[committed] libstdc++: Fix std::__uninitialized_default_n for constant evaluation [PR110542]

[committed] libstdc++: Fix std::__uninitialized_default_n for constant evaluation [PR110542]

[Jonathan Wakely]

This is a weird one as described in the PR. GCC doesn't complain about
this bug, but seems to have a bogus error elsewhere. I'll add a testcase
once I've understood/fixed the GCC error.

Tested x86_64-linux. Pushed to trunk.

-- >8 --

libstdc++-v3/ChangeLog:

	PR libstdc++/110542
	* include/bits/stl_uninitialized.h (__uninitialized_default_n):
	Do not use std::fill_n during constant evaluation.
---
 libstdc++-v3/include/bits/stl_uninitialized.h | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/libstdc++-v3/include/bits/stl_uninitialized.h b/libstdc++-v3/include/bits/stl_uninitialized.h
index 6752c6b5746..be7b4afdd05 100644
--- a/libstdc++-v3/include/bits/stl_uninitialized.h
+++ b/libstdc++-v3/include/bits/stl_uninitialized.h
@@ -695,6 +695,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     inline _ForwardIterator
     __uninitialized_default_n(_ForwardIterator __first, _Size __n)
     {
+#ifdef __cpp_lib_is_constant_evaluated
+      if (std::is_constant_evaluated())
+	return __uninitialized_default_n_1<false>::
+		 __uninit_default_n(__first, __n);
+#endif
+
       typedef typename iterator_traits<_ForwardIterator>::value_type
 	_ValueType;
       // See uninitialized_fill_n for the conditions for using std::fill_n.
-- 
2.41.0