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 82c3657)

Author: jwakely

libstdc++-v3/include/bits/ranges_algobase.h

@@ -525,17 +525,25 @@ 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;

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
 }