CMake C++ library and Fortran main program LINKER_LANGUAGE heuristic issue

Readme.md

CMake LINKER_LANGUAGE heuristic: C++ lib and Fortran main

With Linux and Intel oneAPI or NVIDIA HPC SDK, regardless of CMake version or compiler version, this CMakeLists.txt will fail to build with error as below if the "LINKER_LANGUAGE" property is not set.

/lib/../lib64/crt1.o: in function `_start':
(.text+0x1b): undefined reference to `main'

This Discourse seemed to indicate that's just the way it is.

However, this issue may indicate there could be a CMake-internal fix.

CMakeLists.txt

cmake_minimum_required(VERSION 3.19)

project(link LANGUAGES CXX Fortran)

file(GENERATE OUTPUT lib.cpp CONTENT "extern \"C\" int addtwo(int N){ return N + 2; }")

file(GENERATE OUTPUT main.f90 CONTENT "program adder

use, intrinsic :: iso_c_binding, only : C_INT

implicit none

interface
integer(C_INT) function addtwo(N) bind(C)
import C_INT
integer(C_INT), intent(in), value :: N
end function
end interface

if (addtwo(2) /= 4) error stop 'failed to add 2+2 with C++ library'

end program
")

add_executable(main_f main.f90 lib.cpp)

# comment out this line, and CMake fails to build with Linux Intel oneAPI or NVHPC
set_property(TARGET main_f PROPERTY LINKER_LANGUAGE Fortran)