diff --git a/libcxx/test/std/input.output/filesystems/fs.op.funcs/fs.op.exists/exists.pass.cpp b/libcxx/test/std/input.output/filesystems/fs.op.funcs/fs.op.exists/exists.pass.cpp
--- a/libcxx/test/std/input.output/filesystems/fs.op.funcs/fs.op.exists/exists.pass.cpp
+++ b/libcxx/test/std/input.output/filesystems/fs.op.funcs/fs.op.exists/exists.pass.cpp
@@ -66,6 +66,17 @@
     const path p = static_env.DNE;
     TEST_CHECK(exists(p) == false);
 
+    TEST_CHECK(exists(static_env.Dir) == true);
+    TEST_CHECK(exists(static_env.Dir / "dne") == false);
+    // Whether <dir>/dne/.. is considered to exist or not is not necessarily
+    // something we need to define, but the platform specific behaviour
+    // does affect a few other tests, so clarify the root cause here.
+#ifdef _WIN32
+    TEST_CHECK(exists(static_env.Dir / "dne" / "..") == true);
+#else
+    TEST_CHECK(exists(static_env.Dir / "dne" / "..") == false);
+#endif
+
     std::error_code ec = GetTestEC();
     TEST_CHECK(exists(p, ec) == false);
     TEST_CHECK(!ec);
diff --git a/libcxx/test/std/input.output/filesystems/fs.op.funcs/fs.op.relative/relative.pass.cpp b/libcxx/test/std/input.output/filesystems/fs.op.funcs/fs.op.relative/relative.pass.cpp
--- a/libcxx/test/std/input.output/filesystems/fs.op.funcs/fs.op.relative/relative.pass.cpp
+++ b/libcxx/test/std/input.output/filesystems/fs.op.funcs/fs.op.relative/relative.pass.cpp
@@ -95,7 +95,17 @@
   static_test_env static_env;
   fs::path p(static_env.SymlinkToDir / "dir2/../dir2/DNE/..");
   const fs::path output = fs::weakly_canonical(p);
+  // weakly_canonical has a quirk - if the path is considered to exist,
+  // it's returned without a trailing slash, otherwise it's returned with
+  // one (see a note in fs.op.weakly_canonical/weakly_canonical.pass.cpp).
+  // On Windows, a path like existent/nonexistentsubdir/.. is considered
+  // to exist, on posix it's considered to not exist. Therefore, the
+  // result here differs in the trailing slash.
+#ifdef _WIN32
+  TEST_CHECK(output == fs::path::string_type(static_env.Dir2));
+#else
   TEST_CHECK(output == fs::path::string_type(static_env.Dir2 / ""));
+#endif
 }
 
 TEST_CASE(test_signature_10) {
diff --git a/libcxx/test/std/input.output/filesystems/fs.op.funcs/fs.op.weakly_canonical/weakly_canonical.pass.cpp b/libcxx/test/std/input.output/filesystems/fs.op.funcs/fs.op.weakly_canonical/weakly_canonical.pass.cpp
--- a/libcxx/test/std/input.output/filesystems/fs.op.funcs/fs.op.weakly_canonical/weakly_canonical.pass.cpp
+++ b/libcxx/test/std/input.output/filesystems/fs.op.funcs/fs.op.weakly_canonical/weakly_canonical.pass.cpp
@@ -46,12 +46,20 @@
       {static_env.Dir, static_env.Dir},
       {static_env.SymlinkToDir, static_env.Dir},
       {static_env.SymlinkToDir / "dir2/.", static_env.Dir / "dir2"},
-      // FIXME? If the trailing separator occurs in a part of the path that exists,
+      // Note: If the trailing separator occurs in a part of the path that exists,
       // it is omitted. Otherwise it is added to the end of the result.
+      // MS STL and libstdc++ behave similarly.
       {static_env.SymlinkToDir / "dir2/./", static_env.Dir / "dir2"},
       {static_env.SymlinkToDir / "dir2/DNE/./", static_env.Dir / "dir2/DNE/"},
       {static_env.SymlinkToDir / "dir2", static_env.Dir2},
+#ifdef _WIN32
+      // On Windows, this path is considered to exist (even though it
+      // passes through a nonexistent directory), and thus is returned
+      // without a trailing slash, see the note above.
+      {static_env.SymlinkToDir / "dir2/../dir2/DNE/..", static_env.Dir2},
+#else
       {static_env.SymlinkToDir / "dir2/../dir2/DNE/..", static_env.Dir2 / ""},
+#endif
       {static_env.SymlinkToDir / "dir2/dir3/../DNE/DNE2", static_env.Dir2 / "DNE/DNE2"},
       {static_env.Dir / "../dir1", static_env.Dir},
       {static_env.Dir / "./.", static_env.Dir},