[clang] Allow constexpr relational comparison of unequal "void*" values

Since C++17, it is no longer the case that applying relational operators
to unequal pointers to "void" always produces an unspecified result.

Author: offsetof

clang/lib/AST/ExprConstant.cpp

@@ -13510,7 +13510,10 @@ EvaluateComparisonBinaryOperator(EvalInfo &Info, const BinaryOperator *E,
     //   operator is <= or >= and false otherwise; otherwise the result is
     //   unspecified.
     // We interpret this as applying to pointers to *cv* void.
-    if (LHSTy->isVoidPointerType() && LHSOffset != RHSOffset && IsRelational)
+    // This only applies until C++14; in C++17 a "cv void*" value can "point to
+    // an object" and is not treated specially in [expr.rel].
+    if (!Info.getLangOpts().CPlusPlus17 &&
+        LHSTy->isVoidPointerType() && LHSOffset != RHSOffset && IsRelational)
       Info.CCEDiag(E, diag::note_constexpr_void_comparison);
 
     // C++11 [expr.rel]p2:

clang/test/AST/Interp/literals.cpp

@@ -1,6 +1,6 @@
 // RUN: %clang_cc1 -fexperimental-new-constant-interpreter -Wno-vla -fms-extensions -std=c++11 -verify=expected,both %s
 // RUN: %clang_cc1 -fexperimental-new-constant-interpreter -Wno-vla -fms-extensions -std=c++20 -verify=expected,both %s
-// RUN: %clang_cc1 -std=c++11 -fms-extensions -Wno-vla -verify=ref,both %s
+// RUN: %clang_cc1 -std=c++11 -fms-extensions -Wno-vla -verify=ref,ref-cxx11,both %s
 // RUN: %clang_cc1 -std=c++20 -fms-extensions -Wno-vla -verify=ref,both %s
 
 #define INT_MIN (~__INT_MAX__)
@@ -193,10 +193,10 @@ namespace PointerComparison {
 
   /// FIXME: These two are rejected by the current interpreter, but
   ///   accepted by GCC.
-  constexpr bool v5 = qv >= pv; // ref-error {{constant expression}} \
-                                // ref-note {{unequal pointers to void}}
-  constexpr bool v8 = qv > (void*)&s.a; // ref-error {{constant expression}} \
-                                        // ref-note {{unequal pointers to void}}
+  constexpr bool v5 = qv >= pv; // ref-cxx11-error {{constant expression}} \
+                                // ref-cxx11-note {{unequal pointers to void}}
+  constexpr bool v8 = qv > (void*)&s.a; // ref-cxx11-error {{constant expression}} \
+                                        // ref-cxx11-note {{unequal pointers to void}}
   constexpr bool v6 = qv > null; // both-error {{must be initialized by a constant expression}} \
                                  // both-note {{comparison between '&s.b' and 'nullptr' has unspecified value}}
 

clang/test/CXX/drs/dr25xx.cpp

@@ -83,6 +83,7 @@ using ::cwg2521::operator""_div;
 #endif
 } // namespace cwg2521
 
+// cwg2526: sup 2749
 
 #if __cplusplus >= 202302L
 namespace cwg2553 { // cwg2553: 18 review 2023-07-14

clang/test/CXX/drs/dr26xx.cpp

@@ -238,3 +238,5 @@ void test() {
 }
 }
 #endif
+
+// cwg2696: dup 2749

clang/test/CXX/drs/dr27xx.cpp

@@ -10,6 +10,28 @@
 // expected-no-diagnostics
 #endif
 
+namespace cwg2749 { // cwg2749: 19
+#if __cplusplus >= 201703L
+
+constexpr bool f() {
+  int arr[2] {};
+  return (void*)arr < arr + 1;
+}
+static_assert(f());
+
+constexpr bool g() {
+  struct {
+    char c;
+    short s;
+    int i;
+  } s {};
+  return (void*)&s.c < &s.i;
+}
+static_assert(g());
+
+#endif
+}
+
 namespace cwg2759 { // cwg2759: 19
 #if __cplusplus >= 201103L
 

