[vcpkg] Visual Studio 2026 (v18) not recognized as a valid instance in Triplet x64-windows

[blouflashdb]

Description:
When trying to install dependencies via manifest mode in Visual Studio 2026 (v18.1.1), vcpkg fails to recognize the Visual Studio instance as "complete", even though the C++ workload (v145) is fully installed and the compiler (cl.exe) is functional via the Developer Command Prompt.

Environment:

• OS: Windows 11
• Visual Studio Version: 2026 (v18.1.1 Professional)
• Toolset: v145
• vcpkg version: (The one bundled with VS 2026)
• CMake version: 4.2.1 (System installed)

Steps to Reproduce:

1. Open a project with a vcpkg.json in Visual Studio 2026.
2. Ensure "Desktop development with C++" is installed.
3. Attempt to Build/Restore.

Observed Behavior:
vcpkg exits with the following error:

error: in Triplet x64-windows: Es wurde keine gültige Visual Studio-Instanz gefunden
Es konnte keine vollständige Visual Studio-Instanz gefunden werden.
Die folgenden Pfade wurden für Visual Studio-Instanzen untersucht:
  C:\Program Files\Microsoft SQL Server Management Studio 21\Release\VC\Auxiliary/Build\vcvarsall.bat
  C:\Program Files\Microsoft Visual Studio\18\Professional\VC\Auxiliary/Build\vcvarsall.bat

Diagnostic Logs (relevant snippet):
The debug log shows that vswhere.exe is called but vcpkg fails to identify the VS 18.0 instance as valid immediately after:

[DEBUG] 1010: CreateProcessW("C:\Program Files (x86)\Microsoft Visual Studio\Installer\vswhere.exe" -all -prerelease -legacy -products * -format xml)
[DEBUG] 1010: cmd_execute_and_stream_data() returned 0 after    47854 us
error: in Triplet x64-windows: Es wurde keine gültige Visual Studio-Instanz gefunden

Additional Context:

• cl.exe works fine in the Developer Command Prompt (Version 19.50.35721).
• Setting VCPKG_FORCE_SYSTEM_BINARIES=1 and using CMake 4.2.1 (which supports VS 2026) does not bypass the instance check.
• Setting VCPKG_VISUAL_STUDIO_PATH manually does not resolve the "instance not complete" error.