general#nt:type-specifier)
or
[ptr-operator](https://eel.is/c++draft/dcl.decl.general#nt:ptr-operator)
of a
[conversion-type-id](https://eel.is/c++draft/class.conv.fct#nt:conversion-type-id)
is looked up in the same fashion as the
[conversion-function-id](https://eel.is/c++draft/class.conv.fct#nt:conversion-function-id)
in which it
appears[.](https://eel.is/c++draft/basic.lookup#unqual-5.sentence-1)
If that lookup finds nothing, it undergoes unqualified name lookup; in
each case, only names that denote types or templates whose
specializations are types are
considered[.](https://eel.is/c++draft/basic.lookup#unqual-5.sentence-2)

Per resolution of
[CWG1111](https://cplusplus.github.io/CWG/issues/1111.html), additional
lookup in the context of the entire postfix-expression, which originally
was intended to cross-check lookup in the context of object-expression,
was effectively turned into a fallback for it.

Check out "Calling a conversion function" example in
[P1787R6](https://wg21.link/p1787r6) for step-by-step explanation of the
current lookup mechanics for conversion functions.

Clang rejects one of the well-formed examples, hence partial status.
Clang is the only implementation which rejects it:
https://godbolt.org/z/ohhbx8Mfs

clang/test/CXX/drs/cwg11xx.cpp

@@ -19,7 +19,7 @@ decltype(return_T<B<int>>())* b;
 #endif
 } // namespace cwg1110
 
-namespace cwg1111 { // cwg1111: 3.2
+namespace cwg1111 { // cwg1111: partial
 namespace example1 {
 template <typename> struct set; // #cwg1111-struct-set
 
@@ -57,6 +57,48 @@ void baz() {
   a.operator A();
 }
 } // namespace example2
+
+namespace example3 {
+struct A {
+  operator int();
+} a;
+void foo() {
+  typedef int T;
+  a.operator T(); // T is found using unqualified lookup
+                  // after qualified lookup in A fails.
+}
+} // namespace example3
+
+namespace example4 {
+struct A {
+  typedef int T; // #cwg1111-A-T
+  operator T();
+};
+struct B : A {
+  operator T();
+} b;
+void foo() {
+  b.A::operator T(); // FIXME: qualified lookup should find T in A.
+  // expected-error@-1 {{unknown type name 'T'}}
+  //   expected-note@#cwg1111-A-T {{'A::T' declared here}}
+}
+} // namespace example4
+
+namespace example5 {
+template <class T1> struct A {
+  operator T1();
+};
+template <class T2> struct B : A<T2> {
+  operator T2();
+  void foo() {
+    // In both cases, during instantiation, qualified lookup for T2 wouldn't be able
+    // to find anything, so T2 has to be found by unqualified lookup.
+    // After that, 'operator T2()' is found in A<T2> by qualfied lookup.
+    T2 a = A<T2>::operator T2();
+    T2 b = ((A<T2> *)this)->operator T2();
+  }
+};
+} // namespace example5
 } // namespace cwg1111
 
 namespace cwg1113 { // cwg1113: partial

clang/test/CXX/drs/cwg1xx.cpp

@@ -922,6 +922,7 @@ namespace cwg155 { // cwg155: dup 632
   // expected-warning@-1 {{braces around scalar initializer}}
 }
 
+// cwg156: sup 1111
 // cwg158 is in cwg158.cpp
 
 namespace cwg159 { // cwg159: 3.5

clang/www/cxx_dr_status.html

@@ -981,7 +981,7 @@ <h2 id="cxxdr">C++ defect report implementation status</h2>
     <td><a href="https://cplusplus.github.io/CWG/issues/156.html">156</a></td>
     <td>NAD</td>
     <td>Name lookup for conversion functions</td>
-    <td class="unknown" align="center">Unknown</td>
+    <td class="partial-superseded" align="center">Superseded by <a href="#1111">1111</a></td>
   </tr>
   <tr class="open" id="157">
     <td><a href="https://cplusplus.github.io/CWG/issues/157.html">157</a></td>
@@ -6485,7 +6485,7 @@ <h2 id="cxxdr">C++ defect report implementation status</h2>
     <td><a href="https://cplusplus.github.io/CWG/issues/1111.html">1111</a></td>
     <td>C++11</td>
     <td>Remove dual-scope lookup of member template names</td>
-    <td class="full" align="center">Clang 3.2</td>
+    <td class="partial" align="center">Partial</td>
   </tr>
   <tr id="1112">
     <td><a href="https://cplusplus.github.io/CWG/issues/1112.html">1112</a></td>