VS: Select and save a VS 2017 instance persistently

Visual Studio 2017 supports multiple instances installed on a single
machine.  We use the Visual Studio Installer tool to enumerate instances
and select one.  Once we select an instance for a given build tree, save
the result in `CMAKE_GENERATOR_INSTANCE` so we can re-configure the tree
with the same instance on future re-runs of CMake.

Fixes: #17268

1 files changed, 14 insertions, 0 deletions

diff --git a/Source/cmCMakeHostSystemInformationCommand.cxx b/Source/cmCMakeHostSystemInformationCommand.cxx
index 5106f52..662dd74 100644
--- a/Source/cmCMakeHostSystemInformationCommand.cxx
+++ b/Source/cmCMakeHostSystemInformationCommand.cxx
@@ -8,6 +8,9 @@
 #include "cmsys/SystemInformation.hxx"
 
 #if defined(_WIN32)
+#include "cmAlgorithms.h"
+#include "cmGlobalGenerator.h"
+#include "cmGlobalVisualStudio15Generator.h"
 #include "cmSystemTools.h"
 #include "cmVSSetupHelper.h"
 #define HAVE_VS_SETUP_HELPER
@@ -127,6 +130,17 @@ bool cmCMakeHostSystemInformationCommand::GetValue(
     value = this->ValueToString(info.GetOSPlatform());
 #ifdef HAVE_VS_SETUP_HELPER
   } else if (key == "VS_15_DIR") {
+    // If generating for the VS 15 IDE, use the same instance.
+    cmGlobalGenerator* gg = this->Makefile->GetGlobalGenerator();
+    if (cmHasLiteralPrefix(gg->GetName(), "Visual Studio 15 ")) {
+      cmGlobalVisualStudio15Generator* vs15gen =
+        static_cast<cmGlobalVisualStudio15Generator*>(gg);
+      if (vs15gen->GetVSInstance(value)) {
+        return true;
+      }
+    }
+
+    // Otherwise, find a VS 15 instance ourselves.
     cmVSSetupAPIHelper vsSetupAPIHelper;
     if (vsSetupAPIHelper.GetVSInstanceInfo(value)) {
       cmSystemTools::ConvertToUnixSlashes(value);