Genex: Fix COMPILE_LANGUAGE in SYSTEM include directories

When evaluating `INTERFACE_SYSTEM_INCLUDE_DIRECTORIES`, or evaluating
`INTERFACE_INCLUDE_DIRECTORIES` on an imported target, thread the
compile language through to the generator expression evaluator so
that it can support `$<COMPILE_LANGUAGE:...>`.

Fixes: #17811

1 files changed, 7 insertions, 0 deletions

diff --git a/Tests/IncludeDirectories/SystemIncludeDirectories/CMakeLists.txt b/Tests/IncludeDirectories/SystemIncludeDirectories/CMakeLists.txt
index 5078f30..c1059a9 100644
--- a/Tests/IncludeDirectories/SystemIncludeDirectories/CMakeLists.txt
+++ b/Tests/IncludeDirectories/SystemIncludeDirectories/CMakeLists.txt
@@ -52,6 +52,13 @@ add_library(otherlib upstream.cpp)
 target_link_libraries(otherlib PUBLIC somelib)
 target_compile_options(somelib PRIVATE -Werror=unused-variable)
 
+add_library(iface_lang IMPORTED INTERFACE)
+set_property(TARGET iface_lang PROPERTY INTERFACE_INCLUDE_DIRECTORIES
+  "$<$<COMPILE_LANGUAGE:CXX>:${CMAKE_CURRENT_SOURCE_DIR}/systemlib_header_only>"
+  )
+add_library(imported_consumer_lang imported_consumer.cpp)
+target_link_libraries(imported_consumer_lang iface_lang)
+
 macro(do_try_compile error_option)
   set(TC_ARGS
     IFACE_TRY_COMPILE_${error_option}