VS: Add CMAKE_GENERATOR_PLATFORM field to control Windows SDK selection

Add a `version=` field to explicitly control the SDK version selection
without relying on `CMAKE_SYSTEM_VERSION`.

Fixes: #16713

5 files changed, 50 insertions, 12 deletions

diff --git a/Help/manual/cmake-toolchains.7.rst b/Help/manual/cmake-toolchains.7.rst
index 9feb4d2..a831fa6 100644
--- a/Help/manual/cmake-toolchains.7.rst
+++ b/Help/manual/cmake-toolchains.7.rst
@@ -273,7 +273,7 @@ supported out of the box.  Other versions may require one to set
 Cross Compiling for Windows 10 Universal Applications
 -----------------------------------------------------
 
-A toolchain file to configure a Visual Studio generator for a
+A toolchain file to configure :ref:`Visual Studio Generators` for a
 Windows 10 Universal Application may look like this:
 
 .. code-block:: cmake
@@ -283,9 +283,10 @@ Windows 10 Universal Application may look like this:
 
 A Windows 10 Universal Application targets both Windows Store and
 Windows Phone.  Specify the :variable:`CMAKE_SYSTEM_VERSION` variable
-to be ``10.0`` to build with the latest available Windows 10 SDK.
-Specify a more specific version (e.g. ``10.0.10240.0`` for RTM)
-to build with the corresponding SDK.
+to be ``10.0`` or higher.
+
+CMake selects a Windows SDK as described by documentation of the
+:variable:`CMAKE_VS_WINDOWS_TARGET_PLATFORM_VERSION` variable.
 
 Cross Compiling for Windows Phone
 ---------------------------------
diff --git a/Help/release/dev/vs-sdk-selection.rst b/Help/release/dev/vs-sdk-selection.rst
new file mode 100644
index 0000000..856a203
--- /dev/null
+++ b/Help/release/dev/vs-sdk-selection.rst
@@ -0,0 +1,7 @@
+vs-sdk-selection
+----------------
+
+* The :ref:`Visual Studio Generators` for VS 2015 and above learned to
+  select the Windows SDK version explicitly using a ``version=`` field
+  in the :variable:`CMAKE_GENERATOR_PLATFORM` variable.
+  See :ref:`Visual Studio Platform Selection`.
diff --git a/Help/variable/CMAKE_GENERATOR_PLATFORM.rst b/Help/variable/CMAKE_GENERATOR_PLATFORM.rst
index 0238183..3b6932a 100644
--- a/Help/variable/CMAKE_GENERATOR_PLATFORM.rst
+++ b/Help/variable/CMAKE_GENERATOR_PLATFORM.rst
@@ -26,6 +26,8 @@ Platform specification is supported only on specific generators:
 
 See native build system documentation for allowed platform names.
 
+.. _`Visual Studio Platform Selection`:
+
 Visual Studio Platform Selection
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
@@ -42,4 +44,24 @@ name is provided in the :variable:`CMAKE_VS_PLATFORM_NAME` variable.
 
 The ``key=value`` pairs form a comma-separated list of options to
 specify generator-specific details of the platform selection.
-There are no supported pairs: this syntax is reserved for future use.
+Supported pairs are:
+
+``version=<version>``
+  .. versionadded:: 3.27
+
+  Specify the Windows SDK version to use.  This is supported by VS 2015 and
+  above when targeting Windows 10.0+ or Windows Store.  CMake will set the
+  :variable:`CMAKE_VS_WINDOWS_TARGET_PLATFORM_VERSION` variable to the
+  selected SDK version.
+
+  The ``<version>`` may be one of:
+
+  ``10.0.<build>.<increment>``
+    Specify the exact 4-component SDK version, e.g., ``10.0.19041.0``.
+    The specified version of the SDK must be installed.  It may not exceed
+    the value of :variable:`CMAKE_VS_WINDOWS_TARGET_PLATFORM_VERSION_MAXIMUM`,
+    if that variable is set.
+
+  If the ``version`` field is not specified, CMake selects a version as
+  described in the :variable:`CMAKE_VS_WINDOWS_TARGET_PLATFORM_VERSION`
+  variable documentation.
diff --git a/Help/variable/CMAKE_VS_WINDOWS_TARGET_PLATFORM_VERSION.rst b/Help/variable/CMAKE_VS_WINDOWS_TARGET_PLATFORM_VERSION.rst
index eb71049..4a3ef47 100644
--- a/Help/variable/CMAKE_VS_WINDOWS_TARGET_PLATFORM_VERSION.rst
+++ b/Help/variable/CMAKE_VS_WINDOWS_TARGET_PLATFORM_VERSION.rst
@@ -5,11 +5,19 @@ CMAKE_VS_WINDOWS_TARGET_PLATFORM_VERSION
 
 Visual Studio Windows Target Platform Version.
 
-When targeting Windows 10 and above Visual Studio 2015 and above support
-specification of a target Windows version to select a corresponding SDK.
-The :variable:`CMAKE_SYSTEM_VERSION` variable may be set to specify a
-version.  Otherwise CMake computes a default version based on the Windows
-SDK versions available.  The chosen Windows target version number is provided
+When targeting Windows 10 and above, :ref:`Visual Studio Generators` for
+VS 2015 and above support specification of a Windows SDK version:
+
+* If :variable:`CMAKE_GENERATOR_PLATFORM` specifies a ``version=`` field,
+  as documented by :ref:`Visual Studio Platform Selection`, that SDK
+  version is selected.
+
+* Otherwise, if :variable:`CMAKE_SYSTEM_VERSION` is set to an available
+  SDK version, that version is selected.
+
+* Otherwise, CMake uses the latest Windows SDK version available.
+
+The chosen Windows target version number is provided
 in ``CMAKE_VS_WINDOWS_TARGET_PLATFORM_VERSION``.  If no Windows 10 SDK
 is available this value will be empty.
 
diff --git a/Help/variable/CMAKE_VS_WINDOWS_TARGET_PLATFORM_VERSION_MAXIMUM.rst b/Help/variable/CMAKE_VS_WINDOWS_TARGET_PLATFORM_VERSION_MAXIMUM.rst
index f1a1977..727ccc9 100644
--- a/Help/variable/CMAKE_VS_WINDOWS_TARGET_PLATFORM_VERSION_MAXIMUM.rst
+++ b/Help/variable/CMAKE_VS_WINDOWS_TARGET_PLATFORM_VERSION_MAXIMUM.rst
@@ -10,5 +10,5 @@ be set to a false value (e.g. ``OFF``, ``FALSE``, or ``0``) or the SDK version
 to use as the maximum (e.g. ``10.0.14393.0``).  If unset, the default depends
 on which version of Visual Studio is targeted by the current generator.
 
-This can be used in conjunction with :variable:`CMAKE_SYSTEM_VERSION`, which
-CMake uses to select :variable:`CMAKE_VS_WINDOWS_TARGET_PLATFORM_VERSION`.
+This can be used to exclude Windows SDK versions from consideration for
+:variable:`CMAKE_VS_WINDOWS_TARGET_PLATFORM_VERSION`.