Improve MinGW toolchain support for Clang and libc++

This patch primarily enables support for LLVM libc++ runtimes (c++_static, c++_shared) in the mingw toolchain, and updates C++20 module rules so that `import std;` resolves to LLVM libc++ instead of hardcoded GNU libstdc++ when using Clang on MinGW (llvm-mingw), which caused build failures before.
Also, configuring for MinGW on Linux or macOS without a cross-compiler present would previously result in fallback to host-native /usr/bin/gcc. Now unprefixed fallback is only used on Windows/MSYS2/Cygwin hosts. The toolchain check now also automatically selects clang when pointing at llvm-mingw or a clang-only installation, avoiding the need to manually pass clang = true.

4 files changed, 30 insertions, 3 deletions

diff --git a/xmake/modules/detect/sdks/find_mingw.lua b/xmake/modules/detect/sdks/find_mingw.lua
index c70bebb11..d6d7e0d8e 100644
--- a/xmake/modules/detect/sdks/find_mingw.lua
+++ b/xmake/modules/detect/sdks/find_mingw.lua
@@ -108,7 +108,8 @@ function _find_mingw(sdkdir, opt)
 
     -- find cross toolchain
     local toolchain = find_cross_toolchain(sdkdir or bindir, {bindir = bindir, cross = cross})
-    if not toolchain then -- fallback, e.g. gcc.exe without cross
+    -- fallback, e.g. gcc.exe without cross
+    if not toolchain and (is_host("windows") or is_subhost("msys", "cygwin")) then
         toolchain = find_cross_toolchain(sdkdir or bindir, {bindir = bindir})
     end
     if toolchain then
diff --git a/xmake/rules/c++/modules/support.lua b/xmake/rules/c++/modules/support.lua
index 126e25c61..38df28a05 100644
--- a/xmake/rules/c++/modules/support.lua
+++ b/xmake/rules/c++/modules/support.lua
@@ -92,7 +92,15 @@ function get_cpplibrary_name(target)
         end
     elseif target:is_plat("macosx", "iphoneos", "watchos", "appletvos", "applexros", "bsd", "harmony") then
         return "c++"
-    elseif target:is_plat("linux", "mingw", "cygwin", "msys", "haiku") then
+    elseif target:is_plat("linux", "cygwin", "msys", "haiku") then
+        return "stdc++"
+    elseif target:is_plat("mingw") then
+        local toolchain_inst = target:toolchain("mingw")
+        local is_clang = (toolchain_inst and toolchain_inst:config("clang")) or
+            target:has_tool("cxx", "clang", "clangxx", "clang_cl")
+        if is_clang then
+            return "c++"
+        end
         return "stdc++"
     elseif target:is_plat("windows") then
         return "msstl"
diff --git a/xmake/toolchains/mingw/check.lua b/xmake/toolchains/mingw/check.lua
index 15cfe510a..104130fee 100644
--- a/xmake/toolchains/mingw/check.lua
+++ b/xmake/toolchains/mingw/check.lua
@@ -44,6 +44,24 @@ function main(toolchain)
         })
     end
     if mingw then
+        local bindir = mingw.bindir or path.join(mingw.sdkdir, "bin")
+        local cross = mingw.cross or ""
+        local is_win = is_host("windows")
+        local gcc = path.join(bindir, cross .. (is_win and "gcc.exe" or "gcc"))
+        local clang = path.join(bindir, cross .. (is_win and "clang.exe" or "clang"))
+        local use_clang = toolchain:config("clang")
+        if use_clang then
+            if not os.isexec(clang) then
+                return false
+            end
+        elseif use_clang == false then
+            if not os.isexec(gcc) then
+                return false
+            end
+        elseif (not os.isexec(gcc) or (mingw.sdkdir and mingw.sdkdir:find("llvm-mingw", 1, true)))
+            and os.isexec(clang) then
+            toolchain:config_set("clang", true)
+        end
         toolchain:config_set("mingw", mingw.sdkdir)
         toolchain:config_set("cross", mingw.cross)
         toolchain:config_set("bindir", mingw.bindir)
diff --git a/xmake/toolchains/mingw/xmake.lua b/xmake/toolchains/mingw/xmake.lua
index d0695dcfe..a75ab3cce 100644
--- a/xmake/toolchains/mingw/xmake.lua
+++ b/xmake/toolchains/mingw/xmake.lua
@@ -22,7 +22,7 @@ toolchain("mingw")
     set_kind("standalone")
     set_homepage("http://www.mingw.org/")
     set_description("Minimalist GNU for Windows")
-    set_runtimes("stdc++_static", "stdc++_shared")
+    set_runtimes("stdc++_static", "stdc++_shared", "c++_static", "c++_shared")
 
     on_check("check")
     on_load(function (toolchain)