9 files changed, 70 insertions, 1 deletions
diff --git a/Help/release/dev/vs-instance.rst b/Help/release/dev/vs-instance.rst
new file mode 100644
index 0000000..0b9ff4b
--- /dev/null
+++ b/Help/release/dev/vs-instance.rst
@@ -0,0 +1,6 @@
+vs-instance
+-----------
+
+* The :ref:`Visual Studio Generators` for VS 2017 and above learned to
+  use portable instances of Visual Studio not known to the VS installer.
+  See the :variable:`CMAKE_GENERATOR_INSTANCE` variable.
diff --git a/Help/variable/CMAKE_GENERATOR_INSTANCE.rst b/Help/variable/CMAKE_GENERATOR_INSTANCE.rst
index 3596bf6..6a35f17 100644
--- a/Help/variable/CMAKE_GENERATOR_INSTANCE.rst
+++ b/Help/variable/CMAKE_GENERATOR_INSTANCE.rst
@@ -44,6 +44,12 @@ Supported pairs are:
 
   Specify the 4-component VS Build Version.
 
+.. versionadded:: 3.23
+
+  A portable VS instance may be specified that is not known to the
+  Visual Studio Installer tool.  The ``location`` and ``version=``
+  values must both be provided.
+
 If the value of ``CMAKE_GENERATOR_INSTANCE`` is not specified explicitly
 by the user or a toolchain file, CMake queries the Visual Studio Installer
 to locate VS instances, chooses one, and sets the variable as a cache entry
diff --git a/Source/cmGlobalVisualStudioVersionedGenerator.cxx b/Source/cmGlobalVisualStudioVersionedGenerator.cxx
index 806871d..f2ce83e 100644
--- a/Source/cmGlobalVisualStudioVersionedGenerator.cxx
+++ b/Source/cmGlobalVisualStudioVersionedGenerator.cxx
@@ -509,7 +509,7 @@ bool cmGlobalVisualStudioVersionedGenerator::SetGeneratorInstance(
           cmSystemTools::FileIsDirectory(this->GeneratorInstance)) {
         e << "\n"
              "The directory exists, but the instance is not known to the "
-             "Visual Studio Installer.";
+             "Visual Studio Installer, and no 'version=' field was given.";
       }
       mf->IssueMessage(MessageType::FATAL_ERROR, e.str());
       return false;
diff --git a/Source/cmVSSetupHelper.cxx b/Source/cmVSSetupHelper.cxx
index 39ddce3..cbd241b 100644
--- a/Source/cmVSSetupHelper.cxx
+++ b/Source/cmVSSetupHelper.cxx
@@ -2,6 +2,8 @@
    file Copyright.txt or https://cmake.org/licensing for details.  */
 #include "cmVSSetupHelper.h"
 
+#include <utility>
+
 #include "cmsys/Encoding.hxx"
 #include "cmsys/FStream.hxx"
 
@@ -342,6 +344,8 @@ bool cmVSSetupAPIHelper::EnumerateAndChooseVSInstance()
 
   std::string const wantVersion = std::to_string(this->Version) + '.';
 
+  bool specifiedLocationNotSpecifiedVersion = false;
+
   SmartCOMPtr<ISetupInstance> instance;
   while (SUCCEEDED(enumInstances->Next(1, &instance, NULL)) && instance) {
     SmartCOMPtr<ISetupInstance2> instance2 = NULL;
@@ -373,6 +377,7 @@ bool cmVSSetupAPIHelper::EnumerateAndChooseVSInstance()
             chosenInstanceInfo = instanceInfo;
             return true;
           }
+          specifiedLocationNotSpecifiedVersion = true;
         }
       } else if (!this->SpecifiedVSInstallVersion.empty()) {
         // We are looking for a specific version.
@@ -398,6 +403,13 @@ bool cmVSSetupAPIHelper::EnumerateAndChooseVSInstance()
     }
   }
 
+  if (!this->SpecifiedVSInstallLocation.empty() &&
+      !specifiedLocationNotSpecifiedVersion) {
+    // The VS Installer does not know about the specified location.
+    // Check for one directly on disk.
+    return this->LoadSpecifiedVSInstanceFromDisk();
+  }
+
   if (vecVSInstances.size() > 0) {
     isVSInstanceExists = true;
     int index = ChooseVSInstance(vecVSInstances);
@@ -460,6 +472,32 @@ int cmVSSetupAPIHelper::ChooseVSInstance(
   return chosenIndex;
 }
 
