[Clang] Warning as error Array Comparisons from C++26 #118872

AmrDeveloper · 2024-12-05T21:28:36Z

Starting from C++26 the array comparison warning should converted to an error.

Fixes: #117859

Starting from C++26 the array comparison warning should converted to an error.

llvmbot · 2024-12-05T21:29:15Z

@llvm/pr-subscribers-clang-tools-extra
@llvm/pr-subscribers-clang-tidy

@llvm/pr-subscribers-clang

Author: Amr Hesham (AmrDeveloper)

Changes

Starting from C++26 the array comparison warning should converted to an error.

Fixes: #117859

Full diff: https://github.com/llvm/llvm-project/pull/118872.diff

7 Files Affected:

(modified) clang/docs/ReleaseNotes.rst (+3)
(modified) clang/include/clang/Basic/DiagnosticSemaKinds.td (+5)
(modified) clang/lib/Sema/SemaExpr.cpp (+3-1)
(modified) clang/test/Sema/warn-stringcompare.c (+5-4)
(modified) clang/test/SemaCXX/warn-array-comparion.cpp (+4-2)
(modified) clang/test/SemaCXX/warn-self-comparisons.cpp (+2)
(modified) clang/www/cxx_status.html (+1-1)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index ad5ef2119d3654..6719cf42970ba4 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/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 warn is error by default.
+
 Deprecated Compiler Flags
 -------------------------
 
diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
index 447358f0a5f382..0bf9303ba50f45 100644
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/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 compare their addresses not their contents; "
+  "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)">,
diff --git a/clang/lib/Sema/SemaExpr.cpp b/clang/lib/Sema/SemaExpr.cpp
index 4ffce2c1236610..14564b99de44c5 100644
--- a/clang/lib/Sema/SemaExpr.cpp
+++ b/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()
diff --git a/clang/test/Sema/warn-stringcompare.c b/clang/test/Sema/warn-stringcompare.c
index 78145cf42578a4..93c2bc31fea0a3 100644
--- a/clang/test/Sema/warn-stringcompare.c
+++ b/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 compare their addresses not their contents}}
   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 compare their addresses not their contents}}
   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 compare their addresses not their contents}}
   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 compare their addresses not their contents}}
 }
diff --git a/clang/test/SemaCXX/warn-array-comparion.cpp b/clang/test/SemaCXX/warn-array-comparion.cpp
index a6eaaab22fc16d..87b5c4fb91e3d7 100644
--- a/clang/test/SemaCXX/warn-array-comparion.cpp
+++ b/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 compare their addresses not their contents}}
   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 compare their addresses not their contents}}
   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 compare their addresses not their contents}}
 }
diff --git a/clang/test/SemaCXX/warn-self-comparisons.cpp b/clang/test/SemaCXX/warn-self-comparisons.cpp
index 3847c2d918bf61..c7bb5fcc8fd00d 100644
--- a/clang/test/SemaCXX/warn-self-comparisons.cpp
+++ b/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 compare their addresses not their contents}}
 }
diff --git a/clang/www/cxx_status.html b/clang/www/cxx_status.html
index fdb9807b1168c7..839b66e7a082d3 100755
--- a/clang/www/cxx_status.html
+++ b/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="none" align="center">Clang 20</td>
  </tr>
  <tr>
   <td>Structured Bindings can introduce a Pack</td>

erichkeane · 2024-12-05T21:52:45Z

clang/docs/ReleaseNotes.rst

  on versions older than C++20.

+- The ``-Warray-compare-cxx26`` warning has been added to warn about array comparison
+  starting from C++26, this warn is error by default.


Suggested change

starting from C++26, this warn is error by default.

starting from C++26, this warning is enabled as an error by default.

erichkeane · 2024-12-05T21:53:33Z

clang/include/clang/Basic/DiagnosticSemaKinds.td

  InGroup<DiagGroup<"array-compare">>;

+def warn_array_comparison_cxx26 : Warning<
+  "comparison between two arrays compare their addresses not their contents; "


I wonder if this should just say 'is ill formed in C++26' or something instead of just repeating the same message?

I agree, "comparison between two arrays is ll-formed in C++26; " will be better in this case

I would prefer "not allowed" instead of "ill-formed".

Example:

llvm-project/clang/include/clang/Basic/DiagnosticSemaKinds.td

Lines 665 to 667 in 02ad623

def err_access_decl : Error<

"ISO C++11 does not allow access declarations; "

"use using declarations instead">;

I found that we have diagnostics with 'not allowed' and 'ill-formed' in the codebase, I am okay with both, and we can agree on opinion.

What do you think the best option here?

@erichkeane @cor3ntin @zwuis

I don't think it matters a lot. ill formed conveys it's a standard requirement pretty well

zwuis · 2024-12-06T04:02:48Z

clang/www/cxx_status.html

  <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="none" align="center">Clang 20</td>


Suggested change

<td class="none" align="center">Clang 20</td>

<td class="unreleased" align="center">Clang 20</td>

Done, thanks

erichkeane

This seems fine to me. Please give folks ~24 hrs to chime in before merging.

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

30eb50c

Starting from C++26 the array comparison warning should converted to an error.

AmrDeveloper requested review from AaronBallman, cor3ntin, erichkeane and shafik December 5, 2024 21:28

llvmbot added clang Clang issues not falling into any other category clang:frontend Language frontend issues, e.g. anything involving "Sema" labels Dec 5, 2024

erichkeane reviewed Dec 5, 2024

View reviewed changes

zwuis reviewed Dec 6, 2024

View reviewed changes

AmrDeveloper added 2 commits December 6, 2024 18:39

Modify release note and CXX status

f91c1a3

Update C++26 Diagnostic message

c3b15ca

erichkeane approved these changes Dec 6, 2024

View reviewed changes

AmrDeveloper added 3 commits December 6, 2024 19:29

Merge branch 'llvm:main' into cpp26_dep_array_compare_werr

a36a3c4

Fix the name of verify prefix

14624e0

Disable checking for array-comparison C++26 in clang tidy check

62b4192

llvmbot added clang-tools-extra clang-tidy labels Dec 6, 2024

cor3ntin approved these changes Dec 7, 2024

View reviewed changes

Fix typo in diagnostic message

5280439

AmrDeveloper merged commit 3f458cd into llvm:main Dec 7, 2024
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[Clang] Warning as error Array Comparisons from C++26 #118872

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	starting from C++26, this warn is error by default.
	starting from C++26, this warning is enabled as an error by default.

	def err_access_decl : Error<
	"ISO C++11 does not allow access declarations; "
	"use using declarations instead">;

	<td class="none" align="center">Clang 20</td>
	<td class="unreleased" align="center">Clang 20</td>

[Clang] Warning as error Array Comparisons from C++26 #118872

[Clang] Warning as error Array Comparisons from C++26 #118872

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