C++20 features detection

[xq114]

你在什么场景下需要该功能？

https://xmake.io/#/manual/extension_modules?id=compilerfeatures

现有的features detection缺少C++20的concepts、text format、modules等

描述可能的解决方案

添加c++20的features detection

[waruqi]

特性探测，我看没多少人用，所以一直没咋更新，如果你有兴趣，也可以帮忙改进下，其实也是从 cmake 那边 port 过来的。。

https://github.com/xmake-io/xmake/blob/master/xmake/modules/detect/tools/gcc/cxxfeatures.lua

[xq114]

https://en.cppreference.com/w/cpp/feature_test 似乎c++20之后直接用check_macro就能完成这个检查了

[waruqi]

那倒是省事了不少，就先用 check_macro 吧。。而且我看 cmake 的 cxxfeatures 也停留在 c++14 常年不更新了，感觉用的人不是很多。

[waruqi]

不过既然有现成的命名和宏，后面有时间放到 check_features 里面也是可以的，也就是个体力活。。毕竟 check_feature 只需要调用一遍编译器，check_macros 每次都要执行一遍。效率上来讲，check_feature 会快不少

[waruqi]

不对，都有现成宏了。。直接用不就完事了，为啥还要 check_macro 一遍再去定义个 HAS_XXX 宏。。这不有点多此一举么 = =

[xq114]

不对，都有现成宏了。。直接用不就完事了，为啥还要 check_macro 一遍再去定义个 HAS_XXX 宏。。这不有点多此一举么 = =

编译成库的时候就确定好符号，这样之后换高版本的编译器也能链接，头文件不随编译器变化

[waruqi]

c++17/20 的 我搞进去了。。

xmake/tests/apis/check_xxx/xmake.lua

Lines 45 to 93 in de3e0ce

	local features_cxx17 = {
	"cxx_aggregate_bases",
	"cxx_aligned_new",
	"cxx_capture_star_this",
	"cxx_constexpr",
	"cxx_deduction_guides",
	"cxx_enumerator_attributes",
	"cxx_fold_expressions",
	"cxx_guaranteed_copy_elision",
	"cxx_hex_float",
	"cxx_if_constexpr",
	"cxx_inheriting_constructors",
	"cxx_inline_variables",
	"cxx_namespace_attributes",
	"cxx_noexcept_function_type",
	"cxx_nontype_template_args",
	"cxx_nontype_template_parameter_auto",
	"cxx_range_based_for",
	"cxx_static_assert",
	"cxx_structured_bindings",
	"cxx_template_template_args",
	"cxx_variadic_using"}
	for _, feature in ipairs(features_cxx17) do
	check_features("HAS_17_" .. feature:upper(), feature, {languages = "c++17"})
	end
	local features_cxx20 = {
	"cxx_aggregate_paren_init",
	"cxx_char8_t",
	"cxx_concepts",
	"cxx_conditional_explicit",
	"cxx_consteval",
	"cxx_constexpr",
	"cxx_constexpr_dynamic_alloc",
	"cxx_constexpr_in_decltype",
	"cxx_constinit",
	"cxx_deduction_guides",
	"cxx_designated_initializers",
	"cxx_generic_lambdas",
	"cxx_impl_coroutine",
	"cxx_impl_destroying_delete",
	"cxx_impl_three_way_comparison",
	"cxx_init_captures",
	"cxx_modules",
	"cxx_nontype_template_args",
	"cxx_using_enum"}
	for _, feature in ipairs(features_cxx20) do
	check_features("HAS_20_" .. feature:upper(), feature, {languages = "c++20"})
	end