Variables set with PARENT_SCOPE are empty in the corresponding child scope. Why?

Question

Consider the following minimal example:

.
├── bar
│   └── CMakeLists.txt
└── CMakeLists.txt

where ./CMakeLists.txt is

Answer 1

I do not see anything that is not consistent with the SET command documentation

If PARENT_SCOPE is present, the variable will be set in the scope above the current scope. Each new directory or function creates a new scope. This command will set the value of a variable into the parent directory or calling function (whichever is applicable to the case at hand).

./bar/CMakeLists.txt

set( BAR "This is bar." PARENT_SCOPE ) #<-- Variable is set only in the PARENT scope
message( STATUS "Variable BAR in ./bar/ = ${BAR}" ) #<--- Still undefined/empty

You can always do:

set( BAR "This is bar." ) #<-- set in this scope
set( BAR ${BAR} PARENT_SCOPE ) #<-- set in the parent scope too

Grep for PARENT_SCOPE in the delivered modules in your installation, for example FindGTK2

if(GTK2_${_var}_FOUND)
   set(GTK2_LIBRARIES ${GTK2_LIBRARIES} ${GTK2_${_var}_LIBRARY})
   set(GTK2_LIBRARIES ${GTK2_LIBRARIES} PARENT_SCOPE)
endif()