From 63fef6d33778f72592202a4938ee8f8bbac21951 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Tomasz=20Kami=C5=84ski?= <tkaminsk@redhat.com>
Date: Tue, 17 Mar 2026 09:34:37 +0100
Subject: [PATCH] libstdc++: Fix is_explict checks in extents test.
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The condition was checking if extents<int, ...> is convertible to index_type,
and not the reverse. This patch tests implicit conversion from braced
initializer list (to cover multiple arguments) and is_convertible in case of
single index.

libstdc++-v3/ChangeLog:

	* testsuite/23_containers/mdspan/extents/ctor_ints.cc: Updated
	is_explicit, and added more test cases.

Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
Signed-off-by: Tomasz Kamiński <tkaminsk@redhat.com>
---
 .../23_containers/mdspan/extents/ctor_ints.cc | 20 +++++++++++++++----
 1 file changed, 16 insertions(+), 4 deletions(-)

diff --git a/libstdc++-v3/testsuite/23_containers/mdspan/extents/ctor_ints.cc b/libstdc++-v3/testsuite/23_containers/mdspan/extents/ctor_ints.cc
index fdbcb707bbe..eefeab0e6a0 100644
--- a/libstdc++-v3/testsuite/23_containers/mdspan/extents/ctor_ints.cc
+++ b/libstdc++-v3/testsuite/23_containers/mdspan/extents/ctor_ints.cc
@@ -24,18 +24,30 @@ static_assert(std::is_constructible_v<std::extents<unsigned __int128, 1, 2>,
 #endif
 
 // No implicit conversion from integer-like objects.
-template<typename Extent, typename... OExtents>
-  constexpr bool
+template<typename ExtentsType, typename OIndex, typename... RIndicies>
+  consteval bool
   is_explicit()
   {
-    return std::is_nothrow_constructible_v<Extent, OExtents...>
-	   && !std::is_convertible_v<Extent, OExtents...>;
+    if (!std::is_nothrow_constructible_v<ExtentsType, OIndex, RIndicies...>)
+      return false;
+    if constexpr (sizeof...(RIndicies) == 0)
+      if (std::is_convertible_v<OIndex, ExtentsType>)
+	return false;
+
+    extern void testConv(ExtentsType);
+    return !requires (OIndex index, RIndicies... rindicies)
+      { testConv({index, rindicies...}); };
   }
 
 static_assert(is_explicit<std::extents<int, 1>, int>());
 static_assert(is_explicit<std::extents<int, 1>, unsigned int>());
 static_assert(is_explicit<std::extents<unsigned int, 1>, int>());
 
+static_assert(is_explicit<std::dextents<int, 1>, int>());
+static_assert(is_explicit<std::dextents<int, 2>, int, int>());
+static_assert(is_explicit<std::dextents<int, 3>, int, int, int>());
+
+	
 constexpr bool
 test_all()
 {