MSBuild - Visual Studio 2015 Update 1 (ish)

This tag corresponds to the version of MSBuild packaged with Visual Studio 2015 Update 1.

Important: the actual Update 1 release was not produced by compiling the GitHub source at this commit. It was built from a mirror of MSBuild in the Microsoft-internal VS repository, which was manually synced to contain the changes made here. These sources are instructive for behavior and understanding purposes, but not suitable for debugging the released binaries.