CMAKE_TOOLCHAIN_FILE: Account for empty values when detecting changes

Initially empty CMAKE_TOOLCHAIN_FILE values should not result in an
include() statement but should be recorded to later detect changes.

Fixes: #28231

Modules/CMakeDetermineSystem.cmake

@@ -225,12 +225,16 @@ if(CMAKE_BINARY_DIR)
   # if a toolchain file is used, it needs to be included in the configured file,
   # so settings done there are also available if they don't go in the cache and in try_compile()
   set(INCLUDE_CMAKE_TOOLCHAIN_FILE_IF_REQUIRED)
-  if(CMAKE_TOOLCHAIN_FILE)
+  if(DEFINED CMAKE_TOOLCHAIN_FILE)
     string(CONCAT INCLUDE_CMAKE_TOOLCHAIN_FILE_IF_REQUIRED
       "set(_CMAKE_INPUT_TOOLCHAIN_FILE [==[${_CMAKE_INPUT_TOOLCHAIN_FILE}]==])\n"
-      "set(_CMAKE_SYSTEM_TOOLCHAIN_FILE \"${CMAKE_TOOLCHAIN_FILE}\")\n"
-      "include(\"\${_CMAKE_SYSTEM_TOOLCHAIN_FILE}\")"
     )
+    if(_INCLUDED_TOOLCHAIN_FILE)
+      string(APPEND INCLUDE_CMAKE_TOOLCHAIN_FILE_IF_REQUIRED
+        "set(_CMAKE_SYSTEM_TOOLCHAIN_FILE \"${CMAKE_TOOLCHAIN_FILE}\")\n"
+        "include(\"\${_CMAKE_SYSTEM_TOOLCHAIN_FILE}\")"
+      )
+    endif()
   endif()
 
   # configure variables set in this file for fast reload, the template file is defined at the top of this file

Tests/RunCMake/CompilerChange/RunCMakeTest.cmake

@@ -45,6 +45,17 @@ if(UNIX AND "${RunCMake_GENERATOR}" MATCHES "Unix Makefiles|Ninja|FASTBuild")
   endblock()
 endif()
 
+# Running with empty toolchain argument does not trigger reconfigure
+block()
+  set(RunCMake_TEST_BINARY_DIR ${RunCMake_BINARY_DIR}/ToolchainEmpty-build)
+  set(RunCMake-stdout-file
+    ${RunCMake_SOURCE_DIR}/ToolchainEmpty-stdout.txt
+  )
+  run_cmake_with_options(ToolchainEmpty-step1 "-DCMAKE_TOOLCHAIN_FILE=")
+  set(RunCMake_TEST_NO_CLEAN 1)
+  run_cmake(ToolchainEmpty-step2)
+endblock()
+
 # Set up "toolchain" files
 file(WRITE "${RunCMake_BINARY_DIR}/foo.cmake" "set(toolchain_var foo)\n")
 file(WRITE "${RunCMake_BINARY_DIR}/bar.cmake" "set(toolchain_var bar)\n")

Tests/RunCMake/CompilerChange/ToolchainEmpty-stdout.txt

@@ -0,0 +1,2 @@
+-- Toolchain file: $
+-- Using toolchain: $

Tests/RunCMake/CompilerChange/ToolchainEmpty-step1.cmake

@@ -0,0 +1,2 @@
+message(STATUS "Toolchain file: ${CMAKE_TOOLCHAIN_FILE}")
+message(STATUS "Using toolchain: ${toolchain_var}")

Tests/RunCMake/CompilerChange/ToolchainEmpty-step2.cmake

@@ -0,0 +1,2 @@
+message(STATUS "Toolchain file: ${CMAKE_TOOLCHAIN_FILE}")
+message(STATUS "Using toolchain: ${toolchain_var}")