VS: Provide the default platform name to project code

The value of `CMAKE_VS_PLATFORM_NAME` is computed by Visual Studio
generators based on `CMAKE_GENERATOR_PLATFORM` or some default.
Prior to the VS 2019 generator, the default was always `Win32`.
However, for the `Visual Studio 16 2019` generator, the default is
based on the host platform.

Store the default in a new `CMAKE_VS_PLATFORM_NAME_DEFAULT` variable for
use by project code.  This is particularly useful in toolchain files
because they are allowed to set `CMAKE_GENERATOR_PLATFORM` and so
`CMAKE_VS_PLATFORM_NAME` is not yet known.  Of course the toolchain file
author knows whether it will set `CMAKE_GENERATOR_PLATFORM`, and if not
then `CMAKE_VS_PLATFORM_NAME_DEFAULT` provides the platform name that
will be used.

Fixes: #19177

Help/generator/Visual Studio 16 2019.rst

@@ -28,7 +28,8 @@ by default.
 Platform Selection
 ^^^^^^^^^^^^^^^^^^
 
-The default target platform name (architecture) is that of the host.
+The default target platform name (architecture) is that of the host
+and is provided in the :variable:`CMAKE_VS_PLATFORM_NAME_DEFAULT` variable.
 
 The :variable:`CMAKE_GENERATOR_PLATFORM` variable may be set, perhaps
 via the :manual:`cmake(1)` ``-A`` option, to specify a target platform

Help/manual/cmake-variables.7.rst

@@ -105,6 +105,7 @@ Variables that Provide Information
    /variable/CMAKE_VS_MSBUILD_COMMAND
    /variable/CMAKE_VS_NsightTegra_VERSION
    /variable/CMAKE_VS_PLATFORM_NAME
+   /variable/CMAKE_VS_PLATFORM_NAME_DEFAULT
    /variable/CMAKE_VS_PLATFORM_TOOLSET
    /variable/CMAKE_VS_PLATFORM_TOOLSET_CUDA
    /variable/CMAKE_VS_PLATFORM_TOOLSET_HOST_ARCHITECTURE

Help/release/3.14.rst

@@ -137,6 +137,10 @@ Variables
   relocatable and reproducible builds that are invariant of the build
   directory.
 
+* A :variable:`CMAKE_VS_PLATFORM_NAME_DEFAULT` variable was added for
+  :ref:`Visual Studio Generators` to report their default platform used
+  when :variable:`CMAKE_GENERATOR_PLATFORM` is not set explicitly.
+
 Properties
 ----------
 
@@ -391,3 +395,10 @@ Changes made since CMake 3.14.0 include the following.
   our conventions.  3.14.1 revises the module to use ``Fontconfig_*``
   variable names.  This is incompatible with 3.14.0 but since the
   module is new in the 3.14 series usage should not yet be widespread.
+
+3.14.3
+------
+
+* The :variable:`CMAKE_VS_PLATFORM_NAME_DEFAULT` variable was added
+  to help toolchain files work with the :generator:`Visual Studio 16 2019`
+  generator where the default platform now depends on the host platform.

Help/variable/CMAKE_VS_PLATFORM_NAME.rst

@@ -1,8 +1,10 @@
 CMAKE_VS_PLATFORM_NAME
 ----------------------
 
-Visual Studio target platform name.
+Visual Studio target platform name used by the current generator.
 
 VS 8 and above allow project files to specify a target platform.
 CMake provides the name of the chosen platform in this variable.
 See the :variable:`CMAKE_GENERATOR_PLATFORM` variable for details.
+
+See also the :variable:`CMAKE_VS_PLATFORM_NAME_DEFAULT` variable.

Help/variable/CMAKE_VS_PLATFORM_NAME_DEFAULT.rst

@@ -0,0 +1,9 @@
+CMAKE_VS_PLATFORM_NAME_DEFAULT
+------------------------------
+
+Default for the Visual Studio target platform name for the current generator
+without considering the value of the :variable:`CMAKE_GENERATOR_PLATFORM`
+variable.  For :ref:`Visual Studio Generators` for VS 2017 and below this is
+always ``Win32``.  For VS 2019 and above this is based on the host platform.
+
+See also the :variable:`CMAKE_VS_PLATFORM_NAME` variable.

Source/cmGlobalVisualStudio7Generator.cxx

@@ -101,7 +101,7 @@ void cmGlobalVisualStudio7Generator::EnableLanguage(
   }
 
   // Create list of configurations requested by user's cache, if any.
-  this->cmGlobalGenerator::EnableLanguage(lang, mf, optional);
+  this->cmGlobalVisualStudioGenerator::EnableLanguage(lang, mf, optional);
 
   // if this environment variable is set, then copy it to
   // a static cache entry.  It will be used by

Source/cmGlobalVisualStudioGenerator.cxx

@@ -52,6 +52,14 @@ void cmGlobalVisualStudioGenerator::SetVersion(VSVersion v)
   this->Version = v;
 }
 
+void cmGlobalVisualStudioGenerator::EnableLanguage(
+  std::vector<std::string> const& lang, cmMakefile* mf, bool optional)
+{
+  mf->AddDefinition("CMAKE_VS_PLATFORM_NAME_DEFAULT",
+                    this->DefaultPlatformName.c_str());
+  this->cmGlobalGenerator::EnableLanguage(lang, mf, optional);
+}
+
 bool cmGlobalVisualStudioGenerator::SetGeneratorPlatform(std::string const& p,
                                                          cmMakefile* mf)
 {

Source/cmGlobalVisualStudioGenerator.h

@@ -50,6 +50,9 @@ public:
   /** Is the installed VS an Express edition?  */
   bool IsExpressEdition() const { return this->ExpressEdition; }
 
+  void EnableLanguage(std::vector<std::string> const& languages, cmMakefile*,
+                      bool optional) override;
+
   bool SetGeneratorPlatform(std::string const& p, cmMakefile* mf) override;
 
   /**

Tests/RunCMake/GeneratorPlatform/TestPlatform-toolchain.cmake

@@ -1 +1,2 @@
+message("CMAKE_VS_PLATFORM_NAME_DEFAULT is \"${CMAKE_VS_PLATFORM_NAME_DEFAULT}\"")
 set(CMAKE_GENERATOR_PLATFORM "Test Platform")

Tests/RunCMake/GeneratorPlatform/TestPlatformToolchain-stderr.txt

@@ -1,3 +1,4 @@
+CMAKE_VS_PLATFORM_NAME_DEFAULT is "[^"]+"
 CMake Error at TestPlatformToolchain.cmake:[0-9]+ \(message\):
   CMAKE_GENERATOR_PLATFORM is "Test Platform" as expected.
 Call Stack \(most recent call first\):