Genex: Add the PLATFORM_ID expression.

Source/cmDocumentGeneratorExpressions.h

@@ -40,6 +40,9 @@
   "is exported using export(), or when the target is used by another "  \
   "target in the same buildsystem. Expands to the empty string "        \
   "otherwise.\n"                                                        \
+  "  $<PLATFORM_ID>            = The CMake-id of the platform "         \
+  "  $<PLATFORM_ID:comp>       = '1' if the The CMake-id of the "       \
+  "platform matches comp, otherwise '0'.\n"                             \
   "  $<C_COMPILER_ID>          = The CMake-id of the C compiler "       \
   "used.\n"                                                             \
   "  $<C_COMPILER_ID:comp>     = '1' if the CMake-id of the C "         \

Source/cmGeneratorExpressionEvaluator.cxx

@@ -437,6 +437,39 @@ static const struct CxxCompilerVersionNode : public CompilerVersionNode
 } cxxCompilerVersionNode;
 
 
+//----------------------------------------------------------------------------
+struct PlatformIdNode : public cmGeneratorExpressionNode
+{
+  PlatformIdNode() {}
+
+  virtual int NumExpectedParameters() const { return ZeroOrMoreParameters; }
+
+  std::string Evaluate(const std::vector<std::string> &parameters,
+                       cmGeneratorExpressionContext *context,
+                       const GeneratorExpressionContent *,
+                       cmGeneratorExpressionDAGChecker *) const
+  {
+    const char *platformId = context->Makefile ?
+                              context->Makefile->GetSafeDefinition(
+                        "CMAKE_SYSTEM_NAME") : "";
+    if (parameters.size() == 0)
+      {
+      return platformId ? platformId : "";
+      }
+
+    if (!platformId)
+      {
+      return parameters.front().empty() ? "1" : "0";
+      }
+
+    if (cmsysString_strcasecmp(parameters.begin()->c_str(), platformId) == 0)
+      {
+      return "1";
+      }
+    return "0";
+  }
+} platformIdNode;
+
 //----------------------------------------------------------------------------
 static const struct VersionGreaterNode : public cmGeneratorExpressionNode
 {
@@ -1355,6 +1388,8 @@ cmGeneratorExpressionNode* GetNode(const std::string &identifier)
     return &cCompilerVersionNode;
   else if (identifier == "CXX_COMPILER_VERSION")
     return &cxxCompilerVersionNode;
+  else if (identifier == "PLATFORM_ID")
+    return &platformIdNode;
   else if (identifier == "CONFIGURATION")
     return &configurationNode;
   else if (identifier == "CONFIG")

Tests/GeneratorExpression/CMakeLists.txt

@@ -186,6 +186,11 @@ add_custom_target(check-part3 ALL
     -Dtest_alias_target_name=$<STREQUAL:$<TARGET_PROPERTY:Alias::SomeLib,NAME>,$<TARGET_PROPERTY:empty1,NAME>>
     -Dtest_early_termination_1=$<$<1:>:
     -Dtest_early_termination_2=$<$<1:>:,
+    -Dsystem_name=${CMAKE_HOST_SYSTEM_NAME}
+    -Dtest_platform_id=$<PLATFORM_ID>
+    -Dtest_platform_id_Linux=$<PLATFORM_ID:Linux>
+    -Dtest_platform_id_Windows=$<PLATFORM_ID:Windows>
+    -Dtest_platform_id_Darwin=$<PLATFORM_ID:Darwin>
     -P ${CMAKE_CURRENT_SOURCE_DIR}/check-part3.cmake
   COMMAND ${CMAKE_COMMAND} -E echo "check done (part 3 of 3)"
   VERBATIM

Tests/GeneratorExpression/check-part3.cmake

@@ -26,3 +26,11 @@ check(test_alias_file_lib "1")
 check(test_alias_target_name "1")
 check(test_early_termination_1 "$<:")
 check(test_early_termination_2 "$<:,")
+check(test_platform_id "${system_name}")
+foreach(system Linux Windows Darwin)
+  if(system_name STREQUAL system)
+    check(test_platform_id_${system} 1)
+  else()
+    check(test_platform_id_${system} 0)
+  endif()
+endforeach()