+bool cmVSSetupAPIHelper::LoadSpecifiedVSInstanceFromDisk()
+{
+  if (!cmSystemTools::FileIsDirectory(this->SpecifiedVSInstallLocation)) {
+    return false;
+  }
+  VSInstanceInfo vsInstanceInfo;
+  vsInstanceInfo.VSInstallLocation = this->SpecifiedVSInstallLocation;
+  // FIXME: Is there a better way to get SDK information?
+  vsInstanceInfo.IsWin10SDKInstalled = true;
+  vsInstanceInfo.IsWin81SDKInstalled = false;
+
+  if (!this->SpecifiedVSInstallVersion.empty()) {
+    // Assume the version specified by the user is correct.
+    vsInstanceInfo.Version = this->SpecifiedVSInstallVersion;
+  } else {
+    return false;
+  }
+
+  if (!LoadVSInstanceVCToolsetVersion(vsInstanceInfo)) {
+    return false;
+  }
+
+  chosenInstanceInfo = std::move(vsInstanceInfo);
+  return true;
+}
+
 bool cmVSSetupAPIHelper::Initialize()
 {
   if (initializationFailure)
diff --git a/Source/cmVSSetupHelper.h b/Source/cmVSSetupHelper.h
index d7e82d0..44c883b 100644
--- a/Source/cmVSSetupHelper.h
+++ b/Source/cmVSSetupHelper.h
@@ -117,6 +117,7 @@ private:
                                bool& bWin10SDK, bool& bWin81SDK);
   int ChooseVSInstance(const std::vector<VSInstanceInfo>& vecVSInstances);
   bool EnumerateAndChooseVSInstance();
+  bool LoadSpecifiedVSInstanceFromDisk();
 
   unsigned int Version;
 
diff --git a/Tests/RunCMake/GeneratorInstance/PortableNoVersion-result.txt b/Tests/RunCMake/GeneratorInstance/PortableNoVersion-result.txt
new file mode 100644
index 0000000..d00491f
--- /dev/null
+++ b/Tests/RunCMake/GeneratorInstance/PortableNoVersion-result.txt
@@ -0,0 +1 @@
+1
diff --git a/Tests/RunCMake/GeneratorInstance/PortableNoVersion-stderr.txt b/Tests/RunCMake/GeneratorInstance/PortableNoVersion-stderr.txt
new file mode 100644
index 0000000..baa17aa
--- /dev/null
+++ b/Tests/RunCMake/GeneratorInstance/PortableNoVersion-stderr.txt
@@ -0,0 +1,13 @@
+^CMake Error at CMakeLists.txt:[0-9]+ \(project\):
+  Generator
+
+    Visual Studio [^
+]+
+
+  could not find specified instance of Visual Studio:
+
+    [^
+]+/Tests/RunCMake/GeneratorInstance
+
+  The directory exists, but the instance is not known to the Visual Studio
+  Installer, and no 'version=' field was given\.$
diff --git a/Tests/RunCMake/GeneratorInstance/PortableNoVersion.cmake b/Tests/RunCMake/GeneratorInstance/PortableNoVersion.cmake
new file mode 100644
index 0000000..2fc38e5
--- /dev/null
+++ b/Tests/RunCMake/GeneratorInstance/PortableNoVersion.cmake
@@ -0,0 +1 @@
+message(FATAL_ERROR "This should not be reached!")
diff --git a/Tests/RunCMake/GeneratorInstance/RunCMakeTest.cmake b/Tests/RunCMake/GeneratorInstance/RunCMakeTest.cmake
index cdcaac8..dfcdcf8 100644
--- a/Tests/RunCMake/GeneratorInstance/RunCMakeTest.cmake
+++ b/Tests/RunCMake/GeneratorInstance/RunCMakeTest.cmake
@@ -32,6 +32,9 @@ if("${RunCMake_GENERATOR}" MATCHES "^Visual Studio (1[56789])")
     set(RunCMake_GENERATOR_INSTANCE "${default_instance},version=${vs_major}.999.99999.999")
     run_cmake(WrongVersion)
   endif()
+
+  set(RunCMake_GENERATOR_INSTANCE "${RunCMake_SOURCE_DIR}")
+  run_cmake(PortableNoVersion)
 else()
   set(RunCMake_GENERATOR_INSTANCE "")
   run_cmake(NoInstance)