[libc++] Fix LWG 4265: std::midpoint should not accept const bool

[eiytoq]

Fixes: #171324

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@llvm/pr-subscribers-libcxx

Author: eiytoq (eiytoq)

Changes

Fixes: #171324

---

Full diff: https://github.com/llvm/llvm-project/pull/174579.diff

3 Files Affected:

• (modified) libcxx/docs/Status/Cxx2cIssues.csv (+1-1)
• (modified) libcxx/include/__numeric/midpoint.h (+3-2)
• (modified) libcxx/test/std/numerics/numeric.ops/numeric.ops.midpoint/midpoint.verify.cpp (+8-7)

diff --git a/libcxx/docs/Status/Cxx2cIssues.csv b/libcxx/docs/Status/Cxx2cIssues.csv
index 9445149a203a2..101a708b7b2de 100644
--- a/libcxx/docs/Status/Cxx2cIssues.csv
+++ b/libcxx/docs/Status/Cxx2cIssues.csv
@@ -164,7 +164,7 @@
 "`LWG4256 <https://wg21.link/LWG4256>`__","Incorrect constrains for ``function_ref`` constructors from ``nontype_t``","2025-11 (Kona)","","","`#171321 <https://github.com/llvm/llvm-project/issues/171321>`__",""
 "`LWG4257 <https://wg21.link/LWG4257>`__","Stream insertion for ``chrono::local_time`` should be constrained","2025-11 (Kona)","","","`#171322 <https://github.com/llvm/llvm-project/issues/171322>`__",""
 "`LWG4260 <https://wg21.link/LWG4260>`__","Query objects must be default constructible","2025-11 (Kona)","","","`#171323 <https://github.com/llvm/llvm-project/issues/171323>`__",""
-"`LWG4265 <https://wg21.link/LWG4265>`__","``std::midpoint`` should not accept ``const bool``","2025-11 (Kona)","","","`#171324 <https://github.com/llvm/llvm-project/issues/171324>`__",""
+"`LWG4265 <https://wg21.link/LWG4265>`__","``std::midpoint`` should not accept ``const bool``","2025-11 (Kona)","|Complete|","22","`#171324 <https://github.com/llvm/llvm-project/issues/171324>`__",""
 "`LWG4266 <https://wg21.link/LWG4266>`__","``layout_stride::mapping`` should treat empty mappings as exhaustive","2025-11 (Kona)","","","`#171325 <https://github.com/llvm/llvm-project/issues/171325>`__",""
 "`LWG4269 <https://wg21.link/LWG4269>`__","``unique_copy`` passes arguments to its predicate backwards","2025-11 (Kona)","","","`#171326 <https://github.com/llvm/llvm-project/issues/171326>`__",""
 "`LWG4272 <https://wg21.link/LWG4272>`__","For ``rank == 0``, ``layout_stride`` is atypically convertible","2025-11 (Kona)","","","`#171327 <https://github.com/llvm/llvm-project/issues/171327>`__",""
diff --git a/libcxx/include/__numeric/midpoint.h b/libcxx/include/__numeric/midpoint.h
index 2ba80e5cca07d..07ade9d5e1c55 100644
--- a/libcxx/include/__numeric/midpoint.h
+++ b/libcxx/include/__numeric/midpoint.h
@@ -35,8 +35,9 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 
 #if _LIBCPP_STD_VER >= 20
 template <class _Tp>
-_LIBCPP_HIDE_FROM_ABI constexpr enable_if_t<is_integral_v<_Tp> && !is_same_v<bool, _Tp> && !is_null_pointer_v<_Tp>, _Tp>
-midpoint(_Tp __a, _Tp __b) noexcept _LIBCPP_DISABLE_UBSAN_UNSIGNED_INTEGER_CHECK {
+_LIBCPP_HIDE_FROM_ABI constexpr enable_if_t<
+    is_integral_v<_Tp> && !is_same_v<bool, __remove_cv_t<_Tp>> && !is_null_pointer_v<_Tp>,
+    _Tp> midpoint(_Tp __a, _Tp __b) noexcept _LIBCPP_DISABLE_UBSAN_UNSIGNED_INTEGER_CHECK {
   using _Up                = make_unsigned_t<_Tp>;
   constexpr _Up __bitshift = numeric_limits<_Up>::digits - 1;
 
diff --git a/libcxx/test/std/numerics/numeric.ops/numeric.ops.midpoint/midpoint.verify.cpp b/libcxx/test/std/numerics/numeric.ops/numeric.ops.midpoint/midpoint.verify.cpp
index 7028e0f32945c..0218770ded4ed 100644
--- a/libcxx/test/std/numerics/numeric.ops/numeric.ops.midpoint/midpoint.verify.cpp
+++ b/libcxx/test/std/numerics/numeric.ops/numeric.ops.midpoint/midpoint.verify.cpp
@@ -27,13 +27,14 @@ void       *vp = nullptr;
 
 int main(int, char**)
 {
-    (void) std::midpoint(false, true);  // expected-error {{no matching function for call to 'midpoint'}}
+  (void)std::midpoint(false, true);             // expected-error {{no matching function for call to 'midpoint'}}
+  (void)std::midpoint<const bool>(false, true); // expected-error {{no matching function for call to 'midpoint'}}
 
-//  A couple of odd pointer types that should fail
-    (void) std::midpoint(nullptr, nullptr);  // expected-error {{no matching function for call to 'midpoint'}}
-    (void) std::midpoint(func1, func2);      // expected-error {{no matching function for call to 'midpoint'}}
-    (void) std::midpoint(ip, ip);            // expected-error {{no matching function for call to 'midpoint'}}
-    (void) std::midpoint(vp, vp);            // expected-error {{no matching function for call to 'midpoint'}}
+  //  A couple of odd pointer types that should fail
+  (void)std::midpoint(nullptr, nullptr); // expected-error {{no matching function for call to 'midpoint'}}
+  (void)std::midpoint(func1, func2);     // expected-error {{no matching function for call to 'midpoint'}}
+  (void)std::midpoint(ip, ip);           // expected-error {{no matching function for call to 'midpoint'}}
+  (void)std::midpoint(vp, vp);           // expected-error {{no matching function for call to 'midpoint'}}
 
-    return 0;
+  return 0;
 }

[H-G-Hristov]

I prepared a PR with my suggestions: #174596 which should be merged first eventually.

[eiytoq]

I prepared a PR with my suggestions: #174596 which should be merged first eventually.

Thanks for putting this together!

[eiytoq]

Superseded by #174596

[H-G-Hristov]

LGTM. Please wait for another maintainer to approve and let us know if you need help with landing this.

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[philnik777]

These are constraints, so they should really be SFINAE tests.

[eiytoq]

Thanks for the feedback. Since this PR is already merged, I will open a follow-up PR to convert these into proper SFINAE tests. I'll also include some general cleanup of midpoint.

[frederick-vs-ja]

It was pre-exiting to test constraints of midpoint in .verify.cpp. I think it would be clearer to leave the refactoring in a separated PR.