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What you can do as suggested by @usr1234567 is to compile a short code that includes _Float16.

Here's a minimal working code :

cmake_minimum_required(VERSION 3.12)

project(MyProject C)

include(CheckSourceCompiles)

check_source_compiles(C "
    #define __STDC_WANT_IEC_60559_TYPES_EXT__
    #include <float.h>
    int main() {
        _Float16 x = 1.0f16;
        return 0;
    }
" HAVE_FLOAT16)

if(HAVE_FLOAT16)
    message(STATUS "_Float16 is supported by the compiler.")
else()
    message(WARNING "_Float16 is not supported by the compiler.")
endif()

I've found __STDC_WANT_IEC_60559_TYPES_EXT__ here : https://gcc.gnu.org/onlinedocs/cpp/Common-Predefined-Macros.html

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