[Clang] Warning as error Array Comparisons from C++26

clang/docs/ReleaseNotes.rst

@@ -428,6 +428,9 @@ New Compiler Flags
 - The ``-Warray-compare`` warning has been added to warn about array comparison
   on versions older than C++20.
 
+- The ``-Warray-compare-cxx26`` warning has been added to warn about array comparison
+  starting from C++26, this warning is enabled as an error by default.
+
 Deprecated Compiler Flags
 -------------------------
 

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -10274,6 +10274,11 @@ def warn_array_comparison : Warning<
   "to compare array addresses, use unary '+' to decay operands to pointers">,
   InGroup<DiagGroup<"array-compare">>;
 
+def warn_array_comparison_cxx26 : Warning<
+  "comparison between two arrays is ll-formed in C++26; "
+  "to compare array addresses, use unary '+' to decay operands to pointers">,
+  InGroup<DiagGroup<"array-compare-cxx26">>, DefaultError;
+
 def warn_stringcompare : Warning<
   "result of comparison against %select{a string literal|@encode}0 is "
   "unspecified (use an explicit string comparison function instead)">,

clang/lib/Sema/SemaExpr.cpp

@@ -11843,7 +11843,9 @@ static void diagnoseTautologicalComparison(Sema &S, SourceLocation Loc,
       RHSStripped->getType()->isArrayType()) {
     auto IsDeprArrayComparionIgnored =
         S.getDiagnostics().isIgnored(diag::warn_depr_array_comparison, Loc);
-    auto DiagID = !S.getLangOpts().CPlusPlus20 || IsDeprArrayComparionIgnored
+    auto DiagID = S.getLangOpts().CPlusPlus26
+                      ? diag::warn_array_comparison_cxx26
+                  : !S.getLangOpts().CPlusPlus20 || IsDeprArrayComparionIgnored
                       ? diag::warn_array_comparison
                       : diag::warn_depr_array_comparison;
     S.Diag(Loc, DiagID) << LHS->getSourceRange() << RHS->getSourceRange()

clang/test/Sema/warn-stringcompare.c

@@ -1,5 +1,6 @@
 // RUN: %clang_cc1 -x c -fsyntax-only -verify %s
 // RUN: %clang_cc1 -x c++ -fsyntax-only -verify=expected,cxx %s
+// RUN: %clang_cc1 -x c++ -std=c++26 -fsyntax-only -verify=expected,cxx-26 %s
 
 #define DELIM "/"
 #define DOT "."
@@ -15,15 +16,15 @@ void test(const char *d) {
   if (NULL == "/")
     return;
   if ("/" != DELIM) // expected-warning {{result of comparison against a string literal is unspecified (use an explicit string comparison function instead)}}
-    return;         // cxx-warning@-1 {{comparison between two arrays}}
+    return;         // cxx-warning@-1 {{comparison between two arrays}} cxx26-error@-1 {{comparison between two arrays is ll-formed in C++26}}
   if (DELIM == "/") // expected-warning {{result of comparison against a string literal is unspecified (use an explicit string comparison function instead)}}
-    return;         // cxx-warning@-1 {{comparison between two arrays}}
+    return;         // cxx-warning@-1 {{comparison between two arrays}} cxx26-error@-1 {{comparison between two arrays is ll-formed in C++26}}
   if (DELIM != NULL)
     return;
   if (NULL == DELIM)
     return;
   if (DOT != DELIM) // expected-warning {{result of comparison against a string literal is unspecified (use an explicit string comparison function instead)}}
-    return;         // cxx-warning@-1 {{comparison between two arrays}}
+    return;         // cxx-warning@-1 {{comparison between two arrays}} cxx26-error@-1 {{comparison between two arrays is ll-formed in C++26}}
   if (DELIM == DOT) // expected-warning {{result of comparison against a string literal is unspecified (use an explicit string comparison function instead)}}
-    return;         // cxx-warning@-1 {{comparison between two arrays}}
+    return;         // cxx-warning@-1 {{comparison between two arrays}} cxx26-error@-1 {{comparison between two arrays is ll-formed in C++26}}
 }

clang/test/SemaCXX/warn-array-comparion.cpp

@@ -1,6 +1,7 @@
 // RUN: %clang_cc1 -std=c++17 -fsyntax-only -verify %s -verify=expected,not-cxx20
 // RUN: %clang_cc1 -std=c++20 -fsyntax-only -Wno-deprecated-array-compare -verify %s -verify=expected,not-cxx20
 // RUN: %clang_cc1 -std=c++20 -fsyntax-only -Wdeprecated -verify %s -verify=expected,cxx20
+// RUN: %clang_cc1 -std=c++26 -fsyntax-only -Wdeprecated -verify %s -verify=expected,cxx26
 
 typedef struct {
   char str[16];
@@ -9,13 +10,14 @@ typedef struct {
 
 bool object_equal(const Object &obj1, const Object &obj2) {
   if (obj1.str != obj2.str) // not-cxx20-warning {{comparison between two arrays compare their addresses}} cxx20-warning {{comparison between two arrays is deprecated}}
-    return false;
+    return false;           // cxx26-error@-1 {{comparison between two arrays is ll-formed in C++26}}
   if (obj1.id != obj2.id) // not-cxx20-warning {{comparison between two arrays compare their addresses}} cxx20-warning {{comparison between two arrays is deprecated}}
-    return false;
+    return false;         // cxx26-error@-1 {{comparison between two arrays is ll-formed in C++26}}
   return true;
 }
 
 
 void foo(int (&array1)[2], int (&array2)[2]) {
   if (array1 == array2) { } // not-cxx20-warning {{comparison between two arrays compare their addresses}} cxx20-warning {{comparison between two arrays is deprecated}}
+                            // cxx26-error@-1 {{comparison between two arrays is ll-formed in C++26}}
 }

clang/test/SemaCXX/warn-self-comparisons.cpp

@@ -1,6 +1,8 @@
 // RUN: %clang_cc1 -std=c++17 -fsyntax-only -verify %s -verify=expected,not-cxx20
 // RUN: %clang_cc1 -std=c++20 -fsyntax-only -Wdeprecated -verify %s -verify=expected,cxx20
+// RUN: %clang_cc1 -std=c++26 -fsyntax-only -Wdeprecated -verify %s -verify=expected,cxx26
 
 void f(int (&array1)[2], int (&array2)[2]) {
   if (array1 == array2) { } // not-cxx20-warning {{comparison between two arrays compare their addresses}} cxx20-warning {{comparison between two arrays is deprecated}}
+                            // cxx26-error@-1 {{comparison between two arrays is ll-formed in C++26}}
 }

clang/www/cxx_status.html

@@ -239,7 +239,7 @@ <h2 id="cxx26">C++2c implementation status</h2>
  <tr>
   <td>Remove Deprecated Array Comparisons from C++26</td>
   <td><a href="https://wg21.link/P2865R6">P2865R6</a></td>
-  <td class="none" align="center">No</td>
+  <td class="unreleased" align="center">Clang 20</td>
  </tr>
  <tr>
   <td>Structured Bindings can introduce a Pack</td>