Allow building with VS 2017 when VS 2013/2015 are in path

build/toolchain/win/setup_toolchain.py

 # Copyright (c) 2013 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 #
 # Copies the given "win tool" (which the toolchain uses to wrap compiler
 # invocations) and the environment blocks for the 32-bit and 64-bit builds on
 # Windows to the build directory.
 #
 # The arguments are the visual studio install location and the location of the
 # win tool. The script assumes that the root build directory is the current dir
@@ skipping 109 matching lines @@
       assert _ExtractImportantEnvironment(variables) == \
              _ExtractImportantEnvironment(_LoadEnvFromBat([script, '/' + cpu]))
   else:
     if 'GYP_MSVS_OVERRIDE_PATH' not in os.environ:
       os.environ['GYP_MSVS_OVERRIDE_PATH'] = _DetectVisualStudioPath()
     # We only support x64-hosted tools.
     script_path = os.path.normpath(os.path.join(
                                        os.environ['GYP_MSVS_OVERRIDE_PATH'],
                                        'VC/vcvarsall.bat'))
     if not os.path.exists(script_path):
+      # vcvarsall.bat for VS 2017 fails if run after running vcvarsall.bat from
+      # VS 2013 or VS 2015. Fix this by clearing the vsinstalldir environment
+      # variable.
+      if 'VSINSTALLDIR' in os.environ:
+        del os.environ['VSINSTALLDIR']
       other_path = os.path.normpath(os.path.join(
                                         os.environ['GYP_MSVS_OVERRIDE_PATH'],
                                         'VC/Auxiliary/Build/vcvarsall.bat'))
       if not os.path.exists(other_path):
         raise Exception('%s is missing - make sure VC++ tools are installed.' %
                         script_path)
       script_path = other_path
     args = [script_path, 'amd64_x86' if cpu == 'x86' else 'amd64']
     variables = _LoadEnvFromBat(args)
   return _ExtractImportantEnvironment(variables)
@@ skipping 60 matching lines @@
 
   assert vc_bin_dir
   print 'vc_bin_dir = ' + gn_helpers.ToGNString(vc_bin_dir)
   assert include_I
   print 'include_flags_I = ' + gn_helpers.ToGNString(include_I)
   assert include_imsvc
   print 'include_flags_imsvc = ' + gn_helpers.ToGNString(include_imsvc)
 
 if __name__ == '__main__':
   main()