Tests/RunCMake/Syntax/AtWithVariableEmptyExpansion.cmake - third_party/github.com/Kitware/CMake - Git at Google

 # Literal since 'var' is not defined.
 set(ref "@var@")
 set(right "wrong")
 set(var "\${right}")

 # 'var' is dereferenced here.
 string(CONFIGURE "${ref}" output)
 message("-->${output}<--")
	# Literal since 'var' is not defined.
	set(ref "@var@")
	set(right "wrong")
	set(var "\${right}")

	# 'var' is dereferenced here.
	string(CONFIGURE "${ref}" output)
	message("-->${output}<--")