From 60fcd0db87747f8ae189f07dd7d2e1344b46c4ed Mon Sep 17 00:00:00 2001
From: Krystian Stasiowski <sdkrystian@gmail.com>
Date: Sat, 21 Dec 2019 19:06:19 -0500
Subject: [PATCH] Make data_impl constexpr for zero-size strings

---
 .../static_string/detail/static_string.hpp    | 21 +++++++++++--------
 test/static_string.cpp                        |  7 ++++---
 2 files changed, 16 insertions(+), 12 deletions(-)

diff --git a/include/boost/static_string/detail/static_string.hpp b/include/boost/static_string/detail/static_string.hpp
index c50ffa7..14b53e9 100644
--- a/include/boost/static_string/detail/static_string.hpp
+++ b/include/boost/static_string/detail/static_string.hpp
@@ -119,7 +119,7 @@ public:
 #ifdef BOOST_STATIC_STRING_ALLOW_UNINIT_MEM
   CharT data_[N + 1];
 #else
-  CharT data_[N + 1]{};
+  CharT data_[N + 1]{0};
 #endif
 };
 
@@ -129,17 +129,17 @@ class static_string_base_zero<0, CharT, Traits>
 {
 public:
   BOOST_STATIC_STRING_CPP11_CONSTEXPR
-  static_string_base_zero() noexcept {  }
+  static_string_base_zero() noexcept { }
 
   BOOST_STATIC_STRING_CPP11_CONSTEXPR
   static_string_base_zero(std::size_t) noexcept { }
 
-  // not possible to constexpr with the static there
+  // Modifying the null terminator is UB
+  BOOST_STATIC_STRING_CPP11_CONSTEXPR
   CharT*
   data_impl() const noexcept
   {
-    static CharT null{};
-    return &null;
+    return const_cast<CharT*>(&null_);
   }
 
   BOOST_STATIC_STRING_CPP11_CONSTEXPR
@@ -158,12 +158,15 @@ public:
 
   BOOST_STATIC_STRING_CPP14_CONSTEXPR
   void
-  term_impl() noexcept
-  {
+  term_impl() noexcept { }
 
-  }
+private:
+  static constexpr CharT null_{0};
 };
 
+template<typename CharT, typename Traits>
+constexpr CharT static_string_base_zero<0, CharT, Traits>::null_;
+
 // Optimization for storing the size in the last element
 template<std::size_t N, typename CharT, typename Traits>
 class static_string_base_null
@@ -213,7 +216,7 @@ public:
 #ifdef BOOST_STATIC_STRING_ALLOW_UNINIT_MEM
   CharT data_[N + 1];
 #else
-  CharT data_[N + 1]{};
+  CharT data_[N + 1]{0};
 #endif
 };
 
diff --git a/test/static_string.cpp b/test/static_string.cpp
index ecfbb82..f8b7164 100644
--- a/test/static_string.cpp
+++ b/test/static_string.cpp
@@ -6971,6 +6971,10 @@ constexpr bool testConstexpr()
     constexpr auto h = s2.back();
   }
 
+  static_string<0> a;
+
+  constexpr auto b = a.data();
+
   //constexpr auto g = s1.assign(1, '1');
   //constexpr auto h = s1.assign({'1'});
   //constexpr auto i = s1.assign("", 1);
@@ -7021,9 +7025,6 @@ runTests()
 
   testHash();
 
-  static_string<1>() + static_string<3>();
-  static_string<3>() + "ggggg";
-
   return report_errors();
 }