clang-win: toolset fails to initialize when only Visual Studio Build Tools is installed #620

clang-win.init locates Visual Studio with vswhere but does not pass -products *. On a machine with only Visual Studio Build Tools it gets an empty path and crashes in path.make. It neither configures the toolset nor reports a useful error.

Use case

boostorg/url runs its CI matrix through cpp-actions. For the clang-cl entry, the CMake build passes, and then the b2 step runs:

b2.exe -j 8 --toolset=clang-win address-model=64 cxxstd=20,23,latest variant=release warnings-as-errors=on embed-manifest-via=linker --abbreviate-paths libs/url/test

That step fails immediately in clang-win.init. Full log here (step "B2 Workflow", group "Build and Test"):

https://github.com/boostorg/url/actions/runs/38317560556/job/115038635318

This is from boostorg/url#1007. The runner is a C++ Alliance self-hosted windows-2025 machine with Visual Studio 2022 Build Tools only. The same log shows that. Every MSVC path is under C:\Program Files (x86)\Microsoft Visual Studio\2022\BuildTools\. The environment comes from BuildTools\Common7\Tools\VsDevCmd.bat ("Visual Studio 2022 Developer Command Prompt v17.14.39").

The identical step gets past clang-win.init on GitHub's hosted windows-2025 image, which has Visual Studio Enterprise installed (it later fails on an unrelated warning in our own code):

https://github.com/alandefreitas/url/actions/runs/38305599956/job/114999283406

The only difference between the two machines that matters here is which Visual Studio product they have.

Environment

• Boost.Build from the Boost superproject develop (2026-10-12)
• Windows Server 2025, Visual Studio 2022 Build Tools 17.14 only (no Community/Professional/Enterprise)
• LLVM 22 at C:\Program Files\LLVM\bin (clang-cl 22)
• Seen on the C++ Alliance self-hosted GitHub Actions runners (windows-2025-aws) in boostorg/url PR 1007, job "Clang-CL 22: C++20-23"

Steps to reproduce

On a machine with only Build Tools and LLVM installed:

b2 --toolset=clang-win address-model=64 cxxstd=20 variant=release libs/url/test

Actual result

tools/build/src/tools/clang-win.jam:148: in clang-win.init
error: rule path.make ( native )
error: called with: ( )
error: missing argument native

Expected result

The toolset initializes using the Build Tools install, as the msvc toolset already does with the same install. Failing that, it stops with a clear message such as "Cannot configure toolset clang-win: no Visual Studio installation with the VC tools found".

Cause

In src/tools/clang-win.jam, init runs:

vswhere -latest -requires Microsoft.VisualStudio.Component.VC.Tools.x86.x64 -property installationPath

Per the vswhere documentation, -products defaults to Community, Professional and Enterprise. vswhere lists Build Tools only when given -products *. On a Build Tools-only machine the result is empty, and the next line passes that empty value to path.make:

vspath = $(vspath[1]) ;
local msvcpath = [ path.glob [ path.make "$(vspath)\\VC\\Tools\\MSVC" ] : * ] ;

Suggested fix

1. Add -products * to the vswhere invocation.
2. Check vspath before calling path.make and raise errors.error with a readable message when it is empty.

Workaround

We're currently skipping the clang-win toolset on such machines. boostorg/url currently does this in its CI (if: matrix.compiler != 'clang-cl' on the b2 step) and relies on the CMake build for clang-cl coverage.