diff --git a/src/ghc/filesystem.hpp b/src/ghc/filesystem.hpp
index 0705cf7f..4924c73c 100644
--- a/src/ghc/filesystem.hpp
+++ b/src/ghc/filesystem.hpp
@@ -301,7 +301,7 @@
 //- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
 
 // ghc::filesystem version in decimal (major * 10000 + minor * 100 + patch)
-#define GHC_FILESYSTEM_VERSION 10508L
+#define GHC_FILESYSTEM_VERSION 10509L
 
 #if !defined(GHC_WITH_EXCEPTIONS) && (defined(__EXCEPTIONS) || defined(__cpp_exceptions) || defined(_CPPUNWIND))
 #define GHC_WITH_EXCEPTIONS
@@ -4875,7 +4875,7 @@ GHC_INLINE space_info space(const path& p, std::error_code& ec) noexcept
         ec = detail::make_system_error();
         return {static_cast<uintmax_t>(-1), static_cast<uintmax_t>(-1), static_cast<uintmax_t>(-1)};
     }
-    return {static_cast<uintmax_t>(sfs.f_blocks * sfs.f_frsize), static_cast<uintmax_t>(sfs.f_bfree * sfs.f_frsize), static_cast<uintmax_t>(sfs.f_bavail * sfs.f_frsize)};
+    return {static_cast<uintmax_t>(sfs.f_blocks) * static_cast<uintmax_t>(sfs.f_frsize), static_cast<uintmax_t>(sfs.f_bfree) * static_cast<uintmax_t>(sfs.f_frsize), static_cast<uintmax_t>(sfs.f_bavail) * static_cast<uintmax_t>(sfs.f_frsize)};
 #endif
 }
 
diff --git a/src/nonstd/optional.hpp b/src/nonstd/optional.hpp
index 8b371e5a..2c9f1224 100644
--- a/src/nonstd/optional.hpp
+++ b/src/nonstd/optional.hpp
@@ -1,5 +1,5 @@
 //
-// Copyright (c) 2014-2018 Martin Moene
+// Copyright (c) 2014-2021 Martin Moene
 //
 // https://github.com/martinmoene/optional-lite
 //
@@ -12,7 +12,7 @@
 #define NONSTD_OPTIONAL_LITE_HPP
 
 #define optional_lite_MAJOR  3
-#define optional_lite_MINOR  4
+#define optional_lite_MINOR  5
 #define optional_lite_PATCH  0
 
 #define optional_lite_VERSION  optional_STRINGIFY(optional_lite_MAJOR) "." optional_STRINGIFY(optional_lite_MINOR) "." optional_STRINGIFY(optional_lite_PATCH)
@@ -47,7 +47,7 @@
 // Control presence of exception handling (try and auto discover):
 
 #ifndef optional_CONFIG_NO_EXCEPTIONS
-# if _MSC_VER
+# if defined(_MSC_VER)
 # include <cstddef>     // for _HAS_EXCEPTIONS
 # endif
 # if defined(__cpp_exceptions) || defined(__EXCEPTIONS) || (_HAS_EXCEPTIONS)
@@ -313,11 +313,11 @@ namespace nonstd {
 #define optional_CPP14_000  (optional_CPP14_OR_GREATER)
 #define optional_CPP17_000  (optional_CPP17_OR_GREATER)
 
-// gcc >= 4.9, msvc >= vc14.1 (vs17):
-#define optional_CPP11_140_G490     ((optional_CPP11_OR_GREATER_ && optional_COMPILER_GNUC_VERSION >= 490) || (optional_COMPILER_MSVC_VER >= 1910))
+// clang >= 2.9, gcc >= 4.9, msvc >= vc14.0/1900 (vs15):
+#define optional_CPP11_140_C290_G490    ((optional_CPP11_OR_GREATER_ && (optional_COMPILER_CLANG_VERSION >= 290 || optional_COMPILER_GNUC_VERSION >= 490)) || (optional_COMPILER_MSVC_VER >= 1900))
 
