libstdc++: Fix constexpr functions to conform to older standards

Some constexpr functions were inadvertently relying on relaxed constexpr
rules from later standards.

libstdc++-v3/ChangeLog:

	* include/bits/chrono.h (duration_cast): Do not use braces
	around statements for C++11 constexpr rules.
	* include/bits/stl_algobase.h (__lg): Rewrite as a single
	statement for C++11 constexpr rules.
	* include/experimental/bits/fs_path.h (path::string): Use
	_GLIBCXX17_CONSTEXPR not _GLIBCXX_CONSTEXPR for 'if constexpr'.
	* include/std/charconv (__to_chars_8): Initialize variable for
	C++17 constexpr rules.

(cherry picked from commit b3a2b307b9deea719fb725a86df43b82176fe459)

4 files changed, 12 insertions, 13 deletions

diff --git a/libstdc++-v3/include/bits/chrono.h b/libstdc++-v3/include/bits/chrono.h
index f4573d27b03..0aeea0ae57e 100644
--- a/libstdc++-v3/include/bits/chrono.h
+++ b/libstdc++-v3/include/bits/chrono.h
@@ -276,8 +276,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	if constexpr (is_same_v<_ToDur, duration<_Rep, _Period>>)
 	  return __d;
 	else
+	  {
 #endif
-	{
 	  using __to_period = typename _ToDur::period;
 	  using __to_rep = typename _ToDur::rep;
 	  using __cf = ratio_divide<_Period, __to_period>;
@@ -285,7 +285,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	  using __dc = __duration_cast_impl<_ToDur, __cf, __cr,
 					    __cf::num == 1, __cf::den == 1>;
 	  return __dc::__cast(__d);
-	}
+#if __cpp_inline_variables && __cpp_if_constexpr
+	  }
+#endif
       }
 
     /** Trait indicating whether to treat a type as a floating-point type.
diff --git a/libstdc++-v3/include/bits/stl_algobase.h b/libstdc++-v3/include/bits/stl_algobase.h
index 4a6f8195d98..5a8faedf92a 100644
--- a/libstdc++-v3/include/bits/stl_algobase.h
+++ b/libstdc++-v3/include/bits/stl_algobase.h
@@ -1518,15 +1518,12 @@ _GLIBCXX_END_NAMESPACE_CONTAINER
       return std::__bit_width(make_unsigned_t<_Tp>(__n)) - 1;
 #else
       // Use +__n so it promotes to at least int.
-      const int __sz = sizeof(+__n);
-      int __w = __sz * __CHAR_BIT__ - 1;
-      if (__sz == sizeof(long long))
-	__w -= __builtin_clzll(+__n);
-      else if (__sz == sizeof(long))
-	__w -= __builtin_clzl(+__n);
-      else if (__sz == sizeof(int))
-	__w -= __builtin_clz(+__n);
-      return __w;
+      return (sizeof(+__n) * __CHAR_BIT__ - 1)
+	       - (sizeof(+__n) == sizeof(long long)
+		    ? __builtin_clzll(+__n)
+		    : (sizeof(+__n) == sizeof(long)
+			 ? __builtin_clzl(+__n)
+			 : __builtin_clz(+__n)));
 #endif
     }
 
diff --git a/libstdc++-v3/include/experimental/bits/fs_path.h b/libstdc++-v3/include/experimental/bits/fs_path.h
index e0e47188bb9..bf07754fd84 100644
--- a/libstdc++-v3/include/experimental/bits/fs_path.h
+++ b/libstdc++-v3/include/experimental/bits/fs_path.h
@@ -1042,7 +1042,7 @@ namespace __detail
     inline std::basic_string<_CharT, _Traits, _Allocator>
     path::string(const _Allocator& __a) const
     {
-      if _GLIBCXX_CONSTEXPR (is_same<_CharT, value_type>::value)
+      if _GLIBCXX17_CONSTEXPR (is_same<_CharT, value_type>::value)
 	return { _M_pathname.begin(), _M_pathname.end(), __a };
 
       using _WString = basic_string<_CharT, _Traits, _Allocator>;
diff --git a/libstdc++-v3/include/std/charconv b/libstdc++-v3/include/std/charconv
index b34d672f5bd..cf2b1161014 100644
--- a/libstdc++-v3/include/std/charconv
+++ b/libstdc++-v3/include/std/charconv
@@ -242,7 +242,7 @@ namespace __detail
       static_assert(__integer_to_chars_is_unsigned<_Tp>, "implementation bug");
 
       to_chars_result __res;
-      unsigned __len;
+      unsigned __len = 0;
 
       if _GLIBCXX17_CONSTEXPR (__gnu_cxx::__int_traits<_Tp>::__digits <= 16)
 	{