From 00bca89bfc4220191997de6c61225e90c5330225 Mon Sep 17 00:00:00 2001
From: Alexander Medvednikov <alexander@medvednikov.com>
Date: Tue, 21 Apr 2026 06:17:21 +0300
Subject: [PATCH] os: fix prebuild release and manual installation crash on
 windows (fixes #19466)

---
 v_make.bat                 |  4 +++-
 vlib/os/filepath_test.v    | 10 ++++++++++
 vlib/os/filepath_windows.v | 20 ++++++++++++++++++++
 vlib/os/os.c.v             |  2 +-
 4 files changed, 34 insertions(+), 2 deletions(-)

diff --git a/v_make.bat b/v_make.bat
index 489b165c4..76685a9f9 100644
--- a/v_make.bat
+++ b/v_make.bat
@@ -16,6 +16,8 @@ set V_BOOTSTRAP=./v_win_bootstrap.exe
 set V_OLD=./v_old.exe
 set V_UPDATED=./v_up.exe
 set V_C_FILE=./vc/v_win.c
+set where_exe=where.exe
+if not ["%SystemRoot%"] == [""] if exist "%SystemRoot%\System32\where.exe" set where_exe=%SystemRoot%\System32\where.exe
 
 REM TCC variables
 set tcc_url=https://github.com/vlang/tccbin
@@ -150,7 +152,7 @@ call :move_updated_to_v
 goto :success
 
 :clang_strap
-where /q clang
+"%where_exe%" /q clang
 if %ERRORLEVEL% NEQ 0 (
 	echo  ^> Clang not found
 	if not [!compiler!] == [] goto :error
diff --git a/vlib/os/filepath_test.v b/vlib/os/filepath_test.v
index 1769739de..ddf0eb93f 100644
--- a/vlib/os/filepath_test.v
+++ b/vlib/os/filepath_test.v
@@ -185,3 +185,13 @@ fn test_windows_volume() {
 		assert windows_volume('/') == ''
 	}
 }
+
+fn test_trim_extended_length_path_prefix() {
+	$if windows {
+		assert trim_extended_length_path_prefix(r'\\?\C:\path\to\file.v') == r'C:\path\to\file.v'
+		assert trim_extended_length_path_prefix(r'\\?\UNC\host\share\file.v') == r'\\host\share\file.v'
+		assert trim_extended_length_path_prefix(r'\\?\Volume{01234567-89ab-cdef-0123-456789abcdef}\file.v') == r'\\?\Volume{01234567-89ab-cdef-0123-456789abcdef}\file.v'
+		assert trim_extended_length_path_prefix(r'C:\path\to\file.v') == r'C:\path\to\file.v'
+		assert trim_extended_length_path_prefix('') == ''
+	}
+}
diff --git a/vlib/os/filepath_windows.v b/vlib/os/filepath_windows.v
index 776c8f3bc..fd6173be2 100644
--- a/vlib/os/filepath_windows.v
+++ b/vlib/os/filepath_windows.v
@@ -15,3 +15,23 @@ pub fn windows_volume(path string) string {
 	}
 	return path[..volume_len]
 }
+
+// trim_extended_length_path_prefix converts Win32 extended-length DOS/UNC
+// paths returned by APIs like GetFinalPathNameByHandleW into regular paths.
+// Paths that are not standard DOS or UNC paths are left unchanged.
+fn trim_extended_length_path_prefix(path string) string {
+	$if !windows {
+		return path
+	}
+	if path.len < 4 || !starts_w_slash_slash(path) || path[2] != qmark || !is_slash(path[3]) {
+		return path
+	}
+	if path.len >= 8 && (path[4] == `U` || path[4] == `u`) && (path[5] == `N` || path[5] == `n`)
+		&& (path[6] == `C` || path[6] == `c`) && is_slash(path[7]) {
+		return '\\\\' + path[8..]
+	}
+	if path.len >= 7 && path[4].is_letter() && path[5] == `:` && is_slash(path[6]) {
+		return path[4..]
+	}
+	return path
+}
diff --git a/vlib/os/os.c.v b/vlib/os/os.c.v
index 13ffd4d0e..d496a604c 100644
--- a/vlib/os/os.c.v
+++ b/vlib/os/os.c.v
@@ -974,7 +974,7 @@ pub fn real_path(fpath string) string {
 				return fpath.clone()
 			}
 			unsafe { res.free() }
-			res = unsafe { string_from_wide(pu16_fullpath) }
+			res = normalize_windows_extended_path_prefix(unsafe { string_from_wide(pu16_fullpath) })
 		}
 	} $else {
 		ret := &char(C.realpath(&char(fpath.str), &char(&fullpath[0])))
-- 
2.39.5