STM8 build errors

I’ve been trying to build an stm8 project using SPL library but I get some errors. First, stm8s_conf.h is missing and has to be manually copied into the project directory.
Next, I get a build error:
?ASlink-Warning-Undefined Global ‘_assert_failed’ referenced by module ‘stm8s_adc1’

?ASlink-Warning-Undefined Global '_assert_failed' referenced by module 'stm8s_awu'

?ASlink-Warning-Undefined Global '_assert_failed' referenced by module 'stm8s_beep'

?ASlink-Warning-Undefined Global '_assert_failed' referenced by module 'stm8s_clk'

?ASlink-Warning-Undefined Global '_assert_failed' referenced by module 'stm8s_exti'

?ASlink-Warning-Undefined Global '_assert_failed' referenced by module 'stm8s_flash'

?ASlink-Warning-Undefined Global '_assert_failed' referenced by module 'stm8s_gpio'

?ASlink-Warning-Undefined Global '_assert_failed' referenced by module 'stm8s_i2c'

?ASlink-Warning-Undefined Global '_assert_failed' referenced by module 'stm8s_itc'

?ASlink-Warning-Undefined Global '_assert_failed' referenced by module 'stm8s_iwdg'

?ASlink-Warning-Undefined Global '_assert_failed' referenced by module 'stm8s_rst'

?ASlink-Warning-Undefined Global '_assert_failed' referenced by module 'stm8s_spi'

?ASlink-Warning-Undefined Global '_assert_failed' referenced by module 'stm8s_tim1'

?ASlink-Warning-Undefined Global '_assert_failed' referenced by module 'stm8s_tim2'

?ASlink-Warning-Undefined Global '_assert_failed' referenced by module 'stm8s_tim4'

?ASlink-Warning-Undefined Global '_assert_failed' referenced by module 'stm8s_uart1'

?ASlink-Warning-Undefined Global '_assert_failed' referenced by module 'stm8s_wwdg'
*** [.pio/build/stm8sblue/firmware.elf] Error 1

I remember getting this error back when I was using sdcc+makefile toolchain, it has to do with linking and libraries, but I tried looking at the build process that platformio has and I don’t really know what to modify.

Bug reports for the STM8 platform to Issues · platformio/platform-ststm8 · GitHub with your full platformio.ini, project source code and OS information for reproduction, so that developers like @valeros can have a look at it