Add tests for CWG issues 6, 212, 232, 2823. (#165633)

Unfortunately this adds two more "no"s to cxx_dr_status for 232 and
2823.

---------

Co-authored-by: Vlad Serebrennikov <serebrennikov.vladislav@gmail.com>

Author: zygoloid

clang/test/CXX/drs/cwg0xx.cpp

@@ -90,6 +90,8 @@ namespace cwg5 { // cwg5: 3.1
   const C c = e;
 } // namespace cwg5
 
+// cwg6 is in cwg6.cpp
+
 namespace cwg7 { // cwg7: 3.4
   class A { public: ~A(); };
   class B : virtual private A {}; // #cwg7-B

clang/test/CXX/drs/cwg28xx.cpp

@@ -61,6 +61,24 @@ namespace cwg2819 { // cwg2819: 19 c++26
 #endif
 } // namespace cwg2819
 
+namespace cwg2823 { // cwg2823: no
+#if __cplusplus >= 201103L
+  constexpr int *p = 0;
+  constexpr int *q1 = &*p;
+  // expected-error@-1 {{constexpr variable 'q1' must be initialized by a constant expression}}
+  //   expected-note@-2 {{dereferencing a null pointer is not allowed in a constant expression}}
+  // FIXME: invalid: dereferencing a null pointer.
+  constexpr int *q2 = &p[0];
+
+  int arr[32];
+  constexpr int *r = arr;
+  // FIXME: invalid: dereferencing a past-the-end pointer.
+  constexpr int *s1 = &*(r + 32);
+  // FIXME: invalid: dereferencing a past-the-end pointer.
+  constexpr int *s2 = &r[32];
+#endif
+}
+
 namespace cwg2847 { // cwg2847: 19 review 2024-03-01
 
 #if __cplusplus >= 202002L

clang/test/CXX/drs/cwg2xx.cpp

@@ -230,6 +230,38 @@ namespace cwg211 { // cwg211: 2.7
   };
 } // namespace cwg211
 
+namespace cwg212 { // cwg212: 2.7
+  template<typename T> struct Base;
+  template<typename T> struct Derived;
+
+  int *overload(void*);
+  float *overload(Base<int>*);
+  double *overload(Base<long>*);
+
+  void f(Derived<int> *p) {
+    // OK, calls void* overload.
+    int *a = overload(p);
+
+    Base<int> *q = p;
+    // expected-error@-1 {{cannot initialize a variable of type 'Base<int> *' with an lvalue of type 'Derived<int> *'}}
+  }
+
+  template<typename T> struct Base {};
+  template<typename T> struct Derived : Base<T> {};
+
+  void g(Derived<long> *p) {
+    // OK, instantiates and calls Base<long>* overlod.
+    double *b = overload(p);
+    (void)b;
+  }
+
+  void h(Derived<float> *p) {
+    // OK, instantiates and converts.
+    Base<float> *q = p;
+    (void)q;
+  }
+}
+
 namespace cwg213 { // cwg213: 2.7
   template <class T> struct A : T {
     void h(T t) {
@@ -593,6 +625,9 @@ namespace cwg231 { // cwg231: 2.7
   }
 } // namespace cwg231
 
+// 232 is NAD; the desired behavior is described in 2823.
+// cwg232: dup 2823
+
 // cwg234: na
 // cwg235: na
 

