CMP0054

Only interpret if() arguments as variables or keywords when unquoted.

CMake 3.1 and above no longer implicitly dereference variables or interpret keywords in an if() command argument when it is a Quoted Argument or a Bracket Argument.

The OLD behavior for this policy is to dereference variables and interpret keywords even if they are quoted or bracketed. The NEW behavior is to not dereference variables or interpret keywords that have been quoted or bracketed.

Given the following partial example:

set(A E)
set(E "")

if("${A}" STREQUAL "")
  message("Result is TRUE before CMake 3.1 or when CMP0054 is OLD")
else()
  message("Result is FALSE in CMake 3.1 and above if CMP0054 is NEW")
endif()

After explicit expansion of variables this gives:

if("E" STREQUAL "")

With the policy set to OLD implicit expansion reduces this semantically to:

if("" STREQUAL "")

With the policy set to NEW the quoted arguments will not be further dereferenced:

if("E" STREQUAL "")

This policy was introduced in CMake version 3.1. CMake version 3.19.0-rc3 warns when the policy is not set and uses OLD behavior. Use the cmake_policy() command to set it to OLD or NEW explicitly.