Source: Enhance check for C++17 filesystem support

* Add `lexically_normal` test for all platforms.

* On Windows, MinGW does not currently handle `lexically_normal()`
  correctly on UNC path, but MSVC and IntelLLVM do--add a comment
  on this to avoid future confusion.

* Add test with `\\?\` notation and `weakly_canonical` that also triggers
  the MinGW bug, but is fine with MSVC and oneAPI, for a more robust and
  comprehensive test.

1 files changed, 9 insertions, 2 deletions

diff --git a/Source/Checks/cm_cxx_filesystem.cxx b/Source/Checks/cm_cxx_filesystem.cxx
index c8df589..bdc7c6d 100644
--- a/Source/Checks/cm_cxx_filesystem.cxx
+++ b/Source/Checks/cm_cxx_filesystem.cxx
@@ -9,12 +9,13 @@ int main()
   std::filesystem::path p0(L"/a/b/c");
 
   std::filesystem::path p1("/a/b/c");
-  std::filesystem::path p2("/a/b/c");
-  if (p1 != p2) {
+  std::filesystem::path p2("/a/b//c");
+  if (p1 != p2.lexically_normal()) {
     return 1;
   }
 
 #if defined(_WIN32)
+  // "//host/" is not preserved in some environments like GNU under MinGW.
   std::filesystem::path p3("//host/a/b/../c");
   if (p3.lexically_normal().generic_string() != "//host/a/c") {
     return 1;
@@ -24,6 +25,12 @@ int main()
   if (p4.lexically_normal().generic_string() != "c:/a/") {
     return 1;
   }
+
+  std::filesystem::path b1("C:\\path\\y\\..\\");
+  if (std::filesystem::weakly_canonical("\\\\?\\C:\\path\\x\\..") !=
+      b1.lexically_normal()) {
+    return 1;
+  }
 #endif
 
   // If std::string is copy-on-write, the std::filesystem::path