`py -V:X.Y script.py` fails with "No suitable Python runtime found" when script arguments contain forward slashes

Summary

When using the Python Launcher (py.exe) with a version flag (-V:X.Y), any script argument containing a forward slash (e.g. ./output, /tmp, C:/foo) causes exit code 103 ("No suitable Python runtime found"), even though the specified Python version is installed.

Reproduction

# test_simple.py
import sys
print('ran ok, argv:', sys.argv[1:])

py -V:3.14 test_simple.py files      # OK
py -V:3.14 test_simple.py ./files    # FAIL: "No suitable Python runtime found"
py -V:3.14 test_simple.py --help     # OK  (no slash)
py -V:3.14 test_simple.py .\files    # OK  (backslash is fine)
py -V:3.14 test_simple.py /files     # FAIL
py -V:3.14 test_simple.py C:/files   # FAIL

py test_simple.py ./files            # OK  (no -V: flag = unaffected)

Root cause

In PC/launcher2.c, parseCommandLine() parses -V:company/tag as follows:

const wchar_t *argStart = wcschr(arg, L':') + 1;   // points past ':' in "-V:3.14"
const wchar_t *tagStart = wcschr(argStart, L'/');   // BUG: scans entire remaining cmdline

arg points into the full command-line string (from GetCommandLineW), not just the current token. tail (the whitespace-bounded end of the -V: token) is computed earlier but is not used to bound the wcschr search.

For py -V:3.14 script.py ./files, argStart is "3.14 script.py ./files...". The unbounded wcschr finds the / in ./files, and misparses:

company = "3.14 script.py ."   (should be empty)
tag     = "files"               (should be "3.14")

selectEnvironment then finds no Python matching this bogus company/tag and returns RC_NO_PYTHON.

Fix

Limit the / search to within tail:

const wchar_t *argStart = wcschr(arg, L':') + 1;
// Search for '/' only within the current token boundary
const wchar_t *tagStart = NULL;
for (const wchar_t *p = argStart; p < tail; ++p) {
    if (*p == L'/') { tagStart = p; break; }
}

Environment

• Python 3.14.4 (Windows 11, version 25H2)
• py.exe version: 3.14.4 (C:\Windows\py.EXE)
• Also present in launcher2.c on main and 3.13 branches (same code)

Bắt đầu trong PC/launcher2.c tại parseCommandLine(), sau đó tái hiện vấn đề bằng các lệnh py -V:3.14 được liệt kê và test_simple.py trên Windows. Hoàn tất có nghĩa là các đối số script chứa dấu gạch chéo không còn tạo ra “No suitable Python runtime found” khi sử dụng cờ phiên bản, đồng thời việc phân tích company/tag hiện có vẫn hoạt động.