Is this really a better style to use generator expressions every time?

Usage
1 
# either
target_compile_options(Greeter PUBLIC "$<$<BOOL:${MSVC}>:/permissive->")
# or
if(MSVC)
   target_compile_options(Greeter PUBLIC /permissive)
endif()

the first line generates on unix the GreeterTarget.cmake contents:

# Create imported target Greeter::Greeter
add_library(Greeter::Greeter STATIC IMPORTED)

set_target_properties(Greeter::Greeter PROPERTIES
  INTERFACE_COMPILE_OPTIONS "\$<\$<BOOL:>:/permissive->"
  INTERFACE_INCLUDE_DIRECTORIES "${_IMPORT_PREFIX}/include/Greeter-1.0"
)
2

These two options are 100% equivalent, because ${MSVC} in the genex will be evaluated immediately (i.e. at configure time). So it comes down fully to code style preference, which is subjective (but I find the if-based form more readable).

For a case where configure-time vs. generate-time can be different, see a recent question of mine.

3

But the generator expression pollute on unix hosts the installed cmake config package!

4

Eh, “no one” looks at those. In any case, the right thing, especially for compiler and linker flags, is to use a genex. The consumer of your package might not be using MSVC. MinGW isn’t going to understand /permissive. You should do:

target_compile_options(Greeter
  PUBLIC
    "$<$<COMPILE_LANG_AND_ID:C,MSVC>:/permissive>") # or CXX
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