clang/test/CXX/drs/cwg6.cpp

@@ -0,0 +1,51 @@
+// RUN: %clang_cc1 -std=c++98 %s -triple x86_64-linux-gnu -emit-llvm -o - -fexceptions -fcxx-exceptions -pedantic-errors | FileCheck %s --check-prefixes CHECK
+// RUN: %clang_cc1 -std=c++11 %s -triple x86_64-linux-gnu -emit-llvm -o - -fexceptions -fcxx-exceptions -pedantic-errors | FileCheck %s --check-prefixes CHECK
+// RUN: %clang_cc1 -std=c++14 %s -triple x86_64-linux-gnu -emit-llvm -o - -fexceptions -fcxx-exceptions -pedantic-errors | FileCheck %s --check-prefixes CHECK
+// RUN: %clang_cc1 -std=c++17 %s -triple x86_64-linux-gnu -emit-llvm -o - -fexceptions -fcxx-exceptions -pedantic-errors | FileCheck %s --check-prefixes CHECK
+// RUN: %clang_cc1 -std=c++20 %s -triple x86_64-linux-gnu -emit-llvm -o - -fexceptions -fcxx-exceptions -pedantic-errors | FileCheck %s --check-prefixes CHECK
+// RUN: %clang_cc1 -std=c++23 %s -triple x86_64-linux-gnu -emit-llvm -o - -fexceptions -fcxx-exceptions -pedantic-errors | FileCheck %s --check-prefixes CHECK
+// RUN: %clang_cc1 -std=c++2c %s -triple x86_64-linux-gnu -emit-llvm -o - -fexceptions -fcxx-exceptions -pedantic-errors | FileCheck %s --check-prefixes CHECK
+
+#if __cplusplus == 199711L
+#define static_assert(expr) __extension__ _Static_assert(expr)
+#define noexcept throw()
+#endif
+
+namespace cwg6 { // cwg6: 2.7
+#if __cplusplus >= 201103L
+struct Counter {
+  int copies;
+  constexpr Counter(int copies) : copies(copies) {}
+  constexpr Counter(const Counter& other) : copies(other.copies + 1) {}
+};
+
+// Passing an lvalue by value makes a non-elidable copy.
+constexpr int PassByValue(Counter c) { return c.copies; }
+constexpr int PassByValue2(Counter c) { return PassByValue(c); }
+constexpr int PassByValue3(Counter c) { return PassByValue2(c); }
+static_assert(PassByValue(Counter(0)) == 0, "expect no copies");
+static_assert(PassByValue2(Counter(0)) == 1, "expect 1 copy");
+static_assert(PassByValue3(Counter(0)) == 2, "expect 2 copies");
+#endif
+
+struct A {
+  A() noexcept;
+  A(const A&) noexcept;
+  ~A() noexcept;
+};
+
+inline void f(A a) noexcept {}
+
+// CHECK-LABEL: define {{.*}} @_ZN4cwg64callEv
+void call() {
+  A a;
+  // We copy the parameter here, even though object is not mutated by f and
+  // otherwise satisfies the criteria for the proposed CWG6 optimization.
+  // CHECK: call {{.*}} @_ZN4cwg61AC1ERKS0_(
+  // CHECK: call {{.*}} @_ZN4cwg61fENS_1AE(
+  f(a);
+  // CHECK: call {{.*}} @_ZN4cwg61AD1Ev(
+  // CHECK: call {{.*}} @_ZN4cwg61AD1Ev(
+}
+
+} // namespace cwg6

clang/www/cxx_dr_status.html

@@ -81,7 +81,7 @@ <h2 id="cxxdr">C++ defect report implementation status</h2>
     <td><a href="https://cplusplus.github.io/CWG/issues/6.html">6</a></td>
     <td>NAD</td>
     <td>Should the optimization that allows a class object to alias another object also allow the case of a parameter in an inline function to alias its argument?</td>
-    <td class="unknown" align="center">Unknown</td>
+    <td class="full" align="center">Yes</td>
   </tr>
   <tr id="7">
     <td><a href="https://cplusplus.github.io/CWG/issues/7.html">7</a></td>
@@ -1318,7 +1318,7 @@ <h2 id="cxxdr">C++ defect report implementation status</h2>
     <td><a href="https://cplusplus.github.io/CWG/issues/212.html">212</a></td>
     <td>CD4</td>
     <td>Implicit instantiation is not described clearly enough</td>
-    <td class="unknown" align="center">Unknown</td>
+    <td class="full" align="center">Yes</td>
   </tr>
   <tr id="213">
     <td><a href="https://cplusplus.github.io/CWG/issues/213.html">213</a></td>
@@ -1438,7 +1438,7 @@ <h2 id="cxxdr">C++ defect report implementation status</h2>
     <td><a href="https://cplusplus.github.io/CWG/issues/232.html">232</a></td>
     <td>NAD</td>
     <td>Is indirection through a null pointer undefined behavior?</td>
-    <td class="unknown" align="center">Unknown</td>
+    <td class="none" align="center">Duplicate of <a href="#2823">2823</a></td>
   </tr>
   <tr id="233">
     <td><a href="https://cplusplus.github.io/CWG/issues/233.html">233</a></td>
@@ -16790,7 +16790,7 @@ <h2 id="cxxdr">C++ defect report implementation status</h2>
     <td><a href="https://cplusplus.github.io/CWG/issues/2823.html">2823</a></td>
     <td>CD7</td>
     <td>Implicit undefined behavior when dereferencing pointers</td>
-    <td class="unknown" align="center">Unknown</td>
+    <td class="none" align="center">No</td>
   </tr>
   <tr id="2824">
     <td><a href="https://cplusplus.github.io/CWG/issues/2824.html">2824</a></td>