Visual Studio: ToolsVersion vs PlatformToolset. Do they need to correspond?

Question

The MSDN documentation says that you need to use the Runtime version that correpsonds to the Platform Toolset version, hence it leads to UB otherwise.
I have a legacy project that was migrated to Visual Studio 2019, and has a <PlatformToolset>v142</PlatformToolset> but <Project DefaultTargets="Build" ToolsVersion="4.0" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">.
I know that there should be ToolsVersion="Current" or ToolsVersion="16.0" which is generated by CMake.
I get no warnings from Visual Studio when building the project, but I am concerned if specifing incompatible versions will lead to linking the wrong CRT version.

When I build with /MD it seems that everything is ok:

  Image has the following dependencies:

    msi.dll
    MSVCP140D.dll
    VCRUNTIME140D.dll
    VCRUNTIME140_1D.dll
    ucrtbased.dll
    KERNEL32.dll

But the legacy project uses /MT and I can't determine which CRT is used from the dumpbin output for .obj file:

   Linker Directives
   -----------------
   /FAILIFMISMATCH:_MSC_VER=1900
   /FAILIFMISMATCH:_ITERATOR_DEBUG_LEVEL=2
   /FAILIFMISMATCH:RuntimeLibrary=MTd_StaticDebug
   /DEFAULTLIB:libcpmtd
   /FAILIFMISMATCH:_CRT_STDIO_ISO_WIDE_SPECIFIERS=0
   /DEFAULTLIB:uuid.lib
   /DEFAULTLIB:uuid.lib
   /DEFAULTLIB:LIBCMTD
   /DEFAULTLIB:OLDNAMES

Is ToolsVersion even meaningfull and can it lead to linking the wrong CRT version?
How can I inspect which version of Runtime is linked when compiled with /MT?

---

From the page about binary compatibility:

You can mix binaries built by different versions of the v140, v141, v142, and v143 toolsets. However, you must link by using a toolset at least as recent as the most recent binary in your app. Here's an example: you can link an app compiled using any 2017 toolset (v141, versions 15.0 through 15.9) to a static library compiled using, say, Visual Studio 2019 version 16.2 (v142). You just have to link them by using a version 16.2 or later toolset. You can link a version 16.2 library to a version 16.4 app as long as you use a 16.4 or later toolset.

So my concern is about some libraries to be built with ToolsVersion="4.0", which will not be compatible with binaries build by versions v140..v142.
Hence the question: will anything bad happen if instead of version 15.0 with v141 PlatformToolset a 4.0 ToolsVersion is be used?

Answer 1

ToolsVersion is an optional attribute and used only by msbuild. It is the version of the Toolset MSBuild uses to determine the values for $(MSBuildBinPath) and $(MSBuildToolsPath). You can safely omit this attribute, it won't affect your ABI.