[gcc r12-8657] libstdc++: Support constexpr global std::string for size < 15 [PR105995]

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

https://gcc.gnu.org/g:e562236851e06091256593aa0d3fbda60a28e45b

commit r12-8657-ge562236851e06091256593aa0d3fbda60a28e45b
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Jun 16 14:57:32 2022 +0100

    libstdc++: Support constexpr global std::string for size < 15 [PR105995]
    
    I don't think this is required by the standard, but it's easy to
    support.
    
    libstdc++-v3/ChangeLog:
    
            PR libstdc++/105995
            * include/bits/basic_string.h (_M_use_local_data): Initialize
            the entire SSO buffer.
            * testsuite/21_strings/basic_string/cons/char/105995.cc: New test.
    
    (cherry picked from commit 98a0d72a610a87e8e383d366e50253ddcc9a51dd)

Diff:
---
 libstdc++-v3/include/bits/basic_string.h                      |  3 ++-
 .../testsuite/21_strings/basic_string/cons/char/105995.cc     | 11 +++++++++++
 2 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/libstdc++-v3/include/bits/basic_string.h b/libstdc++-v3/include/bits/basic_string.h
index 6041d05815b..9777deaf2e9 100644
--- a/libstdc++-v3/include/bits/basic_string.h
+++ b/libstdc++-v3/include/bits/basic_string.h
@@ -352,7 +352,8 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       {
 #if __cpp_lib_is_constant_evaluated
 	if (std::is_constant_evaluated())
-	  _M_local_buf[0] = _CharT();
+	  for (_CharT& __c : _M_local_buf)
+	    __c = _CharT();
 #endif
 	return _M_local_data();
       }
diff --git a/libstdc++-v3/testsuite/21_strings/basic_string/cons/char/105995.cc b/libstdc++-v3/testsuite/21_strings/basic_string/cons/char/105995.cc
new file mode 100644
index 00000000000..4764ceff72a
--- /dev/null
+++ b/libstdc++-v3/testsuite/21_strings/basic_string/cons/char/105995.cc
@@ -0,0 +1,11 @@
+// { dg-options "-std=gnu++20" }
+// { dg-do compile { target c++20 } }
+// { dg-require-effective-target cxx11_abi }
+
+#include <string>
+
+// PR libstdc++/105995
+// Not required by the standard, but supported for QoI.
+constexpr std::string pr105995_empty;
+constexpr std::string pr105995_partial = "0";
+constexpr std::string pr105995_full = "0123456789abcde";