 // clang >= 3.5, msvc >= vc11 (vs12):
-#define optional_CPP11_110_C350     ( optional_CPP11_110 && !optional_BETWEEN( optional_COMPILER_CLANG_VERSION, 1, 350 ) )
+#define optional_CPP11_110_C350         ( optional_CPP11_110 && !optional_BETWEEN( optional_COMPILER_CLANG_VERSION, 1, 350 ) )
 
 // clang >= 3.5, gcc >= 5.0, msvc >= vc11 (vs12):
 #define optional_CPP11_110_C350_G500 \
@@ -331,7 +331,8 @@ namespace nonstd {
 #define optional_HAVE_IS_DEFAULT        optional_CPP11_140
 #define optional_HAVE_NOEXCEPT          optional_CPP11_140
 #define optional_HAVE_NULLPTR           optional_CPP11_100
-#define optional_HAVE_REF_QUALIFIER     optional_CPP11_140_G490
+#define optional_HAVE_REF_QUALIFIER     optional_CPP11_140_C290_G490
+#define optional_HAVE_STATIC_ASSERT     optional_CPP11_110
 #define optional_HAVE_INITIALIZER_LIST  optional_CPP11_140
 
 // Presence of C++14 language features:
@@ -405,6 +406,12 @@ namespace nonstd {
 # define optional_refref_qual  /*&&*/
 #endif
 
+#if optional_HAVE( STATIC_ASSERT )
+# define optional_static_assert(expr, text)    static_assert(expr, text);
+#else
+# define optional_static_assert(expr, text)  /*static_assert(expr, text);*/
+#endif
+
 // additional includes:
 
 #if optional_CONFIG_NO_EXCEPTIONS
@@ -917,6 +924,15 @@ public:
 template< typename T>
 class optional
 {
+    optional_static_assert(( !std::is_same<typename std::remove_cv<T>::type, nullopt_t>::value  ),
+        "T in optional<T> must not be of type 'nullopt_t'.")
+
+    optional_static_assert(( !std::is_same<typename std::remove_cv<T>::type, in_place_t>::value ),
+        "T in optional<T> must not be of type 'in_place_t'.")
+
+    optional_static_assert(( std::is_object<T>::value && std::is_destructible<T>::value && !std::is_array<T>::value ),
+        "T in optional<T> must meet the Cpp17Destructible requirements.")
+
 private:
     template< typename > friend class optional;
 
@@ -925,7 +941,7 @@ private:
 public:
     typedef T value_type;
 
-    // x.x.3.1, constructors
+     // x.x.3.1, constructors
 
     // 1a - default construct
     optional_constexpr optional() optional_noexcept
@@ -1090,7 +1106,7 @@ public:
     >
     optional_constexpr explicit optional( nonstd_lite_in_place_t(T), Args&&... args )
     : has_value_( true )
-    , contained( T( std::forward<Args>(args)...) )
+    , contained( in_place, std::forward<Args>(args)... )
     {}
 
     // 7 (C++11) - in-place construct,  initializer-list
@@ -1443,7 +1459,7 @@ public:
 
 #endif
 
-#if optional_CPP11_OR_GREATER
+#if optional_HAVE( REF_QUALIFIER )
 
     template< typename U >
     optional_constexpr value_type value_or( U && v ) const optional_ref_qual
@@ -1454,7 +1470,11 @@ public:
     template< typename U >
     optional_constexpr14 value_type value_or( U && v ) optional_refref_qual
     {
+#if optional_COMPILER_CLANG_VERSION
+        return has_value() ? /*std::move*/( contained.value() ) : static_cast<T>(std::forward<U>( v ) );
+#else
         return has_value() ? std::move( contained.value() ) : static_cast<T>(std::forward<U>( v ) );
+#endif
     }
 
 #else