Test if() boolean coersion and CMP0012 behaviors

We introduce the "CMake.If" test to try out conversion of constants and
variables to boolean values in the if() command.  We cover both OLD and
NEW behavior for policy CMP0012.

Tests/CMakeTests/CMakeLists.txt

@@ -22,6 +22,7 @@ AddCMakeTest(ConfigureFile "")
 AddCMakeTest(SeparateArguments "")
 AddCMakeTest(ImplicitLinkInfo "")
 AddCMakeTest(ModuleNotices "")
+AddCMakeTest(If "")
 AddCMakeTest(String "")
 AddCMakeTest(Math "")
 AddCMakeTest(CMakeMinimumRequired "")

Tests/CMakeTests/IfTest.cmake.in

@@ -0,0 +1,158 @@
+# Prepare variable definitions.
+set(VAR_UNDEFINED)
+set(VAR_PATH /some/path/to/a/file.txt)
+set(FALSE_NAMES OFF NO FALSE N FOO-NOTFOUND IGNORE Off No False Ignore off n no false ignore)
+set(TRUE_NAMES ON YES TRUE Y On Yes True on yes true y)
+foreach(_arg "" 0 1 2 ${TRUE_NAMES} ${FALSE_NAMES})
+  set(VAR_${_arg} "${_arg}")
+endforeach()
+
+macro(test_vars _old)
+  # Variables set to false or not set.
+  foreach(_var "" 0 ${FALSE_NAMES} UNDEFINED)
+    if(VAR_${_var})
+      message(FATAL_ERROR "${_old}if(VAR_${_var}) is true!")
+    else()
+      message(STATUS "${_old}if(VAR_${_var}) is false")
+    endif()
+
+    if(NOT VAR_${_var})
+      message(STATUS "${_old}if(NOT VAR_${_var}) is true")
+    else()
+      message(FATAL_ERROR "${_old}if(NOT VAR_${_var}) is false!")
+    endif()
+  endforeach()
+
+  # Variables set to true.
+  foreach(_var 1 2 ${TRUE_NAMES} PATH)
+    if(VAR_${_var})
+      message(STATUS "${_old}if(VAR_${_var}) is true")
+    else()
+      message(FATAL_ERROR "${_old}if(VAR_${_var}) is false!")
+    endif()
+
+    if(NOT VAR_${_var})
+      message(FATAL_ERROR "${_old}if(NOT VAR_${_var}) is true!")
+    else()
+      message(STATUS "${_old}if(NOT VAR_${_var}) is false")
+    endif()
+  endforeach()
+endmacro()
+
+#-----------------------------------------------------------------------------
+# Test the OLD behavior of CMP0012.
+cmake_policy(SET CMP0012 OLD)
+
+# False constants not recognized (still false).
+foreach(_false "" ${FALSE_NAMES})
+  if("${_false}")
+    message(FATAL_ERROR "OLD if(${_false}) is true!")
+  else()
+    message(STATUS "OLD if(${_false}) is false")
+  endif()
+
+  if(NOT "${_false}")
+    message(STATUS "OLD if(NOT ${_false}) is true")
+  else()
+    message(FATAL_ERROR "OLD if(NOT ${_false}) is false!")
+  endif()
+endforeach()
+
+# True constants not recognized.
+foreach(_false ${TRUE_NAMES})
+  if(${_false})
+    message(FATAL_ERROR "OLD if(${_false}) is true!")
+  else()
+    message(STATUS "OLD if(${_false}) is false")
+  endif()
+
+  if(NOT ${_false})
+    message(STATUS "OLD if(NOT ${_false}) is true")
+  else()
+    message(FATAL_ERROR "OLD if(NOT ${_false}) is false!")
+  endif()
+endforeach()
+
+# Numbers not recognized properly.
+foreach(_num 2 -2 2.0 -2.0 2x -2x)
+  if(${_num})
+    message(FATAL_ERROR "OLD if(${_num}) is true!")
+  else()
+    message(STATUS "OLD if(${_num}) is false")
+  endif()
+
+  if(NOT ${_num})
+    message(FATAL_ERROR "OLD if(NOT ${_num}) is true!")
+  else()
+    message(STATUS "OLD if(NOT ${_num}) is false")
+  endif()
+endforeach()
+
+test_vars("OLD ")
+
+#-----------------------------------------------------------------------------
+
+# Test the NEW behavior of CMP0012.
+cmake_policy(SET CMP0012 NEW)
+
+# Test false constants.
+foreach(_false "" 0 ${FALSE_NAMES})
+  if("${_false}")
+    message(FATAL_ERROR "if(${_false}) is true!")
+  else()
+    message(STATUS "if(${_false}) is false")
+  endif()
+
+  if(NOT "${_false}")
+    message(STATUS "if(NOT ${_false}) is true")
+  else()
+    message(FATAL_ERROR "if(NOT ${_false}) is false!")
+  endif()
+endforeach()
+
+# Test true constants.
+foreach(_true 1 ${TRUE_NAMES})
+  if(${_true})
+    message(STATUS "if(${_true}) is true")
+  else()
+    message(FATAL_ERROR "if(${_true}) is false!")
+  endif()
+
+  if(NOT ${_true})
+    message(FATAL_ERROR "if(NOT ${_true}) is true!")
+  else()
+    message(STATUS "if(NOT ${_true}) is false")
+  endif()
+endforeach()
+
+# Numbers recognized properly.
+foreach(_num 2 -2 2.0 -2.0)
+  if(${_num})
+    message(STATUS "if(${_num}) is true")
+  else()
+    message(FATAL_ERROR "if(${_num}) is false!")
+  endif()
+
+  if(NOT ${_num})
+    message(FATAL_ERROR "if(NOT ${_num}) is true!")
+  else()
+    message(STATUS "if(NOT ${_num}) is false")
+  endif()
+endforeach()
+
+# Bad numbers not recognized.
+foreach(_bad 2x -2x)
+  if(${_bad})
+    message(FATAL_ERROR "if(${_bad}) is true!")
+  else()
+    message(STATUS "if(${_bad}) is false")
+  endif()
+
+  if(NOT ${_bad})
+    message(STATUS "if(NOT ${_bad}) is true")
+  else()
+    message(FATAL_ERROR "if(NOT ${_bad}) is false!")
+  endif()
+endforeach()
+
+test_vars("")