[gcc r10-10296] libstdc++: Do not use memset in constexpr calls to ranges::fill_n [PR101608]

[Jonathan Wakely <redi@gcc.gnu.org>]

https://gcc.gnu.org/g:d0ac292c6083fab4bad79a08d23533f537a885d4

commit r10-10296-gd0ac292c6083fab4bad79a08d23533f537a885d4
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Nov 24 13:17:54 2021 +0000

    libstdc++: Do not use memset in constexpr calls to ranges::fill_n [PR101608]
    
    libstdc++-v3/ChangeLog:
    
            PR libstdc++/101608
            * include/bits/ranges_algobase.h (__fill_n_fn): Check for
            constant evaluation before using memset.
            * testsuite/25_algorithms/fill_n/constrained.cc: Check
            byte-sized values as well.
    
    (cherry picked from commit 82c3657dd74896b39937bb0a2aaeba9b8ca105fd)

Diff:
---
 libstdc++-v3/include/bits/ranges_algobase.h        | 28 ++++++++++++++--------
 .../testsuite/25_algorithms/fill_n/constrained.cc  |  6 +++--
 2 files changed, 22 insertions(+), 12 deletions(-)

diff --git a/libstdc++-v3/include/bits/ranges_algobase.h b/libstdc++-v3/include/bits/ranges_algobase.h
index aef80a38586..2fb558918f8 100644
--- a/libstdc++-v3/include/bits/ranges_algobase.h
+++ b/libstdc++-v3/include/bits/ranges_algobase.h
@@ -526,17 +526,25 @@ namespace ranges
 	if (__n <= 0)
 	  return __first;
 
-	// TODO: Generalize this optimization to contiguous iterators.
-	if constexpr (is_pointer_v<_Out>
-		      // Note that __is_byte already implies !is_volatile.
-		      && __is_byte<remove_pointer_t<_Out>>::__value
-		      && integral<_Tp>)
-	  {
-	    __builtin_memset(__first, static_cast<unsigned char>(__value), __n);
-	    return __first + __n;
-	  }
-	else if constexpr (is_scalar_v<_Tp>)
+	if constexpr (is_scalar_v<_Tp>)
 	  {
+	    // TODO: Generalize this optimization to contiguous iterators.
+	    if constexpr (is_pointer_v<_Out>
+			  // Note that __is_byte already implies !is_volatile.
+			  && __is_byte<remove_pointer_t<_Out>>::__value
+			  && integral<_Tp>)
+	      {
+#ifdef __cpp_lib_is_constant_evaluated
+		if (!std::is_constant_evaluated())
+#endif
+		  {
+		    __builtin_memset(__first,
+				     static_cast<unsigned char>(__value),
+				     __n);
+		    return __first + __n;
+		  }
+	      }
+
 	    const auto __tmp = __value;
 	    for (; __n > 0; --__n, (void)++__first)
 	      *__first = __tmp;
diff --git a/libstdc++-v3/testsuite/25_algorithms/fill_n/constrained.cc b/libstdc++-v3/testsuite/25_algorithms/fill_n/constrained.cc
index e9ce8e8fb0a..09d3bd37630 100644
--- a/libstdc++-v3/testsuite/25_algorithms/fill_n/constrained.cc
+++ b/libstdc++-v3/testsuite/25_algorithms/fill_n/constrained.cc
@@ -73,11 +73,12 @@ test01()
     }
 }
 
+template<typename T>
 constexpr bool
 test02()
 {
   bool ok = true;
-  int x[6] = { 1, 2, 3, 4, 5, 6 };
+  T x[6] = { 1, 2, 3, 4, 5, 6 };
   const int y[6] = { 1, 2, 3, 4, 5, 6 };
   const int z[6] = { 17, 17, 17, 4, 5, 6 };
 
@@ -94,5 +95,6 @@ int
 main()
 {
   test01();
-  static_assert(test02());
+  static_assert(test02<int>());
+  static_assert(test02<unsigned char>()); // PR libstdc++/101608
 }