Update cxx standard; Add C++ latest for gcc; Update GCC tests

modules/d/actions/vcxproj.lua

@@ -456,7 +456,7 @@
 
 		if cfg.cppdialect and cfg.cppdialect ~= "Default" then
 			local cppMap = {
-				["C++latest"] = "c++17", -- TODO: keep this up to date >_<
+				["C++latest"] = "c++20",
 				["C++98"] = "c++98",
 				["C++0x"] = "c++11",
 				["C++11"] = "c++11",

modules/d/tools/dmd.lua

@@ -255,7 +255,7 @@
 			Context = "-checkaction=context",
 		},
 		cppdialect = {
-			["C++latest"] = "-extern-std=c++17", -- TODO: keep this up to date >_<
+			["C++latest"] = "-extern-std=c++20", -- TODO: keep this up to date >_<
 			["C++98"] = "-extern-std=c++98",
 			["C++0x"] = "-extern-std=c++11",
 			["C++11"] = "-extern-std=c++11",

src/tools/gcc.lua

@@ -212,6 +212,7 @@
 			["gnu++17"] = "-std=gnu++17",
 			["gnu++2a"] = "-std=gnu++2a",
 			["gnu++20"] = "-std=gnu++20",
+			["C++latest"] = "-std=c++20",
 		},
 		rtti = {
 			Off = "-fno-rtti"

tests/tools/test_gcc.lua

@@ -830,6 +830,13 @@
 		test.contains({ }, gcc.getcflags(cfg))
 	end
 
+	function suite.cxxflags_onCppLatest()
+		cppdialect "C++latest"
+		prepare()
+		test.contains({ "-std=c++20" }, gcc.getcxxflags(cfg))
+		test.contains({ }, gcc.getcflags(cfg))
+	end
+
 	function suite.cxxflags_onCppGnu98()
 		cppdialect "gnu++98"
 		prepare()