{"id":9205,"date":"2016-06-07T10:17:34","date_gmt":"2016-06-07T17:17:34","guid":{"rendered":"https:\/\/blogs.msdn.microsoft.com\/vcblog\/?p=9205"},"modified":"2019-02-18T18:04:47","modified_gmt":"2019-02-18T18:04:47","slug":"expression-sfinae-improvements-in-vs-2015-update-3","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/cppblog\/expression-sfinae-improvements-in-vs-2015-update-3\/","title":{"rendered":"Expression SFINAE improvements in VS 2017 RC"},"content":{"rendered":"

This post written by Xiang Fan, Andrew Pardoe, and Gabriel Dos Reis<\/em><\/p>\n

This post was updated to reflect progress we’ve made through VS 2017 RC since VS 2015 Update 3.<\/strong> <\/p>\n

Throughout the VS 2015 cycle we’ve been focusing on the quality of our expression SFINAE implementation. Because expression SFINAE issues can be subtle and complex we’ve been using popular libraries such as Boost<\/a> and Microsoft’s fork of Range-v3<\/a>\u00a0to validate our implementation and find remaining bugs. As we shift the compiler team’s focus to Visual Studio 2017 release we’re excited to tell you about the improvements we’ve made in correctly parsing expression SFINAE.<\/p>\n

With VS 2015 Update 3 we told you that Boost<\/a> compiles correctly with MSVC without defining the macro BOOST_NO_SFINAE_EXPR<\/code>. A few libraries in Boost still didn’t compile and Range-v3 support was incomplete. With Visual Studio 2017 Release Candidate Visual C++\u00a0can now compile Boost Spirit X3<\/a>, Sprout<\/a>, and Range-v3<\/a>. We’ve made fantastic progress so far and will soon have a complete and correct implementation of expression SFINAE.<\/p>\n

Our next focus is Boost Hana where over 70% of the tests pass so far with some source workarounds and some compiler fixes.We’re at the point where many of the bugs that we’re discovering aren’t actually expression SFINAE bugs. We’ve uncovered issues with constexpr<\/code>, generic lambdas, pack expansions, variadic macros, special member functions, parsing problems, and other issues. Some of these issues look like expression SFINAE issues but turn out to be SFINAE dependencies on other areas. We’ve fixed about 20 issues, three of which were SFINAE issues, and have about 35 left to fix in Hana. We look forward to Hana compiling cleanly without workarounds during the VS 2017 cycle.<\/p>\n

What is expression SFINAE?<\/h3>\n

SFINAE is an acronym for \u201cs<\/strong>ubstitution f<\/strong>ailure i<\/strong>s n<\/strong>ot a<\/strong>n e<\/strong>rror.\u201d It is derived from an arcane process used by C++ compilers during overload resolution.\u00a0 At its core, the idea is quite simple: if a candidate function template\u2019s specialization would lead to an ill-formed (compiler-internal) declaration of that specialization, just silently ignore that template as is if the compiler has never seen it<\/em><\/strong>. In another words, the compiler will pretend that wasn\u2019t the template it was looking for.\u00a0 It is an old notion that has been part of C++ since it\u2019s the C++98 release. In that version of C++, the condition for \u201cill-formed compiler-internal declaration of a function template specialization\u201d was specified for types<\/strong> only.<\/p>\n

With the advent of type query features such as decltype<\/code> and auto<\/code>, the validity of a function type now entails the validity of expressions, since whether decltype(expr)<\/code> is a valid type depends on whether the operand expr<\/code> is well-formed.\u00a0 Hence the term \u201cExpression SFINAE\u201d. \u00a0It is a little bit more involved because now the compiler has to perform overload resolution including potentially unbounded evaluation of constant expressions while it is checking whether a type makes sense.<\/p>\n

Improvements since Visual Studio 2015 Update 3<\/h3>\n

We now correctly compile code that constructs temporary objects as Range-v3 does extensively:<\/p>\n

