I came across these while looking at #23449.  The output is in the attached file as it's cleaner that way.

I can't reproduce this easily. Could you delete the externals/tcltk folder and the Debug_VC12 folders under tk.../win, tix.../win and tcl-core.../win, rebuild and capture the output from the VS Output window? Apparently something in one of these external projects is not building properly.

I don't have externals/tcltk, only externals/tcltk64.  There is a Debug_VC12 folder under tix.../win.  There is no Debug_VC12 folder under tk.../win, tcl-core.../win or for that matter tcl.../win, only Debug_AMD64_VC12.

Hmm, so maybe it only happens after you build the 64-bit version first. But apparently it's more complex than that, since that doesn't repro either. When I get some more time I'll run though and double check all the rebuild conditions, but it has worked fine for me every time I've done it across 8-10 machines and 3-4 VS versions.

I think you've ended up in a very unlikely situation that is probably going to be fixed by "hg purge --all && PCBuild\build -e -d && PCBuild\build -e -d -p x64".

Having run the "hg purge..." command previously given everything seemed fine.  Both release and debug win32 builds then successfully completed from VS but the 64 bit builds failed.  I now have externals/tcltk but not externals/tcltk64.  I did a bit of digging and in tcltk.props there are these two lines which appear relevant.

<tcltkDir>$(ExternalsDir)tcltk\</tcltkDir>
    <tcltkDir Condition="'$(Platform)' == x64'">$(ExternalsDir)tcltk64\</tcltkDir>

I'm now way out of my depth and hoping this isn't a red herring.

There's nothing wrong with that line, it's probably the Tk or Tcl build failing. Zach may have some ideas, or I'll play with it some more in the next few days.

Unfortunately, I don't have any ideas.  Mark, is this still happening?  If so, I think your real solution is going to be to install VS2015 CTP 6 (and later the real thing, in whatever the free flavor is).  Same for #23449.

Well, in the interests of due diligence, I checked out & built each of the four flavours (32/64 Release/Debug) independently and without any errors. I then rebuilt each with the "wrong" Platform (ie for Win32 in the x64 directory and vice versa) without cleaning first and also without error.

So I'm going to close this on the ground that we can't reproduce it. If someone can come up with a recipe for reproducing the problem we can re-open (or start a new issue).