4 files changed, 31 insertions, 11 deletions
diff --git a/src/internal/filepathlite/path_windows.go b/src/internal/filepathlite/path_windows.go
index 8f34838a98..011baa96f0 100644
--- a/src/internal/filepathlite/path_windows.go
+++ b/src/internal/filepathlite/path_windows.go
@@ -7,6 +7,7 @@ package filepathlite
 import (
 	"internal/bytealg"
 	"internal/stringslite"
+	"internal/syscall/windows"
 	"syscall"
 )
 
@@ -114,13 +115,14 @@ func isReservedName(name string) bool {
 		return true
 	}
 	// The path element is a reserved name with an extension.
-	// Some Windows versions consider this a reserved name,
-	// while others do not. Use FullPath to see if the name is
-	// reserved.
-	if p, _ := syscall.FullPath(name); len(p) >= 4 && p[:4] == `\\.\` {
-		return true
+	// Since Windows 11, reserved names with extensions are no
+	// longer reserved. For example, "CON.txt" is a valid file
+	// name. Use RtlIsDosDeviceName_U to see if the name is reserved.
+	p, err := syscall.UTF16PtrFromString(name)
+	if err != nil {
+		return false
 	}
-	return false
+	return windows.RtlIsDosDeviceName_U(p) > 0
 }
 
 func isReservedBaseName(name string) bool {
@@ -297,11 +299,6 @@ func cutPath(path string) (before, after string, found bool) {
 	return path, "", false
 }
 
-// isUNC reports whether path is a UNC path.
-func isUNC(path string) bool {
-	return len(path) > 1 && IsPathSeparator(path[0]) && IsPathSeparator(path[1])
-}
-
 // postClean adjusts the results of Clean to avoid turning a relative path
 // into an absolute or rooted one.
 func postClean(out *lazybuf) {
diff --git a/src/internal/syscall/windows/syscall_windows.go b/src/internal/syscall/windows/syscall_windows.go
index c848f92d1f..e4d42f3dae 100644
--- a/src/internal/syscall/windows/syscall_windows.go
+++ b/src/internal/syscall/windows/syscall_windows.go
@@ -535,3 +535,4 @@ const (
 //sys   NtOpenFile(handle *syscall.Handle, access uint32, oa *OBJECT_ATTRIBUTES, iosb *IO_STATUS_BLOCK, share uint32, options uint32) (ntstatus error) = ntdll.NtOpenFile
 //sys   rtlNtStatusToDosErrorNoTeb(ntstatus NTStatus) (ret syscall.Errno) = ntdll.RtlNtStatusToDosErrorNoTeb
 //sys   NtSetInformationFile(handle syscall.Handle, iosb *IO_STATUS_BLOCK, inBuffer uintptr, inBufferLen uint32, class uint32) (ntstatus error) = ntdll.NtSetInformationFile
+//sys	RtlIsDosDeviceName_U(name *uint16) (ret uint32) = ntdll.RtlIsDosDeviceName_U
diff --git a/src/internal/syscall/windows/zsyscall_windows.go b/src/internal/syscall/windows/zsyscall_windows.go
index 6a6ea7bdc0..f7b89e9ca3 100644
--- a/src/internal/syscall/windows/zsyscall_windows.go
+++ b/src/internal/syscall/windows/zsyscall_windows.go
@@ -96,6 +96,7 @@ var (
 	procNtOpenFile                        = modntdll.NewProc("NtOpenFile")
 	procNtSetInformationFile              = modntdll.NewProc("NtSetInformationFile")
 	procRtlGetVersion                     = modntdll.NewProc("RtlGetVersion")
+	procRtlIsDosDeviceName_U              = modntdll.NewProc("RtlIsDosDeviceName_U")
 	procRtlNtStatusToDosErrorNoTeb        = modntdll.NewProc("RtlNtStatusToDosErrorNoTeb")
 	procGetProcessMemoryInfo              = modpsapi.NewProc("GetProcessMemoryInfo")
 	procCreateEnvironmentBlock            = moduserenv.NewProc("CreateEnvironmentBlock")
@@ -500,6 +501,12 @@ func rtlGetVersion(info *_OSVERSIONINFOW) {
 	return
 }
 
+func RtlIsDosDeviceName_U(name *uint16) (ret uint32) {
+	r0, _, _ := syscall.Syscall(procRtlIsDosDeviceName_U.Addr(), 1, uintptr(unsafe.Pointer(name)), 0, 0)
+	ret = uint32(r0)
+	return
+}
+
 func rtlNtStatusToDosErrorNoTeb(ntstatus NTStatus) (ret syscall.Errno) {
 	r0, _, _ := syscall.Syscall(procRtlNtStatusToDosErrorNoTeb.Addr(), 1, uintptr(ntstatus), 0, 0)
 	ret = syscall.Errno(r0)
diff --git a/src/path/filepath/path_test.go b/src/path/filepath/path_test.go
index e9cd82d6c5..7ea02a7c28 100644
--- a/src/path/filepath/path_test.go
+++ b/src/path/filepath/path_test.go
@@ -1889,3 +1889,18 @@ func TestEvalSymlinksTooManyLinks(t *testing.T) {
 		t.Fatal("expected error, got nil")
 	}
 }
+
+func BenchmarkIsLocal(b *testing.B) {
+	tests := islocaltests
+	if runtime.GOOS == "windows" {
+		tests = append(tests, winislocaltests...)
+	}
+	if runtime.GOOS == "plan9" {
+		tests = append(tests, plan9islocaltests...)
+	}
+	for b.Loop() {
+		for _, test := range tests {
+			filepath.IsLocal(test.path)
+		}
+	}
+}