\n\t\t#include <type_traits>\n\t\t\n\t\ttemplate<typename T, std::enable_if_t<std::is_integral<T>{}> * = nullptr>\n\t\tchar f(T *);\n\t\t\n\t\ttemplate<typename T>\n\t\tshort f(...);\n\t\t\n\t\tint main()\n\t\t{\n\t\t\tstatic_assert(sizeof(f<int>(nullptr)) == sizeof(char), "fail");\n\t\t\tstatic_assert(sizeof(f<int *>(nullptr)) == sizeof(short), "fail");\n\t\t}\n<\/pre>\n
We’ve also improved access checks for SFINAE which are illustrated in this code sample:<\/p>\n
\n\t\ttemplate <typename T> class S {\n\t\tprivate:\n\t\t\ttypedef T type;\n\t\t};\n\t\t\n\t\ttemplate <typename T> class S<T *> {\n\t\tpublic:\n\t\t\ttypedef T type;\n\t\t};\n\t\t\n\t\ttemplate <typename T, typename S<T>::type * = nullptr>\n\t\tchar f(T);\n\t\t\n\t\ttemplate<typename T>\n\t\tshort f(...);\n\t\t\n\t\tint main()\n\t\t{\n\t\t\tstatic_assert(sizeof(f<int>(0)) == 2, "fail"); \/\/ fails in VS2015\n\t\t\tstatic_assert(sizeof(f<int *>(nullptr)) == 1, "fail");\n\t\t}\n<\/pre>\n
Lastly, we’ve improved support for void_t<\/code> when used inside of a typename as found in Boost Hana:<\/p>\n
\n\t\ttemplate<typename T, typename U>\n\t\tstruct std_common_type {};\n\t\t\n\t\ttemplate<typename T>\n\t\tstruct std_common_type<T, T> { using type = T; };\n\t\t\n\t\ttemplate<typename T, typename U>\n\t\tstruct is_same { static const bool value = false; };\n\t\t\n\t\ttemplate<typename T>\n\t\tstruct is_same<T, T> { static const bool value = true; };\n\t\t\n\t\ttemplate<bool, typename T>\n\t\tstruct enable_if {};\n\t\t\n\t\ttemplate<typename T>\n\t\tstruct enable_if<true, T> { using type = T; };\n\t\t\n\t\ttemplate<typename...> using void_t = void;\n\t\t\n\t\ttemplate <typename T, typename U = T, typename = void>\n\t\tstruct EqualityComparable1 { static const bool value = false; };\n\t\t\n\t\ttemplate <typename T, typename U>\n\t\tstruct EqualityComparable1<T, U, typename enable_if<!is_same<T, U>::value, void_t<typename std_common_type<T, U>::type>>::type>\n\t\t{\n\t\t\tstatic const bool value = true;\n\t\t};\n\t\t\n\t\ttemplate <typename T, typename U = T, typename = void>\n\t\tstruct EqualityComparable2 { static const bool value = false; };\n\t\t\n\t\ttemplate <typename T, typename U>\n\t\tstruct EqualityComparable2<T, U, void_t<typename std_common_type<T, U>::type>>\n\t\t{\n\t\t\tstatic const bool value = true;\n\t\t};\n\t\t\n\t\tvoid f()\n\t\t{\n\t\t\tstruct S1 {};\n\t\t\tstruct S2 {};\n\t\t\tstatic_assert(!EqualityComparable1<S1, S2>::value, "fail"); \/\/ fails in VS2015\n\t\t\tstatic_assert(!EqualityComparable2<S1, S2>::value, "fail");\n\t\t}\n<\/pre>\n
Improvements since Visual Studio 2015 Update 2<\/h3>\n
Continued improvements in the quality of our expression SFINAE implementation enabled our Standard Template Library to begin using it in VS 2015 Update 2. Expression SFINAE is used in our implementations of std::function<\/code> and result_of<\/code>.<\/p>\n
Improvements since Visual Studio 2015 Update 1<\/h3>\n
Because we’re now generating parse trees for\u00a0decltype<\/code> expressions a number of patterns work correctly in Update 3.<\/p>\n