clang/test/CXX/expr/expr.const/p2-0x.cpp

@@ -571,18 +571,21 @@ namespace UnspecifiedRelations {
   // [expr.rel]p3: Pointers to void can be compared [...] if both pointers
   // represent the same address or are both the null pointer [...]; otherwise
   // the result is unspecified.
-  struct S { int a, b; } s;
+  struct S { int a, b; static int c; } s;
   constexpr void *null = 0;
-  constexpr void *pv = (void*)&s.a;
-  constexpr void *qv = (void*)&s.b;
+  constexpr void *av = (void*)&s.a;
+  constexpr void *bv = (void*)&s.b;
+  constexpr void *cv = (void*)&s.c;
   constexpr bool v1 = null < (int*)0;
-  constexpr bool v2 = null < pv; // expected-error {{constant expression}} expected-note {{comparison between 'nullptr' and '&s.a' has unspecified value}}
-  constexpr bool v3 = null == pv; // ok
-  constexpr bool v4 = qv == pv; // ok
-  constexpr bool v5 = qv >= pv; // expected-error {{constant expression}} expected-note {{unequal pointers to void}}
-  constexpr bool v6 = qv > null; // expected-error {{constant expression}} expected-note {{comparison between '&s.b' and 'nullptr' has unspecified value}}
-  constexpr bool v7 = qv <= (void*)&s.b; // ok
-  constexpr bool v8 = qv > (void*)&s.a; // expected-error {{constant expression}} expected-note {{unequal pointers to void}}
+  constexpr bool v2 = null < av; // expected-error {{constant expression}} expected-note {{comparison between 'nullptr' and '&s.a' has unspecified value}}
+  constexpr bool v3 = null == av; // ok
+  constexpr bool v4 = bv == av; // ok
+  constexpr bool v5 = bv >= av; // cxx11-error {{constant expression}} cxx11-note {{unequal pointers to void}}
+  constexpr bool v6 = bv > null; // expected-error {{constant expression}} expected-note {{comparison between '&s.b' and 'nullptr' has unspecified value}}
+  constexpr bool v7 = bv <= (void*)&s.b; // ok
+  constexpr bool v8 = bv > (void*)&s.a; // cxx11-error {{constant expression}} cxx11-note {{unequal pointers to void}}
+  constexpr bool v9 = cv > bv; // expected-error {{constant expression}} expected-note {{comparison between '&c' and '&s.b' has unspecified value}}
+  constexpr bool v10 = cv <= (void*)&s; // expected-error {{constant expression}} expected-note {{comparison between '&c' and '&s' has unspecified value}}
 }
 
 // - an assignment or a compound assignment (5.17); or

clang/www/cxx_dr_status.html

@@ -14964,7 +14964,7 @@ <h2 id="cxxdr">C++ defect report implementation status</h2>
     <td><a href="https://cplusplus.github.io/CWG/issues/2526.html">2526</a></td>
     <td>C++23</td>
     <td>Relational comparison of <TT>void*</TT> pointers</td>
-    <td class="unknown" align="center">Unknown</td>
+    <td class="unreleased" align="center">Superseded by <a href="#2749">2749</a></td>
   </tr>
   <tr id="2527">
     <td><a href="https://cplusplus.github.io/CWG/issues/2527.html">2527</a></td>
@@ -15984,7 +15984,7 @@ <h2 id="cxxdr">C++ defect report implementation status</h2>
     <td><a href="https://cplusplus.github.io/CWG/issues/2696.html">2696</a></td>
     <td>dup</td>
     <td>Relational comparisons of pointers to <TT>void</TT></td>
-    <td class="unknown" align="center">Unknown</td>
+    <td class="unreleased" align="center">Duplicate of <a href="#2749">2749</a></td>
   </tr>
   <tr id="2697">
     <td><a href="https://cplusplus.github.io/CWG/issues/2697.html">2697</a></td>
@@ -16302,7 +16302,7 @@ <h2 id="cxxdr">C++ defect report implementation status</h2>
     <td><a href="https://cplusplus.github.io/CWG/issues/2749.html">2749</a></td>
     <td>DR</td>
     <td>Treatment of "pointer to void" for relational comparisons</td>
-    <td class="unknown" align="center">Unknown</td>
+    <td class="unreleased" align="center">Clang 19</td>
   </tr>
   <tr id="2750">
     <td><a href="https://cplusplus.github.io/CWG/issues/2750.html">2750</a></td>